Chapter - 14 Mendel Notes and FRQ

What is the probability that a couple will have a girl, a boy, a girl, and a boy in this specific order?

(1/2)4 = 1/16 = 6.25%

Dominant alleles are not necessarily more common than recessive alleles in the gene pool. Explain why this is true.

An allele is dominant because it is seen in the phenotype, not because it somehow subdues a recessive allele. For any character, the observed dominant/recessive relationship of alleles depends on the level (organismal, biochemical, or molecular) at which we examine the phenotype. Dominant alleles are less likely to be more frequent than recessive alleles if they lead to conditions that are not evolutionarily selected for, such as some forms of polydactyly.

Explain how incomplete dominance is different from complete dominance, and give an example of incomplete dominance.

Incomplete dominance is when the phenotype is a mixture of the two parental phenotypes, rather than exactly one of the two parental ones. If a flower is a combination of parental colors (red + white = pink) then alleles are incompletely dominant

Explain why you know the genotype of one female in the third generation, but are unsure of the other.

The cross of her heterozygous parents could result in either a homozygous dominant or a heterozygous genotype for the daughter exhibiting the free earlobe phenotype. The second daughter's genotype could be Ff or FF because both variations would lead to free earlobes.

State the multiplication rule and give an original example.

The multiplication rule calculates the probability of two or more independent events occurring together in some specific combination by multiplying the probability of one event by the probability of the other event. If you have a bag of 6 red and 3 green marbles, the probability of successively randomly choosing 2 red marbles out of the bag (if you replace a marble after removing one) would be (6/9)2 = 0.5 = 50%.

Using the figure as your guide, provide the missing notations for the figure below. (P, F1, F2). a. What is the F2 phenotypic and genotypic ratio? b. Which generation is completely heterozygous? c. Which generation has both heterozygous and homozygous offspring?

a) The F2 genotypic ratio is 50% heterozygous, 25% homozygous recessive, and 25% homozygous dominant. (2 : 1 : 1) The F2 phenotypic ratio is 75% dominant and 25% recessive (3 : 1). b) The F1 generation is completely heterozygous c) F2 generation has both heterozygous and homozygous offspring.

Students are expected to have a general knowledge of the pattern of inheritance and the common symptoms of a number of genetic disorders. Provide this information for the disorders listed below. a. cystic fibrosis b. sickle-cell disease c. achondroplasia d. Huntington's disease

a) Among people of European descent, one out of 25 (4%) are carriers of the cystic fibrosis allele. The normal allele for this gene codes for a membrane protein that functions in the transport of chloride ions between certain cells and the extracellular fluid. These chloride transport channels are defective or absent in the plasma membranes of children who inherit two recessive alleles for cystic fibrosis. The result is an abnormally high concentration of extracellular chloride, which causes the mucus that coats certain cells to become thicker and stickier than normal. The mucus builds up in the pancreas, lungs, digestive tract, and other organs, leading to pleiotropic effects, including chronic bronchitis and recurrent bacterial infections. If untreated, most children with cystic fibrosis die before their fifth birthday. b) Sickle-cell disease is caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells; in homozygous individuals, all hemoglobin is of the sickle-cell variety. When the oxygen content of an affected individual's blood is low, the red blood cells deform into a sickle shape. Sickled cells may clump and clog small blood vessels, often leading to other symptoms throughout the body, including physical weakness, organ damage, and even paralysis. Although two sickle-cell alleles are necessary for an individual to manifest full-blown sickle-cell disease, the presence of one sickle-cell allele can affect the phenotype. Thus, at the organismal level, the normal allele is incompletely dominant to the sickle-cell allele. A the molecular level, the two alleles are codominant; both normal and abnormal hemoglobins are made in heterozygotes. c) In achondroplasia, a form of dwarfism, heterozygous individuals have the dwarf phenotype. Therefore, all people who are not achondroplastic dwarfs are homozygous for the recessive allele. Dominant alleles that cause a lethal disease are much less common than recessive alleles that have lethal effects. However, a lethal dominant allele is able to be passed on if it causes death at a relatively advanced age. d) In Huntington's disease, a degenerative disease of the nervous system, the lethal dominant allele has no obvious phenotypic effect until the individual is about 35 to 45 years old. Once the deterioration of the nervous system begins, it is irreversible and inevitably fatal. As with other dominant traits, a child born to a parent with the Huntington's disease allele has a 50% chance of inheriting the allele and the disorder.

Extending Mendelian Genetics for Two or More Genes

- Some traits may be determined by two or more genes

Alternate versions of the same gene, like purple and white flower color, are termed _____________.

alleles

Degrees of Dominance

- Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical - In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties - In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

Pleiotropy

- Most genes have multiple phenotypic effects, a property called pleiotropy - For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

The Relation Between Dominance and Phenotype

- A dominant allele does not subdue a recessive allele; alleles don't interact - Alleles are simply variations in a gene's nucleotide sequence - For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype

Mendel's Experimental, Quantitative Approach Advantages of pea plants for genetic study

- Advantages of pea plants for genetic study: -There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits -Mating of plants can be controlled -Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) -Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another - Mendel chose to track only those characters that varied in an either-or manner - He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)

Useful Genetic Vocabulary

- An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character - An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character - Unlike homozygotes, heterozygotes are not true-breeding - Because of the different effects of dominant and recessive alleles, an organism's traits do not always reveal its genetic composition - Therefore, we distinguish between an organism's phenotype, or physical appearance, and its genotype, or genetic makeup - In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes

Integrating a Mendelian View of Heredity and Variation

- An organism's phenotype includes its physical appearance, internal anatomy, physiology, and behavior - An organism's phenotype reflects its overall genotype and unique environmental history

Nature and Nurture: The Environmental Impact on Phenotype

- Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype - The norm of reaction is the phenotypic range of a genotype influenced by the environment - For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity - Norms of reaction are generally broadest for polygenic characters - Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype

Cystic Fibrosis

- Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent - The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes - Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

Frequency of Dominant Alleles

- Dominant alleles are not necessarily more common in populations than recessive alleles - For example, one baby out of 400 in the United States is born with extra fingers or toes - The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage - In this example, the recessive allele is far more prevalent than the population's dominant allele

Tests for Identifying Carriers and Fetal Testing

- For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately - In amniocentesis, the liquid that bathes the fetus is removed and tested - In chorionic villus sampling (CVS), a sample of the placenta is removed and tested - Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

Genetic Testing and Counseling and Counseling Based on Mendelian Genetics and Probability Rules

- Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease - Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders

The Testcross

- How can we tell the genotype of an individual with the dominant phenotype? - Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous - The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual - If any offspring display the recessive phenotype, the mystery parent must be heterozygous

Huntington's Disease

- Huntington's disease is a degenerative disease of the nervous system - The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age

hybridization P generation F1 generation F2 generation

- In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization - The true-breeding parents are the P generation - The hybrid offspring of the P generation are called the F1 generation - When F1 individuals self-pollinate, the F2 generation is produced

Epistasis

- In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus - For example, in mice and many other mammals, coat color depends on two genes - One gene determines the pigment color (with alleles B for black and b for brown) - The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair

Recessively Inherited Disorders

- Many genetic disorders are inherited in a recessive manner

The Law of Independent Assortment

- Mendel derived the law of segregation by following a single character - The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character - A cross between such heterozygotes is called a monohybrid cross -Mendel identified his second law of inheritance by following two characters at the same time - Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters - A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently - Using a dihybrid cross, Mendel developed the law of independent assortment - The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation - Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes - Genes located near each other on the same chromosome tend to be inherited together

Mendel's Model

- Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring - Four related concepts make up this model - These concepts can be related to what we now know about genes and chromosomes

Mendel's Reasoning

- Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids - Mendel called the purple flower color a dominant trait and the white flower color a recessive trait - Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits - What Mendel called a "heritable factor" is what we now call a gene

The laws of probability govern Mendelian inheritance

- Mendel's laws of segregation and independent assortment reflect the rules of probability - When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss - In the same way, the alleles of one gene segregate into gametes independently of another gene's alleles

Mendel's segregation model (Punnett Square)

- Mendel's segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses - The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup - A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

Polygenic Inheritance

- Quantitative characters are those that vary in the population along a continuum - Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype - Skin color in humans is an example of polygenic inheritance

The Behavior of Recessive Alleles

- Recessively inherited disorders show up only in individuals homozygous for the allele - Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented) - Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair - If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low - Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele •Most societies and cultures have laws or taboos against marriages between close relatives

Sickle-Cell Disease

- Sickle-cell disease affects one out of 400 African-Americans - The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells - Symptoms include physical weakness, pain, organ damage, and even paralysis

Newborn Screening

- Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States

Dominantly Inherited Disorders

- Some human disorders are caused by dominant alleles - Dominant alleles that cause a lethal disease are rare and arise by mutation - Achondroplasia is a form of dwarfism caused by a rare dominant allele

Tay-Sachs disease

- Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain -At the organismal level, the allele is recessive -At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant -At the molecular level, the alleles are codominant

Drawing from the Deck of Genes -What genetic principles account for the passing of traits from parents to offspring?

- The "blending" hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) - The "particulate" hypothesis is the idea that parents pass on discrete heritable units (genes) - Mendel documented a particulate mechanism through his experiments with garden peas

Mendel's Model (1st concept)

- The first concept is that alternative versions of genes account for variations in inherited characters - For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers - These alternative versions of a gene are now called alleles - Each gene resides at a specific locus on a specific chromosome

Mendel's Model (4th concept)

- The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes - Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism - This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

The Multiplication and Addition Rules Applied to Monohybrid Crosses

- The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities - Probability in an F1 monohybrid cross can be determined using the multiplication rule - Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele - The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities - The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous

Inheritance patterns are often more complex than predicted by simple Mendelian genetics

- The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied - Many heritable characters are not determined by only one gene with two alleles - However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

Mendel's Model (2nd concept)

- The second concept is that for each character an organism inherits two alleles, one from each parent - Mendel made this deduction without knowing about the role of chromosomes - The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel's P generation - Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

Mendel's Model (3rd concept)

- The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism's appearance, and the other (the recessive allele) has no noticeable effect on appearance - In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

Solving Complex Genetics Problems with the Rules of Probability

- We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters - A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously - In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together

The Law of Segregation

- When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple - When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white - Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

Extending Mendelian Genetics for a Single Gene

-Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: -When alleles are not completely dominant or recessive -When a gene has more than two alleles -When a gene produces multiple phenotypes

Quantitative variation usually indicates ______________________________.

...polygenetic inheritance.

Explain the difference between a monohybrid cross and a dihybrid cross

A monohybrid cross is for heterozygote mating where one trait is being followed, while a dihybrid cross follows two characteristics of heterozygotes and determines whether the traits are inherited together

In probability, what is an independent event?

An independent event, such as a coin toss, is unaffected by what has happened on previous trials.

In the 1800s the most widely favored explanation of genetics was blending. Explain the concept of blending, and then describe how Mendel's particulate (gene) theory was different.

Blending was the idea that DNA from parents combined to form child's phenotype, which would mean that overtime, a population would become uniform. Mendel's theory was that parents pass heritable units on to their children that do not combine, but form a sort of collection of genes that make up offspring's DNA

Compare and contrast codominance with incomplete dominance.

Codominance is when the alleles affect the phenotype in distinguishable ways. For Example, a red flower and a white flower give rise to a red and white flower.

Explain what is meant when a gene is said to have multiple alleles.

Genes exist in more than two allelic forms often, meaning a gene could have one allele or two or none, each with different characteristics.

Question 2 in the 14.3 Concept Check is a blood type problem. Complete it here, and show your work.

Half the children would have blood type A (genotype IAi) and half would have blood type B (genotype IBi).

Describe what you think is important to know medically about the behavior of recessive alleles.

If recessive alleles lead to a harmful trait or disease, it would be important to know if the parents have the gene for a disorder and could make a child with a disability or a fatal disease

Explain epistasis.

In epistasis, the expression of a gene at one locus alters the phenotypic expression of another gene at a different locus.

Why is height a good example of polygenic inheritance?

Polygenic inheritance is the additive effect of two or more genes on a single phenotypic character. Height could be a good example because of the broad variation in height seen among a population. There is no either or for height.

One of the keys to success for Mendel was using pea plants. Explain how using pea plants allowed Mendel to control mating; that is, how did this approach let Mendel be positive about the exact characteristics of each parent?

Mendel probably choose peas because there were different varieties with different traits that also exhibited complete dominance. This means he could tell clearly which trait the parent and off spring showed fully. Also, pea plants can mate and self fertilize and produce a lot of off spring. Mendel could control mating

What is pleiotropy? Explain why this is important in diseases like cystic fibrosis and sickle- cell disease.

Most genes have pleiotropy, i.e. multiple phenotypic effects. In humans, for example, pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary diseases, such as cystic fibrosis and sickle-cell disease.

Define the following terms. Then, consider your own family. Which generation would your Mom's grandparents be? Your Mom? You? P generation F1 generation F2 generation

P Generation : Parental Generation (true-breeding) F1 Generation : First filial generation after P Generation (hybrids) F2 Generation : Second filial generation after F1 , cross-pollinated with other F1 hybrids

Explain how Mendel's simple cross of purple and white flowers did the following: refuted blending determined dominant and recessive characteristics demonstrated the merit of experiments that covered multiple generations

Refuted Blending : the white flowers were not "deleted" from the genes (came up again in F2) determined dominant and recessive characteristics : F1 generation exhibited only purple, but F2 had a white and "unmasked" flower demonstrated the merit of experiments that covered multiple generations : he showed that observing multiple generations could explain much more about inheritance of traits.

Explain why the dihybrid cross detailed in Figure 14.12 has 4 white mice instead of the 3 that would have been predicted by Mendel's work.

The figure shows 4 white mouse because the gene that determines if pigment will be deposited or not is epistatic to the gene for what color pigment. The E/e gene overrules the gene for pigment color.

Explain the symptoms of phenylketonuria, and describe how newborn screening is used to identify children with this disorder.

Some genetic disorders can be detected at birth by simple biochemical tests that are now routinely performed in most hospitals in the United States. One common screening program is for phenylketonuria (PKU), a recessively inherited disorder. Affected individuals cannot properly metabolize the amino acid phenylalanine, causing the compound and its byproduct to accumulate to toxic levels in the blood, causing severe intellectual disability. However, if PKU is detected in the newborn, a special diet low in phenylalanine will usually allow normal development.

State the addition rule and give an original example.

The addition rule calculates the probability that any one of two or more mutually exclusive events will occur by adding their individual properties. If you have a bag of 6 red, 3 green and 1 blue marbles, the probability of randomly choosing a red or blue marble out of the bag would be (6/10) + (1/10) = 0.7 = 70%.

Pedigree analysis is often used to determine the mode of inheritance (dominant or recessive, for example). Be sure to read the "Tips for pedigree analysis" in Figure 14.15; then complete the unlabeled pedigree by indicating the genotypes for all involved. What is the mode of inheritance for this pedigree?

The first-born daughter in the third generation has attached earlobes, although both of her parents lack that trait. Such a pattern is easily explained if the attached-lobe phenotype is due to a recessive allele. If it were due to a dominant allele, then at least one parent would also have had the trait.

Explain Mendel's law of independent assortment.

The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation. This law applies only to genes located on different chromosomes, or very far apart on the same chromosome.

Using the terms norm of reaction and multifactorial, explain the potential influence of the environment on phenotypic expression.

The norm of the reaction is the range of phenotypic possibilities for a genotype due to environmental influences. This shows how phenotypes can be effected by environmental factors, like red blood cell count. Multifactorial characters can be influenced by genetic and environmental factors, which collectively determine the phenotype

As you start to work word problems in genetics, two things are critical: the parent's genotype must be correct, and the gametes must be formed correctly. Using Figure 14.8 as your guide, explain how the gametes are derived for the following cross. (You should have four different gametes). YyRr × YyRr

The possible gametes from this cross will be : YR, Yr, yR, yr. Each allele can combine with every other allele from the parent in one gamete

What are the strengths and weaknesses of each fetal test?

The rapid analysis offered by CVS is faster than amniocentesis, in which cells must be cultured for several weeks before karyotyping. Another advantage of CVS is that it can be performed at least four weeks earlier.

In sexually reproducing organisms, why are there exactly two chromosomes in each homologue?

There are two because there is one from each parent in the homologous pair

In pea plants, T is the allele for tall plants, while t is the allele for dwarf plants. If you have a tall plant, demonstrate with a test cross how it could be determined if the plant is homozygous tall or heterozygous tall.

We can cross the mystery plant (T?) with a homozygous recessive short plant (tt), which will make only gametes with the recessive allele (t). The allele in the gamete contributed by the mystery plant will therefore determine the appearance of the offspring. If all the offspring of the cross are tall, then the mystery plant must be homozygous dominant (TT). But if both the short and tall phenotypes appear among the offspring, then the mystery plant must be heterozygous (Tt).

An event that is certain to occur has a probability of _______, while an event that is certain not to occur has a probability of ________.

has a probability of 1, while an event that is certain not to occur has a probability of 0.