4.0 Biology

characteristics

an identifiable or measurable feature of an organism, such as size, color, or texture

Characteristics

Characteristics that vary along a range, such as fur color or height, are called quantitative characteristics, and are often controlled by more than one gene influencing a characteristic. Characteristics that appear as either one form or another, such as flower color or blood type, are called discrete characteristics. Both types of characteristics add up to create variation in a population.

Genes are located on chromosomes and control all aspects of a cell.

Genes are the units of heredity: They are passed from parents to offspring on the chromosomes. Chromosomes are structures made of DNA and proteins, and are located in the nucleus of eukaryotic organisms. Different species have different numbers of chromosomes—for example, humans have 23 pairs of chromosomes.

The diversity of life on earth begins with genetic variation.

Genetic variation is crucial to the health of populations of species. Variation may follow different patterns in different populations. Sexual reproduction, mutation, and crossing-over during meiosis are three processes that contribute new combinations of genes to populations, ensuring that populations maintain genetic variability from generation to generation.

homozygous Recall that another term for homozygous is true-breeding. An organism that is true-breeding for a particular trait will always produce offspring that have the phenotype for that trait when it is crossed with itself.

having two identical alleles for a gene being considered

codominant

in genetics, denoting an equal degree of dominance of two genes

Genes direct the production of proteins, which have different functions in the body.

By producing various proteins, genes direct every aspect of your growth and development. In a future lesson, you will learn how a gene's DNA sequence provides the blueprints for making proteins. For now, just remember that genes direct the production of proteins Opens in modal popup window . Different genes produce different proteins, and different proteins have different functions in the body. The genes that govern characteristics such as height and skin, hair, and eye color produce proteins that play a role in how you look. Variations in these genes—which means variation in the very chemistry of the DNA segment—result in how tall you are and what color your skin, hair, or eyes are.

Genes have different forms, or alleles, that differ in their DNA sequence and in the proteins they produce. Genes 1. are specified segments of 1.1. D N A on a chromosome 2. have multiple forms called 2.1. alleles which are variants of genes with slightly different D N A sequences

A gene is a specific segment of DNA on a chromosome. It is defined by its location on the chromosome. Alleles are the various DNA sequences that may be found at that location in different individuals. Different alleles make different proteins because of the variation in DNA sequence between different alleles of the same gene.

In eukaryotes, chromosomes are located in the cell's nucleus.

All 46 chromosomes in a human cell are found in its nucleus. The same holds true for all eukaryotic organisms, or organisms with a nucleus. But what about prokaryotes—the organisms such as bacteria—that lack a nucleus? A bacterial cell contains one circular chromosome. Lacking a nucleus, that chromosome simply fills the space inside the bacterial cell. Like the chromosomes of eukaryotes, bacterial chromosomes also are made up of DNA and contain genes that produce proteins that direct many cellular processes.

From genotypes, you can predict phenotypes.

All of the offspring of the red- and brown-eyed flies are heterozygous, meaning they all have two different alleles for eye color (Rr). Can you predict what these flies will look like? When you think you know the answer, click the buttons on the Punnett square to see if you are correct. Answer The allele for red eyes (R) is dominant, so all of the offspring will have red eyes. In other words, their phenotype is red eyes. Again, keep in mind that Punnett squares show the possible traits in the offspring and the possible percentage of offspring that will have those traits. As you learned earlier, gametes contain one allele for each trait. The actual outcome of a genetic cross depends on the combination of gametes from the two parents.

Genes are located on chromosomes in the cell's nucleus.

An organism's DNA is located inside the nucleus of each of its cells. That DNA contains genes, which contain a certain DNA sequence that is different for each gene. The DNA in a cell is arranged into structures called chromosomes. An organism's genes are located on those chromosomes Opens in modal popup window , and the genes for different characteristics are often found on different chromosomes. All of an organism's cells contain the same chromosomes and, therefore, the same genes. So, the cells that make up your heart contain the same chromosomes and genes as the cells that make up your bones, even though both structures are very different. That difference comes about because different genes are activated in different cells.

Pedigrees also may show a sex-linked trait.

As you learned during your labs, sex-linked traits are carried only on the sex chromosomes. Sex-linked traits occur because an allele on the X chromosome is often not balanced by the second allele on the Y chromosome. A male, who is XY, may express a recessive allele because in his set of chromosomes, he has only one X chromosome with one allele, not two like a female. Sex-linked traits show their own unique pattern of inheritance in a pedigree. For now, focus on X-linked alleles. Since it is a sex-linked trait, color blindness may occur among more of the males than females shown here.

Different types of organisms have different numbers of chromosomes. Chromosome Numbers of Selected Eukaryotic Organisms Humans: 46 Gorilla, chimpanzee, bonobo, orangutan: 48 House mouse: 40 Horse: 64 Cattle, goat: 60 Sheep: 54 Swine: 38 Indian muntjac (a deer): 6 (female), 7 (male) Chinese muntjac: 46 Cat: 38 Dog: 78 Blue fox: 50 Red fox: 34 Chicken: 78 South African clawed frog: 36 Shrimp: 254 Crayfish: 200 Fruit fly (Drosophila melanogaster): 8 Roundworm (Caenorhabditis elegans): 12 Parasitic roundworm (Parascaris equorum var. univalens): 2 Pea: 14 Potato: 48 Sweet potato: 90 Corn (maize): 20 Field horsetail (a plant): 216 Budding yeast: 32

As you've learned, humans have 46 chromosomes, or 23 pairs, in each cell. Different species have different numbers of chromosomes, and many species have more chromosomes than humans do. For example, a camel has 70 chromosomes; a guinea pig, 64; and a king crab, 208. How many pairs of chromosomes do each of those organisms have? A camel has 35 pairs; a guinea pig, 32 pairs; and a king crab, 104 pairs. On the smaller end of the scale, a fruit fly has only 8 chromosomes, making it an ideal model organism in genetic studies. Pea plants, like the ones Mendel studied, have 14 chromosomes, and a mosquito has only 6.

Different types of organisms have different numbers of chromosomes. Chromosome Numbers of Selected Eukaryotic Organisms Humans: 46 Gorilla, chimpanzee, bonobo, orangutan: 48 House mouse: 40 Horse: 64 Cattle, goat: 60 Sheep: 54 Swine: 38 Indian muntjac (a deer): 6 (female), 7 (male) Chinese muntjac: 46 Cat: 38 Dog: 78 Blue fox: 50 Red fox: 34 Chicken: 78 South African clawed frog: 36 Shrimp: 254 Crayfish: 200 Fruit fly (Drosophila melanogaster): 8 Roundworm (Caenorhabditis elegans): 12 Parasitic roundworm (Parascaris equorum var. univalens): 2 Pea: 14 Potato: 48 Sweet potato: 90 Corn (maize): 20 Field horsetail (a plant): 216 Budding yeast: 32

As you've learned, humans have 46 chromosomes, or 23 pairs, in each cell. Different species have different numbers of chromosomes, and many species have more chromosomes than humans do. For example, a camel has 70 chromosomes; a guinea pig, 64; and a king crab, 208. How many pairs of chromosomes do each of those organisms have? Answer A camel has 35 pairs; a guinea pig, 32 pairs; and a king crab, 104 pairs. On the smaller end of the scale, a fruit fly has only 8 chromosomes, making it an ideal model organism in genetic studies. Pea plants, like the ones Mendel studied, have 14 chromosomes, and a mosquito has only 6.

Genes direct the production of proteins, which have different functions in the body.

By producing various proteins, genes direct every aspect of your growth and development. In a future lesson, you will learn how a gene's DNA sequence provides the blueprints for making proteins. For now, just remember that genes direct the production of proteins Opens in modal popup window . Different genes produce different proteins, and different proteins have different functions in the body. The genes that govern characteristics such as height and skin, hair, and eye color produce proteins that play a role in how you look. Variations in these genes—which means variation in the very chemistry of the DNA segment—result in how tall you are and what color your skin, hair, or eyes are. Genes direct the production of many different proteins, which, in turn, produce a wide variety of physical characteristics in human beings.

Genetic crosses help you understand different patterns of inheritance.

By understanding how alleles separate into gametes, you can predict the genotypes and phenotypes of the offspring that a pair of parents will produce. You also will be able to predict the frequencies of those genotypes in that generation. Understanding those principles of inheritance will also allow you to infer the genotypes of a pair of individuals based on the offspring they produce.

Alleles on the chromosomes in an organism's gametes determine the physical traits of that organism's offspring.

Consider one of Mendel's purple-flowered pea plants that is heterozygous Opens in modal popup window for flower color. If you could zoom into one of the plant's cells and examine a pair of homologous chromosomes, you would find an allele for purple flowers in one chromosome and an allele for white flowers in the other. If you happened to be looking at a cell as it goes through meiosis to produce eggs or sperm, the homologous chromosomes and their alleles would first replicate and the DNA would duplicate. After completing meiosis, the alleles would have segregated into four separate gametes, each carrying one allele for flower color. Explore the process that takes place when the pea plant with two different alleles for flower color produces two gametes with an allele for purple flowers and two gametes with an allele for white flowers.

Variation can follow different patterns in a population. Lemur Population Different populations of the same species may show different patterns in the distribution of traits. While lemurs in population A have more dark- and light-colored fur than medium-colored fur, population B shows just the opposite, more medium-colored fur than dark- or light-colored fur.

Consider two imaginary populations of the same species of lemur living in Madagascar. Assume that you and some of your colleagues just completed a survey of these two populations, in which you recorded various characteristics of the populations. In each population, you find lemurs with fur colors ranging from dark, chocolatey brown to light, sandy beige. In population A, you see an even split between dark fur and light fur, but very few in the middle shades. In population B, you see just the opposite: many lemurs with medium-colored fur, but very few with dark- or light-colored fur. Study some graphs showing the patterns of color variation for both groups. Population Variation Opens in modal popup window Why the difference in those two populations? Perhaps in A, female lemurs prefer mates with either the lightest or the darkest fur. Perhaps in B, lemurs with very dark or very light fur are easier for predators to see. Whatever the reason, it's important to recognize that traits show up in different patterns in different populations.

Variation can follow different patterns in a population. Characteristics Characteristics that vary along a range, such as fur color or height, are called quantitative characteristics, and are often controlled by more than one gene influencing a characteristic. Characteristics that appear as either one form or another, such as flower color or blood type, are called discrete characteristics. Both types of characteristics add up to create variation in a population.

Consider two imaginary populations of the same species of lemur living in Madagascar. Assume that you and some of your colleagues just completed a survey of these two populations, in which you recorded various characteristics of the populations. In each population, you find lemurs with fur colors ranging from dark, chocolatey brown to light, sandy beige. In population A, you see an even split between dark fur and light fur, but very few in the middle shades. In population B, you see just the opposite: many lemurs with medium-colored fur, but very few with dark- or light-colored fur. Study some graphs showing the patterns of color variation for both groups. Population Variation Opens in modal popup window Why the difference in those two populations? Perhaps in A, female lemurs prefer mates with either the lightest or the darkest fur. Perhaps in B, lemurs with very dark or very light fur are easier for predators to see. Whatever the reason, it's important to recognize that traits show up in different patterns in different populations.

Mendel showed that inheritance was often a matter of some kind of individual factors that were passed on from parent to offspring.

Did Mendel's work support the theory of blending inheritance? No. In each case, the traits he observed retained their distinct appearance—they didn't blend or average out over generations. Based on his data, Mendel concluded that pea plants have distinct units of inheritance contributed by each parent to the offspring. He called these units factors but was unclear about what they were. One white flower and one purple flower combine to form three purple and one white flowers When Mendel crossed pea plants with purple flowers and white flowers, their offspring did not have pale purple flowers; rather, their flowers remained either purple or white.

Variation in a population begins with variation in genes.

Different individuals in a population have different combinations of genes. You can see proof of this if you just look around you next time you're in a public place: No two people you see will look alike. The difference in genes from one individual to the next is called genetic variation. In any population of organisms, each individual will have a different set of genes from the other individuals in the population. The happy-face spiders of Maui, for example, show the effects of genetic variation in a population. The different patterns on their abdomens are controlled by different alleles. Variation in the alleles among individuals in a population gives rise to the variety in appearance you can see, as well as in characteristics you can't see such as blood type.

Dominant alleles, if present in a family, will show in various generations.

Do you know if you have a widow's peak? A widow's peak is a pattern in your hairline, in which the hair along your forehead comes to a slight point, as you can see in the photo at right. A widow's peak is coded for by a dominant allele. The double recessive condition will result in a person without a widow's peak. If the dominant allele (W) is present in a family, then the trait will show up over time again and again. You might ask, if there is a dominant allele for widow's peak, why don't most people have one? The answer is that, although the allele is dominant, it is not common in a population and thus not present in many families. If it is present, however, you might say that it is "a trait that runs in the family." This young woman exhibits a widow's peak, which is coded for by a dominant allele.

DNA in the cell's nucleus condenses into recognizable chromosomes during cell division.

During much of the cell cycle, a chromosome exists as a single molecule of DNA wrapped around specialized proteins called histones. In this phase, chromosomes are somewhat stretched out, so that the DNA sequences of their genes may be reached by cellular proteins that control gene expression. When the cell is preparing to divide, however, the DNA replicates and begins to coil around itself and around the histone Opens in modal popup window , forming tightly compacted coils. Those coils condense into the distinct stubby structure of a chromosome. To learn more about this process, explore the online activity on chromosome formation. You may also review chromosome formation on pages 70-71 in your reference book.

DNA in the cell's nucleus condenses into recognizable chromosomes during cell division. Histone certain proteins found in association with chromosomes Chromosome formation Nucleosomes: DNA and histones form beadlike structures. Chromatin fiber: DNA strands with their nucleosomes coil around themselves to form a chromatin fiber. Looped chromosome fiber: Chromatin fiber loops along another protein molecule. Metaphase chromosome: Condensed sections of a looped chromatin fiber fold up on itself again.The result is a condensed metaphase chromosome.

During much of the cell cycle, a chromosome exists as a single molecule of DNA wrapped around specialized proteins called histones. In this phase, chromosomes are somewhat stretched out, so that the DNA sequences of their genes may be reached by cellular proteins that control gene expression. When the cell is preparing to divide, however, the DNA replicates and begins to coil around itself and around the histone Opens in modal popup window , forming tightly compacted coils. Those coils condense into the distinct stubby structure of a chromosome. To learn more about this process, explore the online activity on chromosome formation. You may also review chromosome formation on pages 70-71 in your reference book.

Some characteristics do not follow simple patterns of dominance and recessiveness.

Four o'clocks are common garden flowers that exhibit a curious color inheritance pattern. If you cross a homozygous plant with dark pink flowers and a homozygous plant with white flowers, the offspring will have light pink flowers. How can this be? You know that blending inheritance is not consistent with the existence of alleles and genes. How might you explain this phenomenon? Flower color in four o' clocks is an example of an inheritance pattern called incomplete dominance. Neither the white allele nor the dark pink allele is dominant, so a plant that is heterozygous for flower color has an intermediate phenotype—in this case, light pink flowers. Look at the Punnett square, and predict the genotypes and phenotypes of the offspring. When you think you have the answers, click the buttons to see if you are correct.

The DNA found in genes provides instructions to the cell. How Many Genes? The exact number of genes in the human genome is still being determined. Long thought to be in the neighborhood of 30,000, some scholars are revising the number downward to around 25,000.

From bacteria to blue whales, the DNA found in genes provides the cell with the instructions it needs to carry out the many functions of life. Genes themselves do not produce anything in the cell. Instead, they contain the set of DNA instructions that the cell uses to build proteins. Another molecule in the cell called RNA reads the chemical structure of the length of DNA in a gene, and uses that information to begin building a protein. The proteins encoded for in an organism's DNA govern all aspects of the cell's growth, development, and maintenance. Scientists are actively researching the entire set of genes found in many organisms, from humans to worms to trees. The entire set of genes in an organism is called its genome. Identifying all genes in our bodies is one of the first steps to better understanding all processes that keep us alive, and how diseases occur when those processes malfunction.

The DNA found in genes provides instructions to the cell.

From bacteria to blue whales, the DNA found in genes provides the cell with the instructions it needs to carry out the many functions of life. Genes themselves do not produce anything in the cell. Instead, they contain the set of DNA instructions that the cell uses to build proteins. Another molecule in the cell called RNA reads the chemical structure of the length of DNA in a gene, and uses that information to begin building a protein. The proteins encoded for in an organism's DNA govern all aspects of the cell's growth, development, and maintenance. Scientists are actively researching the entire set of genes found in many organisms, from humans to worms to trees. The entire set of genes in an organism is called its genome. Identifying all genes in our bodies is one of the first steps to better understanding all processes that keep us alive, and how diseases occur when those processes malfunction. Scientists studying the human genome estimate that humans have about 25,000 genes.

Individuals can have two of the same alleles or two different alleles. The alleles for a characteristic are usually assigned the same letter: uppercase for the dominate trait and lowercase for the recessive trait.

Furthermore, Mendel stated that for each characteristic, one trait would show dominance over another. In other words, if both forms of a trait—such as seed color—are present in an individual, only one of those traits will be visible. We now say that an individual with two identical alleles for a trait, such as two alleles for yellow seeds, is homozygous for the trait. The prefix homo- means same. An individual with two different alleles, such as one for green seeds and one for yellow seeds, is said to be heterozygous. The prefix hetero- means different. Remember, a zygote is the result of an egg and sperm fusing. Can you explain the meaning of the prefixes in the terms homozygous and heterozygous? Answer Homozygous means that the zygote contains two alleles that are the same, or identical for a trait. Heterozygous means that the zygote contains alleles that are different for a trait.

A slight difference in the two alleles will cause the development of two slightly different proteins. Sickle Cell Disease Individuals who have two S alleles are said to have sickle-cell disease. Because they have two S alleles for the trait, those individuals have red blood cells that contain abnormal hemoglobin. The hemoglobin sticks together, giving the red blood cells that contain these molecules a sickle appearance. The blood cells can clot in the body, causing pain. The body also destroys them more quickly than normal cells, causing anemia.

Gene HBB has two alleles, A and S. Normal adult hemoglobin, like the molecule you just read about, is produced by the allele A. That molecule has a shape that allows it to bind with oxygen molecules. There is another hemoglobin allele. Allele S produces abnormal hemoglobin molecules, which tend to stick to one another and cause red blood cells to become distorted, taking on a puckered or sickle shape. Differences in the DNA sequences of alleles A and S account for the difference in the hemoglobin molecules produced. The alleles A and S are codominant Opens in modal popup window . Earlier, you learned that codominant alleles are both expressed when present in an individual who is heterozygous for the trait. Therefore, someone who has a genotype of AS will exhibit both normal and misshapen blood cells. Individuals who have all normal hemoglobin in their blood have the genotype AA.

A slight difference in the two alleles will cause the development of two slightly different proteins. Sickle Cell Disease Individuals who have two S alleles are said to have sickle-cell disease. Because they have two S alleles for the trait, those individuals have red blood cells that contain abnormal hemoglobin. The hemoglobin sticks together, giving the red blood cells that contain these molecules a sickle appearance. The blood cells can clot in the body, causing pain. The body also destroys them more quickly than normal cells, causing anemia.

Gene HBB has two alleles, A and S. Normal adult hemoglobin, like the molecule you just read about, is produced by the allele A. That molecule has a shape that allows it to bind with oxygen molecules. There is another hemoglobin allele. Allele S produces abnormal hemoglobin molecules, which tend to stick to one another and cause red blood cells to become distorted, taking on a puckered or sickle shape. Differences in the DNA sequences of alleles A and S account for the difference in the hemoglobin molecules produced. The alleles A and S are codominant Opens in modal popup window . Earlier, you learned that codominant alleles are both expressed when present in an individual who is heterozygous for the trait. Therefore, someone who has a genotype of AS will exhibit both normal and misshapen blood cells. Individuals who have all normal hemoglobin in their blood have the genotype AA. Several normal and some sickle red blood cells Sickle-shaped and regular-shaped blood cells indicate the presence of the two codominant alleles, A and S, for hemoglobin.

Genetic variation is important to the survival of species. Cheetah Conservation Maintaining genetic diversity is especially important to the survival of rare and endangered species. Cheetahs are one of the world's most threatened species. Not only are their numbers very low, but there is very little genetic variation among the cheetahs that exist today, making them more susceptible to disease.

Genetic variation is enormously important to the survival of species. Species with genetically diverse populations are better equipped to hedge their bets against environmental variables, such as weather extremes, changes in food supply, or disease. What if a potentially lethal parasite infects a population of turtles living in a pond? If all the turtles are genetically identical, they all would have the same susceptibility to the parasite. If none of those turtles have the genetic means to resist infection, all the turtles would become sick and die. But if that population of turtles is genetically diverse, some turtles may have genes that help them withstand infection by this parasite. Some turtles in the population still may become sick, but those individuals with different genes might survive the infection. And survival means those turtles may live to produce offspring.

Genetic variation is important to the survival of species. Cheetah Conservation Maintaining genetic diversity is especially important to the survival of rare and endangered species. Cheetahs are one of the world's most threatened species. Not only are their numbers very low, but there is very little genetic variation among the cheetahs that exist today, making them more susceptible to disease.

Genetic variation is enormously important to the survival of species. Species with genetically diverse populations are better equipped to hedge their bets against environmental variables, such as weather extremes, changes in food supply, or disease. What if a potentially lethal parasite infects a population of turtles living in a pond? If all the turtles are genetically identical, they all would have the same susceptibility to the parasite. If none of those turtles have the genetic means to resist infection, all the turtles would become sick and die. But if that population of turtles is genetically diverse, some turtles may have genes that help them withstand infection by this parasite. Some turtles in the population still may become sick, but those individuals with different genes might survive the infection. And survival means those turtles may live to produce offspring.

A karyotype is an ordered array of an organism's chromosomes. http://learn.genetics.utah.edu/content/chromosomes/karyotype/

Geneticists rely on a tool called a karyotype to help them identify chromosomal changes that might cause diseases or other conditions. Missing chromosomes, extra chromosomes, or chromosomes with missing parts all can cause various genetic disorders. A karyotype Opens in modal popup window is an ordered visual display of all of a person's chromosomes, arranged from largest to smallest. Chromosomes are numbered according to the karyotype, with chromosome 1 being the largest. Click Making a Karyotype to visit a website to learn more about karyotypes.

A karyotype is an ordered array of an organism's chromosomes.

Geneticists rely on a tool called a karyotype to help them identify chromosomal changes that might cause diseases or other conditions. Missing chromosomes, extra chromosomes, or chromosomes with missing parts all can cause various genetic disorders. A karyotype Opens in modal popup window is an ordered visual display of all of a person's chromosomes, arranged from largest to smallest. Chromosomes are numbered according to the karyotype, with chromosome 1 being the largest. Click Making a Karyotype to visit a website to learn more about karyotypes. http://learn.genetics.utah.edu/content/chromosomes/karyotype/

4.02 Mendelian Inheritance Alleles segregate during meiosis. When gametes form during meiosis, pairs of homologous chromosomes carrying alleles separate, and each gamete receives different genetic information.

Gregor Mendel proposed that organisms have two units of genetic information for each characteristic, such as flower color or pea pod shape, and that organisms receive one unit per characteristic from each parent. Today we call Mendel's genetic units alleles. Alleles Opens in modal popup window are different versions of the same gene. When gametes form during meiosis, pairs of homologous chromosomes carrying alleles separate, and each gamete receives only one allele per characteristic. The homologous chromosomes pair up during prophase I, and one chromosome from each pair ends up in a new cell after cytokinesis. Take a moment to review this process: Meiosis I and II Opens in modal popup window The vast majority of cells that make up any body are called autosomes, which means they are not an egg or sperm cell.

Gregor Mendel's studies of heredity established the field of modern genetics.

Gregor Mendel, a monk experimenting with pea plants in the mid-1800s, helped establish today's vast field of genetics. By applying rigorous experimental methods and mathematical analysis to the breeding of pea plants, he was able to uncover many basic rules of heredity. He established that the units of heredity, which we now know as genes, exist as pairs in organisms and segregate during the formation of gametes. He suggested that for any characteristic, one form would be consistently expressed over another if both were present in an organism. He also showed that the genes for different characteristics assort independently of one another into gametes.

Down Syndrom

Humans normally have 23 pairs of chromosomes in their cells. Occasionally, however, a person may have more than two copies of one of the 23 chromosomes. Down syndrome, or trisomy 21, is a condition that occurs when a person has 3 copies of chromosome 21

Different alleles have different DNA sequences.

If a gene is a specific segment of DNA on a chromosome, and alleles are different forms of a gene, then what makes one allele different from the next? While a gene's alleles have almost identical DNA sequences, the alleles do have slight differences in their chemistries—DNA sequences—that set them apart. The order of DNA chemical units called nitrogenous bases are an allele. Different alleles of the gene have small changes in the sequence of these nitrogen bases.

The laws of probability help you predict the outcome of genetic crosses. Laws of Probibility In this example, two of each fly's gametes could contain the R allele and two could contain the r allele. When the gametes fuse, therefore, some zygotes could contain RR alleles, some could contain Rr, and some could contain rr. Since the R allele is dominant, there is a 3 to 1 chance that the offspring will exhibit the dominant trait. This ratio is reflected in the genotypes of the possible offspring in this Punnett square (RR, Rr, Rr, rr).

Have you ever changed your plans for the day because of the weather forecast? Perhaps there was a 60 percent chance of a snowstorm. A weather forecast is an example of a prediction based on probability—the chance that a specific outcome, like a snowstorm, will occur. Probabilities may be described as percentages, ratios, or fractions. In genetics, probability determines the likelihood that a specific allele will occur in a gamete. It also determines the outcome of genetic crosses. What is the probability that the offspring of a homozygous red-eyed fly and a homozygous brown-eyed fly will have red eyes? In this case, the probability of that outcome is 1, because all of the offspring will be heterozygous and will therefore show the dominant trait. But what is the probability that a fly of the next generation will have red eyes? Here is the Punnett square of the F2 generation that you saw on the last screen. One fly has the genotype RR, two have the genotype Rr, and one has the genotype rr. Three out of four have red eyes, so the probability of offspring with red eyes is 3/4, or a 3:1 ratio. The probability that a fly of the F2 generation will have red eyes is 3/4; the probability that it will have brown eyes is 1/4. In other words, it is a 3:1 ratio.

An individual needs both alleles of a recessive trait to express it. Punnet vs Pedigree While a Punnett square for a cross of the heterozygous parents in the chart at right would predict an F1 generation ratio of 1 FF: 2 Ff: 1 ff, the pedigree chart reveals a different reality. This is a key distinction between Punnett squares and pedigrees: The Punnett square predicts possibilities, the pedigree records reality. In this case, the heterozygous parents actually produced oneFF, oneFf, and twoff offspring.

Here is a pedigree that shows how free earlobes and attached earlobes are transmitted from one generation to the next. Neither parent expresses the recessive attached earlobe trait. Yet, two of their four offspring express it. How can this be? Remember, an individual needs two alleles of a recessive trait to express it. The parents do not express the trait, but somehow two of their children do. Those children each inherited an allele for attached earlobes from each parent, which tells you that the parents must be heterozygous for the trait. F represents the dominant free earlobe allele, and f represents the recessive attached earlobe allele. Shaded individuals exhibit attached earlobes. Remember, an individual needs two alleles of a recessive trait to express it. The parents do not express the trait, but somehow two of their children do. Those children each inherited an allele for attached earlobes from each parent, which tells you that the parents must be heterozygous for the trait. F represents the dominant free earlobe allele, and f represents the recessive attached earlobe allele. Shaded individuals exhibit attached earlobes.

Sexual reproduction gives rise to genetic variation.

How do sexual reproduction and crossing-over contribute to genetic variation in populations? Sexual reproduction Opens in modal popup window results in offspring with genetic variation. In sexual reproduction, gametes (sex cells)—an egg cell from the mother and a sperm cell from the father—join together to produce a new individual organism. Each gamete is haploid (1n), meaning that it carries half of the genetic material found in the other cells in the body. Recall that each cell in your body, except sex cells, contains 23 pairs of chromosomes. Your skin cells, muscle cells, nerve cells, and all the other cells that make up your body each contain 23 pairs of chromosomes, or 46 total chromosomes. When gametes form, the chromosomes assort independently. The two paired chromosomes move apart from one another, with one chromosome going into one gamete, and the other chromosome going into another. Independent assortment occurs for each of the 23 pairs of chromosomes, making 223 possible combinations of chromosomes in a gamete.

Sexual reproduction gives rise to genetic variation.

How do sexual reproduction and crossing-over contribute to genetic variation in populations? Sexual reproduction Opens in modal popup window results in offspring with genetic variation. In sexual reproduction, gametes (sex cells)—an egg cell from the mother and a sperm cell from the father—join together to produce a new individual organism. Each gamete is haploid (1n), meaning that it carries half of the genetic material found in the other cells in the body. Recall that each cell in your body, except sex cells, contains 23 pairs of chromosomes. Your skin cells, muscle cells, nerve cells, and all the other cells that make up your body each contain 23 pairs of chromosomes, or 46 total chromosomes. When gametes form, the chromosomes assort independently. The two paired chromosomes move apart from one another, with one chromosome going into one gamete, and the other chromosome going into another. Independent assortment occurs for each of the 23 pairs of chromosomes, making 2^23 possible combinations of chromosomes in a gamete. These siblings have the same parents, so why don't they all look alike?

Abnormal adult hemoglobin is produced by the allele S. The homozygous SS will have all misshapen blood cells.

How does one allele produce a hemoglobin molecule that behaves differently than the other? A comparison of the DNA sequences of both alleles A and S reveals just one difference between them—the switch of a T for an A in the DNA sequence. That difference produces abnormal hemoglobin by causing the body to replace one amino acid for another when building molecules of hemoglobin.

Abnormal adult hemoglobin is produced by the allele S. The homozygous SS will have all misshapen blood cells. Vernon Ingram Scientist Vernon Ingram developed a new lab technique in 1957 that helped him determine the amino acid substitution that produces the abnormal hemoglobin characteristic of sickle-cell disease

How does one allele produce a hemoglobin molecule that behaves differently than the other? A comparison of the DNA sequences of both alleles A and S reveals just one difference between them—the switch of a T for an A in the DNA sequence. That difference produces abnormal hemoglobin by causing the body to replace one amino acid for another when building molecules of hemoglobin.

A chromosome is made up of DNA and structural proteins.

If you were to stretch out the DNA in one of your cells, it would measure nearly 2 m in length from end to end, which is probably longer than you are tall. How is it possible for that amount of DNA to fit into the nucleus, which is only about 0.5 µm, or 0.000005 m, in diameter? Histones, the structural proteins, play an important role in helping to compact the DNA molecule so that it fits into a nucleus. Strands of DNA wrap around histones to produce structures that look much like beads on a string, making thick fibers that give chromosomes their recognizable tubular structure.

A chromosome is made up of DNA and structural proteins.

If you were to stretch out the DNA in one of your cells, it would measure nearly 2 m in length from end to end, which is probably longer than you are tall. How is it possible for that amount of DNA to fit into the nucleus, which is only about 0.5 µm, or 0.000005 m, in diameter? Histones, the structural proteins, play an important role in helping to compact the DNA molecule so that it fits into a nucleus. Strands of DNA wrap around histones to produce structures that look much like beads on a string, making thick fibers that give chromosomes their recognizable tubular structure.DNA wraps around histones to form a beaded-chainlike structure.

During meiosis, crossing-over produces new combinations of genes in eggs and sperm.

Imagine a mother bird has a yellow beak and long tail feathers that are blue—all dominant traits. Assume that the genes for beak color, tail-feather length, and feather color are linked on the same chromosome. A chromosome from the mother bird then would carry alleles for a yellow beak, long tail feathers, and blue feathers. Based on the arrangement of genes on the chromosome you see in the first image at right, she should produce gametes that contain either a chromosome with alleles Y (yellow beak), L (long tail), B (blue feathers), or with alleles b, l, and y. If her chromosomes undergo crossing-over during meiosis, however, the mother bird will produce four possible combinations of alleles in her gametes, as you can see at right.

Geneticists use pedigrees to study human diseases.

In a research project spanning 20 years, scientist Nancy Wexler has assembled pedigrees of over 18,000 people in Venezuela with Huntington's disease, a disorder causing deterioration of brain tissue in adulthood. Her research led to the identification of the gene responsible for the disorder. Like Dr. Wexler, many other scientists use pedigrees in their study of genetic disorders such as hemophilia, sickle-cell disease, and cystic fibrosis. In the past, pedigree analysis of families with disorders helped geneticists determine how the disorders were transmitted from one generation to the next. Today, pedigrees help in the search for the genes that cause diseases. One Sickle red blood cell and three normal red blood cells Pedigrees help scientists better understand the transmission of hereditary disorders, such as the one that causes sickle-shaped blood cells.

Mendel showed that alleles of different genes assort independently during gamete formation.

In conducting his experiments, Mendel observed another pattern: The inheritance of one characteristic had no influence over the inheritance of other characteristics. For example, the inheritance pattern for seed color was completely independent of the inheritance pattern for pea pod shape. Today, scientists refer to this phenomenon as the law of independent assortment. Genes associated with different characteristics assort independently of one another during gamete formation.

A Punnett square also lets you predict the traits of future generations.

In genetic terms, you would say that the first generation—the F1 generation—of the RR and rr cross is all heterozygous for eye color (Rr). You will also need your Biology reference guide. Keep it handy as you work through the lesson. What would you expect to see in the F2 generation, or a cross between two flies from the F1 generation? Complete a second Punnet square based on this information.

Gregor Mendel experimented with the pea plants in his garden. Because of his groundbreaking work on heredity, Gregor Mendel is considered the father of modern genetics.

In the mid-1800s, an Austrian monk named Gregor Mendel Opens in modal popup window began his own experiments with plants. Unlike his predecessors, he used rigorous scientific methods in his experiments. Mendel conducted his experiments with pea plants, which he grew in the garden of his monastery. The plants are easy to grow, reproduce quickly, and have several distinct characteristics Opens in modal popup window , or features, that are easy to observe. Characteristics of pea plants include flower color, seed color, and pea pod shape. Moreover, Mendel knew that each characteristic exists as one of two traits Opens in modal popup window , or forms of the character. For example, the seed color characteristic exists as one of two traits: green or yellow.

Sex-linked traits do not appear equally in males and females.

Individuals with the rare blood disorder hemophilia Opens in modal popup window have blood that does not clot properly. It's a condition found much more often in males than in females. What does this suggest to you about its pattern of inheritance? Answer Hemophilia is a recessive X-linked disorder. Turn to pages 86-87 of your reference book, and study the pedigree of the British royal family. Then try to answer these questions. What do you notice about the expression of this trait? Answer Most of the male offspring have hemophilia, while the females are carriers. How does that tell you this is linked to an X chromosome? Answer The females in the chart are apparently heterozygous in their genes for hemophilia and, therefore, are only carriers. Do any women express the trait? Answer Because males receive only one X chromosome, the recessive allele for hemophilia will always express. Queen Victoria, a carrier for hemophilia, sits with her large family. Many of her male descendents suffered from the disease.

Each of your cells contains 23 pairs of chromosomes. Gametes, or sex cells, are the only cells in your body that have only 23 chromosomes. They contain only 1 chromosome of each pair. When gametes come together during sexual reproduction, each gamete contributes its 23 chromosomes to the new offspring, giving it a full set of 46 chromosomes, or 23 pairs.

Inside the nucleus of each of your cells, there are 23 pairs of chromosomes, for a total of 46 chromosomes. The first pairs of chromosomes are numbered 1 through 22. The last pair of chromosomes, the sex chromosomes, are labeled X and Y. Each chromosome carries the same genes from one person to the next, though different people may have different alleles of those genes. For example, chromosome 15 contains a gene for eye color, and this gene has two alleles, one for brown eyes and one for blue eyes. The brown allele is dominant to the blue, so a heterozygous individual will always have brown eyes.

Each of your cells contains 23 pairs of chromosomes. This karyotype of a human female shows 23 pairs of chromosomes. A human male would show one X chromosome and one Y chromosome at pair 23. Gametes Gametes, or sex cells, are the only cells in your body that have only 23 chromosomes. They contain only 1 chromosome of each pair. When gametes come together during sexual reproduction, each gamete contributes its 23 chromosomes to the new offspring, giving it a full set of 46 chromosomes, or 23 pairs.

Inside the nucleus of each of your cells, there are 23 pairs of chromosomes, for a total of 46 chromosomes. The first pairs of chromosomes are numbered 1 through 22. The last pair of chromosomes, the sex chromosomes, are labeled X and Y. Each chromosome carries the same genes from one person to the next, though different people may have different alleles of those genes. For example, chromosome 15 contains a gene for eye color, and this gene has two alleles, one for brown eyes and one for blue eyes. The brown allele is dominant to the blue, so a heterozygous individual will always have brown eyes.

Mendel's Pea Plants Transcript (Video with Audio Description) Screen 1: 00:00:00.00 Description: There is a drawing of pea pods, then an image of real pea pods. Two men are standing in a garden with tall plants. 00:00:09.00 Male Narrator 1: Mendel used a painstaking method for fertilizing his plants, transferring pollen from the male part of one plant to the female part of another. 00:00:18.00 Description: One man uses tweezers to remove a piece of a plant, then removes a red flower from another plant. He opens a part of the red flower that has pollen on it and wipes it on a part of the other plant. 00:00:31.00 Male Narrator 2: Ah, you're castrating it. Male Narrator 3: Absolutely. Male Narrator 2: So then we go to the other flower, Male Narrator 3: Yep, so over here. Male Narrator 2: which is the red one. Male Narrator 3: Pull that off. Male Narrator 2: Pull it off? Male Narrator 3: Absolutely. Male Narrator 2: It's not very kind, is it, this sex business? Male Narrator 3: No, it's rough and ready. Here we go. Right. Male Narrator 2: And this is how Mendel did it? Male Narrator 3: Yes, same techniques, really. Right, so this time we use the style of this one, which has already got pollen on it. Male Narrator 2: Yeah. Male Narrator 3: We use it as a paintbrush. All right. That's it. Sex in peas. And there we are. Male Narrator 2: That's it! Male Narrator 3: That's it. 00:01:04.00 Description: An actor portraying Mendel walks out a door and into a garden. He inspects pea plants. There are images of different parts of pea plants. 00:01:16.00 Male Narrator 1: Mendel's research was incredibly thorough. Over 10 years, he studied more than 20,000 plants and kept detailed records. He bred generation after generation of peas, and he looked at how the various characteristics cropped up. Eventually he noticed a consistent and remarkable pattern. 00:01:38.00 Description: The Male Narrator sits at a desk. As he speaks, he places a small picture of a red flower on the desk. Next to it, he places a small picture of a white flower. Below them he places two pictures of a red flower. Below them, in a third row, he places three pictures of red flowers and one picture of a white flower. 00:02:01.00 Male Narrator 1: What Mendel discovered was really weird. He took an absolutely true breeding, red-flowered pea - he bred it for generations and it was always red - and he took also a true breeding white-flowered pea, and then he deliberately crossed these. And what he got was only red peas. It looked as if the white had been lost. But he didn't give up there. He let these ones self-fertilize, and in the next generation, well it began to look as if the white had been lost, but then out popped the white again. And he bred thousands of them, and he got consistently a three-to-one ratio of red to white. 00:02:46.00 Description: Mendel walks in the garden. He inspects the pea plants and looks at the flowers. 00:02:54.00 Male Narrator 1: Mendel had discovered a fundamental law of inheritance. When plants breed, each parent passes on a factor, a set of instructions, for creating every one of the offspring's physical characteristics. Only one of these factors is activated, but the other factor lies dormant and can reappear in later generations.

Learn how Gregor Mendel used a painstaking method of fertilizing many generations of pea plants to discover their patterns of inheritance, setting the foundation for the science of genetics.

An allele's DNA sequence determines the protein that it produces. Adult Hemoglobin Notice how the hemoglobin in this example is adult hemoglobin? That's to make the distinction between adult hemoglobin and fetal hemoglobin, which circulates in a developing embryo. Fetal hemoglobin has different properties than adult hemoglobin, and is replaced by adult hemoglobin shortly after birth.

Let's take a closer look at the relationship among genes. As an example, we'll use a real protein that you have in your body— hemoglobin Opens in modal popup window , which is produced by a gene called HBB. This gene is located on chromosome 11. Hemoglobin is a large protein made of four chains of amino acids wound together. The protein exists in your red blood cells, and it carries oxygen throughout your whole body. The shape of the hemoglobin molecule makes it possible for the protein to bind with oxygen molecules and hold onto them as they travel to your body's tissues. Geneticists call the allele for normal adult hemoglobin HbA. To keep it simple, just call it allele A. The sequence of DNA bases in allele A will direct the development of hemoglobin of a very specific shape.

An allele's DNA sequence determines the protein that it produces. Adult Hemoglobin Notice how the hemoglobin in this example is adult hemoglobin? That's to make the distinction between adult hemoglobin and fetal hemoglobin, which circulates in a developing embryo. Fetal hemoglobin has different properties than adult hemoglobin, and is replaced by adult hemoglobin shortly after birth.

Let's take a closer look at the relationship among genes. As an example, we'll use a real protein that you have in your body— hemoglobin Opens in modal popup window , which is produced by a gene called HBB. This gene is located on chromosome 11. Hemoglobin is a large protein made of four chains of amino acids wound together. The protein exists in your red blood cells, and it carries oxygen throughout your whole body. The shape of the hemoglobin molecule makes it possible for the protein to bind with oxygen molecules and hold onto them as they travel to your body's tissues. Geneticists call the allele for normal adult hemoglobin HbA. To keep it simple, just call it allele A. The sequence of DNA bases in allele A will direct the development of hemoglobin of a very specific shape. Twenty-two strands of chromosomes of different sizes with number one to twenty-two below them with an X chromosome and eleventh strand is highlighted Genes on chromosome 11 affect not only hemoglobin shapes, but also tendencies toward osteoporosis and lung, breast, and other cancers

A pedigree can help identify the genotype of each individual.

Let's use an example to learn how to determine genotypes in a family. The parents are in the top row with a horizontal line connecting them. In this example, W indicates the dominant allele for a widow's peak, and w indicates the recessive allele that corresponds to lack of a widow's peak. Shading indicates the expression of the trait under study—in this case, the presence of a widow's peak. Explore the example.

Mendel's experiments allowed him to glimpse some of the basic patterns of inheritance.

Mendel asked: What happens when you mate, or cross, a true-breeding yellow seed-producing pea plant with a true-breeding green seed-producing pea plant? In Mendel's time, most people believed in blending inheritance, in which traits from both parents are mixed together to create offspring with traits intermediate to those of their parents. According to that idea, a cross between white flowers and purple flowers should have produced pale purple flowers, a shade intermediate between the two parents—as would happen if you mixed white and purple paint together. But that's not what Mendel found. Circles of two colors yellow and blue coming from two directions and blending to form green color circle Before Mendel's studies, heredity was thought to result in an intermediate blend of parental traits; for example, a yellow parent and a blue parent would yield green offspring.

Gregor Mendel applied experimental methods to the study of heredity in plants.

Mendel studied the transmission of several characteristics—including height, seed and flower color, and pea pod shape—in pea plants. He designed a series of well-designed, controlled experiments that explored the transmission of characteristics. For each characteristic of pea plants, he first grew groups of plants that he allowed only to self-pollinate. Mendel maintained those self-pollinating groups for several generations, so that each group of plants would eventually be true-breeding, producing offspring with the same character—in other words, all the offspring of white-flowered plants would also have white flowers. He called those plants the P generation, or parental generation plants. The seven characteristics that Mendel studied are listed in this chart and on page 80 of your reference book.

Mendel then created an F2 generation and observed its traits. A cross between two plants grown from F1-generation seeds will produce three-fourths yellow seeds and one-fourth green seeds in the F2 generation.

Mendel then allowed those F1 plants to self-pollinate. Remember, they all produced yellow seeds. The offspring of those seeds, called the F2 generation, provided another surprise. They produced a ratio of 3 yellow seeds for every 1 green seed. Mendel repeated the same type of cross for all of the other characteristics of pea plants. He found identical results: The F1 generation displayed only one of the two traits. The F2 generation displayed both traits, in a 3:1 ratio, with the trait that was not displayed in the F1 generation making up the minority of the F2 population. How could he explain these findings?

Mendel created an F1 generation and observed its traits. A cross between a true-breeding plant grown from a yellow seed and a true-breeding plant from a green seed in the P generation will produce yellow seeds in the F1 generation. Filial The term filial means generations following the parents, and derives from a Latin term meaning son.

Mendel took plants that were true-breeding for green seeds and crossed them with plants that were true-breeding for yellow seeds. He devised an experiment to test what would happen when he cross-fertilized the two types. He dusted pollen from yellow seed-producing plants onto the flowers of green seed-producing plants, and then dusted pollen from green seed-producing plants onto flowers of yellow seed-producing plants. To his surprise, the next generation of seeds did not come out as an intermediate shade of green and yellow. Instead, all seeds in the next generation were yellow. He called this generation the first filial generation, or F1 generation. Turn to page 81 of your reference book to explore this process as it relates to flower color.

Mendel's factors are now known as genes, which often come in twos and are known as alleles.

Mendel used mathematical analysis to further determine that each plant must have two units of inheritance, or two factors—one from each parent. In addition, he proposed that parents have two units for each characteristic, and that when they form gametes (egg or sperm cells), each gamete receives only one unit per characteristic. Today, we call these units alleles. Alleles Opens in modal popup window are different versions of the same gene Opens in modal popup window . In peas, a gene is responsible for seed color. Two different versions of the gene, or two alleles, determine the specific seed color.

The significance of Mendel's work was not recognized for many years.

Mendel's findings and conclusions are especially impressive because at the time he was observing his pea plants, no one had ever conceived of a gene or knew about DNA or chromosomes. In fact, the scientific community of the day largely ignored his research, despite its rigor. Mendel communicated his results publicly, with a lecture delivered in 1865 and a published scientific article in 1866, but scientists of the day did not recognize or understand the significance of his work. Decades later, scientists using microscopes discovered chromosomes and described the process of meiosis. By 1900, three different scientists studying plant genetics realized that Mendel's theories about how characteristics are transmitted could be explained in terms of chromosomes. Microscopic view of X and Y chromosomes The discovery of chromosomes supported Mendel's theories on how characteristics are transmitted.

Keep Accurate Data

Mendel's meticulous experimental design and record keeping gave his work the credibility to later be recognized as key to explaining many aspects of heredity. As you will learn, Mendel uncovered some of the rules of inheritance that scientists continue to study today. But it was the solid wealth of accurate data that convinced other scientists of his accuracy.

A Punnett square is a grid that shows possible allele combinations.

Next time you see a fruit fly buzzing around some overripe bananas, try to get a close look at its eyes. What color are they? Believe it or not, fruit flies can have many different eye colors. Eye color is controlled by different alleles. You can use a Punnett square to predict the genotype and phenotype of the offspring of a pair of fruit flies. Draw a Punnett square on your Student Guide, and then fill it in as you read the next few screens. In fruit flies, the allele for red eyes (R) is dominant over the allele for brown eyes (r). Assume you're crossing a homozygous Opens in modal popup window red-eyed fly (RR) with a homozygous brown-eyed fly (rr). Draw a box with 4 squares like this one and write an R beside each box on the left to represent the RR fly. Write an r above the two top boxes to represent the rr fly. Use the example on-screen to find out if you did it right.

A Punnett square can help you predict the ratio of offspring that could inherit certain traits. This is one example of how a Punnett square can help you predict the ratio of offspring with particular traits. In this case, all of the RR fly's gametes will contain the R allele; all of the rr fly's gametes will contain the r allele. So when the two flies mate, there is a 100 percent chance that the gametes that fuse to form a zygote will have both alleles.

Now you are ready to fill in the grid. Filling in the grid helps you predict the ratio of offspring that will inherit specific genotypes from their parents. First, write one R from the RR fly inside the top left box in the Punnett square on your Student Guide. Then write one r from the rr fly inside the same box. You should now have Rr inside the top left box. Match up the parents' genotypes in the same way inside the other boxes, and then click the buttons to see if you did it correctly. The genotypes inside the boxes represent the genotypes of the offspring that this particular pair of flies could produce. Look at the genotypes inside the boxes. Do you notice a pattern? Answer All of the offspring are heterozygous, meaning they all have two different alleles for eye color (Rr). No matter how many offspring this pair of flies produces, 100 percent will be heterozygous.

A pedigree shows the flow of genetic information in a family.

Of the four P generation parents, only one mother is homozygous for the recessive allele and thus has attached earlobes. The other three parents are hetereozygous for the trait. In the F1 generation, the first set of children, some have attached earlobes. The parents from the next generation do not have attached earlobes. Based on Punnett square probabilities, you would expect some of their children to exhibit attached earlobes since the parents have a recessive allele. In fact, none of them exhibit the recessive trait.

Pedigrees are helpful tools for showing and predicting how alleles are passed through a group of related individuals over many generations.

Pedigree analysis helps determine modes of inheritance and is especially helpful when considering traits in humans. Pedigrees consider the transmission of alleles from one generation to the next, and by closely examining the frequency of alleles in each generation, it is possible to determine if a trait is coded for by a dominant, recessive, or X-linked allele.

selective breeding

People used their understanding of heredity to domesticate animals. Animals such as cattle, sheep, and dogs were selectively bred to achieve desirable qualities such as disease resistance or docile behavior.

A Punnett square helps you predict possible genotypes of offspring based on the parents' genotypes.

Plant and animal breeders need to be able to predict the characteristics of the offspring of a given set of parents. One tool they rely on is a Punnett square Opens in modal popup window . A Punnett square shows all possible ways in which alleles from both parents may combine. It helps people predict the frequency of offspring genotypes based on parent genotypes. A Punnett square uses the facts that alleles separate during gamete formation, and that new combinations of alleles are created when an egg and sperm join. This Punnet square shows the possible offspring of two purple-flowered pea plants that are heterozygous for flower color (Pp ). Study the square for a few minutes to see how the genotypes and phenotypes of the parents match up in the offspring inside the squares. You can also turn to pages 82-83 of your reference book to read more about Punnett squares.

The combination of alleles in an organism is that organism's genotype.

Scientists refer to the alleles associated with different traits using uppercase and lowercase letters. In general, an uppercase letter represents an allele for a trait that is dominant Opens in modal popup window over another trait. The trait that is not dominant is recessive Opens in modal popup window , and its allele is represented by a lowercase letter. The allele for purple flowers in pea plants, for example, is dominant over the recessive allele for white flowers. So when a plant contains the allele for purple flowers and the allele for white flowers, it will produce purple flowers. In this example, the allele for purple flowers is represented by an uppercase P, and the allele for white flowers is represented by a lowercase p. The combination of alleles governing a certain trait in an organism is called the organism's genotype. The possible genotypes Opens in modal popup window that pea plants can have for flower color are PP, Pp, and pp. An organism's physical appearance is called its phenotype. Knowing an organism's genotype lets you predict its phenotype Opens in modal popup window .

What is the mother's genotype? Answer Opens in modal popup window What about the father's genotype? Answer Opens in modal popup window The children give you a clue. Two children lack the widow's peak. Recall that offspring receive one allele for each trait from each parent. All of this family's children must have received a w allele from their mother. The two children showing the recessive trait must have received a w allele from their father, as well. Therefore, he must be heterozygous, or Ww, for the trait. What are the genotypes of the rest of the children? Check your answer on the right.

She does not have a widow's peak; therefore, her genotype must be ww. You can't tell if his genotype is WW or Ww.

4.08 Genes and Alleles A gene is a specific segment of DNA on a chromosome. Human Genome Project http://web.ornl.gov/sci/techresources/Human_Genome/index.shtml

The DNA in your body's cells is packaged into tightly coiled units called chromosomes. Genes Opens in modal popup window are specific segments of DNA on those chromosomes Opens in modal popup window . Remember that a gene in a body cell actually has two forms called alleles. There are 23 chromosomes found in all human eggs. Each of these has a unique set of alleles for genes. For example, on chromosome 15 of both an egg and a sperm is the gene for some aspects of hair color. You, your parents, and your friends all have this allele, albeit in different variations, on chromosome 15.

4.08 Genes and Alleles A gene is a specific segment of DNA on a chromosome.

The DNA in your body's cells is packaged into tightly coiled units called chromosomes. Genes Opens in modal popup window are specific segments of DNA on those chromosomes Opens in modal popup window . Remember that a gene in a body cell actually has two forms called alleles. There are 23 chromosomes found in all human eggs. Each of these has a unique set of alleles for genes. For example, on chromosome 15 of both an egg and a sperm is the gene for some aspects of hair color. You, your parents, and your friends all have this allele, albeit in different variations, on chromosome 15. For a more detailed look at genes and what they influence, click on the Human Genome Project Information. http://web.ornl.gov/sci/techresources/Human_Genome/index.shtml

Early plant and animal breeders contributed important information to the study of heredity. dog geneology: A recent genetic analysis at the University of California confirms that all dog breeds known today descended from the wolf.

The Westminster Kennel Club sponsors the Westminster Kennel Club Annual All Breed Dog Show, where dog breeders enter more than 150 different breeds to compete for the coveted title Best in Show. Many breeds arose hundreds of years ago as people intentionally mated dogs to produce offspring with traits that would make them ideal companions, working dogs, or hunting dogs. Other hobbyists turned to flower breeding hundreds of years ago. The German botanist Josef Gottlieb Kölreuter experimented with crossbreeding different-colored flowers in the late 1700s. Still other breeders specialized in selectively breeding chickens that exhibit exotic feathered crowns and feet or oversized combs.

People have recognized some aspects of heredity for thousands of years.

The corn on the cob that you eat in the summertime traces its roots back nearly 10,000 years, when Native Americans living in present-day Mexico began domesticating a plant called teosinte. The plant produced a cob that was only 2 to 3 in. long, with about 5 to 12 kernels per cob—a far cry from today's foot-long ears of corn bursting with 500 or more kernels. Through a process called selective breeding, in which plants with desirable traits are bred together, those early Native Americans eventually bred teosinte into the corn we know today. Thousands of years before the rise of the science of genetics, people had an understanding of heredity. They knew that breeding two ideal plants together—such as corn plants that produced more kernels than most others—would likely result in offspring carrying those traits. Just about every crop plant you eat came about because people long ago understood the basics of heredity Opens in modal popup window : how traits from parents are passed along to offspring.

Different alleles with different DNA sequences produce different results.

The middle of one allele might contain the sequence CCAT. Assume that this allele produces red eyes in a fruit fly, because the allele produces a protein that makes red pigment. Now, assume that another allele of this gene has a slightly different sequence in the middle: CAAT, instead of CCAT. In this hypothetical example, this small change in the DNA sequence causes the allele to produce a different protein, resulting in the production of white eyes instead of red eyes in the fruit fly.

4.09 Genetic Variation The individuals in a population of organisms differ from one another.

The pigeons in your backyard or local park make up a population. A population Opens in modal popup window is all members of a species living in the same location. This lesson examines how the organisms in a population differ from one another, and what causes those differences. Scientists call the differences between individuals diversity. Organisms in a population may differ in appearance, in behavior, or in characteristics that you can't easily see but are still important to survival, such as susceptibility to disease. As you will see later in this lesson, genes account for much of the diversity in populations of organisms.

4.05 Pedigrees A pedigree shows how a trait is inherited over several generations.

Transmission of genetic traits is fairly easy to study with subjects like pea plants or fruit flies. You can cross them to record how traits such as flower color or eye color are transmitted through generations. Those species also produce many offspring in just days or weeks, so scientists can quickly produce several generations for genetic analysis. With people, however, it's not as easy. You can't experimentally cross people, humans typically have only a few offspring in their lifetime, and the length of time between generations is about 20 years. Therefore, geneticists use pedigree analysis as one basic tool for investigating how Mendelian Opens in modal popup window inheritance works in people. A pedigree Opens in modal popup window shows how traits are expressed in the individuals in a family. Scientists use pedigree analysis to trace how physical and other traits are passed down through heredity from one generation to the next.

Vernon Ingram developed a new lab technique in 1957 that helped him determine the amino acid substitution that produces the abnormal hemoglobin characteristic of sickle-cell disease

Vernon Ingram

The fusion of egg and sperm in sexual reproduction produces new combinations of alleles in a zygote.

When gametes from a mother and a father come together in sexual reproduction, their chromosomes pair up to provide the new individual with a full set of 46 chromosomes, 23 from each parent. Because chromosomes assort independently during meiosis, each gamete an individual produces will have a different chromosome assembly than the next; therefore, each offspring receives a unique set of chromosomes from each parent. The new combination of chromosomes in an offspring produces new genotypes and, therefore, new phenotypes. Offspring may express new combinations of traits—for example, you might have your mother's eyes and your father's smile. The production of new genotypes in offspring is one of the sources of genetic variation in any population. It explains why, for example, the kittens in a litter look different from each other.

The fusion of egg and sperm in sexual reproduction produces new combinations of alleles in a zygote.

When gametes from a mother and a father come together in sexual reproduction, their chromosomes pair up to provide the new individual with a full set of 46 chromosomes, 23 from each parent. Because chromosomes assort independently during meiosis, each gamete an individual produces will have a different chromosome assembly than the next; therefore, each offspring receives a unique set of chromosomes from each parent. The new combination of chromosomes in an offspring produces new genotypes and, therefore, new phenotypes. Offspring may express new combinations of traits—for example, you might have your mother's eyes and your father's smile. The production of new genotypes in offspring is one of the sources of genetic variation in any population. It explains why, for example, the kittens in a litter look different from each other. Parental gametes unite in new chromosome combinations that yield varying phenotypes.

By studying a pedigree, you can determine how a trait is inherited.

When looking at a pedigree, you should consider a few questions to help you determine how a trait is inherited. If an allele is dominant Opens in modal popup window and common in a population, the trait it codes for most likely will show up in every generation. A trait coded for by an uncommon recessive allele will be less common. A trait showing up more frequently among males is probably sex-linked, the alleles of which are usually carried on the X chromosome. If you understand the basic principles of Mendelian inheritance, you will be able to discern the patterns of inheritance in any pedigree you study. In the screens ahead, you'll see how traits are represented in pedigrees.

Genetic variation occurs when genes are combined in new ways.

When scientists talk about genetic variation, they're talking about more than just flower color, the pattern on a happy-spider's abdomen, or a person's blood type. They're talking about the accumulation of all the genes in all the individuals in a population. That kind of variation is maintained when genes get shuffled around during processes such as sexual reproduction and meiosis. The shuffling creates new combinations of genes in offspring, producing phenotypes with combinations of characteristics not seen in either parent. A mother with type A blood, blue eyes, and brown hair and a father with type B blood, brown eyes, and blond hair might, for example, have a child with type AB blood, blue eyes, and sandy brown hair. Another way that variation occurs is by mutation—an actual chemical or physical change of genes that happens because of some environmental factor. You will learn more about mutation later in the course.

An organism's observable characteristics make up its phenotype.

When you look at a pea plant and see purple flowers, you are viewing the plant's phenotype, or observable characteristics. An organism's phenotype is determined partly by its genotype and partly by its environment, but for now, focus on how genotype influences phenotype. Can you use a plant's phenotype determine its genotype? Not always. For example, could you tell by looking at a pea plant with a purple flower if its genotype is Pp or PP? Answer No, because the dominant trait of purple flowers (P) is the only one you can see. The genotype of the pea plant might also include the recessive allele for white flowers (p). If a pea plant has the alleles P and P for flower color, and it also has two different alleles for pea pod shape—R (rounded pods) and r (constricted pods)—you would write that plant's genotype as PPRr. Knowing the plant's genotype, you would predict that its phenotype would be purple flowers and rounded pea pods since those traits are dominant. If the plant's genotype were pprr, what would you predict its phenotype to be? Explore the example on-screen to find out.

An allele is one or more variations of a gene.

When you studied Mendel and his pea plants, you examined characteristics such as flower color. In genetic terms, the characteristic flower color is governed by a gene. Different colors in flowers, however, are produced by different variants of the gene. Purple flowers, for example, are produced by one variation, or allele Opens in modal popup window , of the flower color gene. White flowers are produced by a different allele of the flower color gene. Most genes have at least two alleles, and many have three or more. In humans, many traits also show inheritance patterns that are more complex than dominance and recessiveness that Mendel showed in his pea plants. Eye color, for example, is governed by several alleles of at least two genes.

Genes occur in different forms or alleles.

When you studied Mendel earlier in this unit, you learned that he studied several different characteristics in pea plants. Consider just one of them: flower color. Mendel's pea plants produced either purple flowers or white flowers. Flower color is governed by a gene, and different forms of that gene, or alleles Opens in modal popup window , tell the cell to produce different colors. While all people share genes for characteristics such as eye color or susceptibility to certain diseases, each person differs in the alleles for those characteristics. If you have blue eyes and your best friend has brown eyes, you know that you have different alleles for eye color.

Sons only inherit X-linked alleles from their mothers.

Why is this? A father's sperm cells carry either an X or a Y chromosome, while all of a mother's egg cells carry X chromosomes. Sons inherit Y chromosomes only from their fathers and X chromosomes only from their mothers. Daughters inherit an X chromosome from each parent. Many sex-linked traits are recessive Opens in modal popup window , meaning daughters need to inherit a recessive X-linked allele from both parents to express it. Daughters who have only one recessive X-linked allele are said to be carriers for the trait. They carry the allele, but do not express it. Sons, however, inherit only one X chromosome and therefore inherit only one X-linked allele for the trait. Even if the trait is recessive, they will express it.

4.01 The work of Gregor Mendel Gregor Mendel's studies of heredity established the field of modern genetics. Gregor Mendel crossed generations of pea plants. He correctly predicted inheritance patterns. He described dominant traits and recessive traits. He defined 'factors', now called genes

You can hardly turn on the TV, pick up the newspaper, or visit your favorite news website without hearing about discoveries in the field of genetics. Whether it's new information about genetic markers associated with disease or a new application of biotechnology in crop production, today's advances in genetics started with the work of an Austrian monk more than 100 years ago.

Attached and free earlobes are traits that can be mapped on a pedigree.

You probably haven't given much thought to your earlobes, but some geneticists have. An easily observed trait called attached earlobes is a trait coded for by two recessive alleles. The presence of a dominate allele will result in an individual with free or nonattached earlobes. Look at the examples of each type of earlobe. Which type do you have? What about the other members of your family? The allele for nonattached earlobes, right, is dominant over the recessive allele for attached earlobes, left.

4.07 Chromosomes and Genes Genes are segments of the molecule DNA. Jumping Genes Modern scientific research has shown that genes are not quite as simply arranged as this screen would indicate. Parts of a gene might be spread across different segments of a chromosome, or across two or more chromosomes. Other genes might even "jump" between chromosomes. For now, just stick with the simplest, and probably the most common, case—that is, a gene is a segment of DNA on a chromosome.

You've just spent several lessons learning how genes are passed from one generation to the next. But what exactly are these genes? Where are they found, and what are they made of? What do they do, and how do they do it? A gene Opens in modal popup window is a segment of the molecule DNA, which you learned about earlier in this course. DNA Opens in modal popup window is an information storage molecule, and it contains the instructions used by the cell to produce proteins. Those proteins perform many essential functions in a cell, such as helping break down food and producing energy, regulating a cell's interaction with the environment and with other cells, and providing structure and building components for growing and dividing cells. Genes, because they are parts of chromosomes, are passed from parents to offspring during sexual reproduction and are considered the agents of heredity.

4.07 Chromosomes and Genes Genes are segments of the molecule DNA. Jumping Genes Modern scientific research has shown that genes are not quite as simply arranged as this screen would indicate. Parts of a gene might be spread across different segments of a chromosome, or across two or more chromosomes. Other genes might even "jump" between chromosomes. For now, just stick with the simplest, and probably the most common, case—that is, a gene is a segment of DNA on a chromosome.

You've just spent several lessons learning how genes are passed from one generation to the next. But what exactly are these genes? Where are they found, and what are they made of? What do they do, and how do they do it? A gene Opens in modal popup window is a segment of the molecule DNA, which you learned about earlier in this course. DNA Opens in modal popup window is an information storage molecule, and it contains the instructions used by the cell to produce proteins. Those proteins perform many essential functions in a cell, such as helping break down food and producing energy, regulating a cell's interaction with the environment and with other cells, and providing structure and building components for growing and dividing cells. Genes, because they are parts of chromosomes, are passed from parents to offspring during sexual reproduction and are considered the agents of heredity.

During meiosis, crossing-over creates new gene combinations.

Your chromosomes are home to thousands of genes. Yet, you have only 23 pairs of chromosomes in your body. What does this tell you? It means that each chromsome must carry hundreds of genes. Genes that are carried on the same chromosome are said to be linked, because those genes are more often than not inherited as a unit on the same chromosome. Sometimes, as you know, pairs of chromosomes will swap material during meiosis in a process called crossing-over. Crossing-over Opens in modal popup window , as you recall, creates new combinations of genes on a chromosome.

dominant

a characteristic of a trait, or of the allele associated with that trait, in which the trait is expressed even if only one copy of the allele is present

recessive

a characteristic of a trait, or of the allele associated with that trait, in which the trait is expressed only if two copies of the allele are present

pedigree

a diagram that shows how a trait is passed throughout a group of related individuals over many generations

Punnet square Punnett squares show the possible genotypes and phenotypes of the offspring of two parents.

a grid used to predict the results of genetic crosses

gene

a segment of DNA that directs the development of some inherited traits

alleles

a specific form of a gene

A pedigree is a diagram made up of symbols indicating traits.

pedigree is a standard visualization composed of symbols. In a typical pedigree, such as the one on pages 86-87 of your reference book, circles represent females and squares represent males. A circle and a square connected with a horizontal line represent a set of parents. A vertical line branching down from that horizontal line represents offspring. Siblings all branch off of the same horizontal line. Furthermore, individuals of the same generation, such as cousins, all share the same row in a pedigree. A pedigree also shows you which individuals express a given trait. Generally, a pedigree comes with a key indicating the trait in question and the color coding associated with that trait. In a simple pedigree, the individuals expressing a trait are represented by a colored circle or square. The individuals who don't express the trait are represented by a blank circle or square.

genotype

the particular alleles present in an organism for a character or set of characters

heredity

the passing of genetic information from parents to their offspring

phenotype

the physical or detectable traits of an organism

traits

the specific observable form of a character, such as tall for height, pink for flower color, or smooth for seed texture