Chapter 6: Meiosis and Mendel

Diploid

A cell that contains two haploid sets of chromosomes. a cell has two copies of each chromosome: one copy from the mother, and one copy from the father. Diploid cells can be represented as 2n. In humans, the diploid chromosome number is 46. Gametes are not diploid cells; they are haploid cells, represented as n

Monohybrid cross

A cross between individuals that involves one pair of contrasting traits. crosses that examine the inheritance of only one specific trait. Results: 1. Homozygous-Homozygous 2. Heterozygous-Heterozygous 3. Heterozygous-Homozygous

Punnett square

A graphic used to predict the results of a genetic cross. is a grid system for predicting all possible genotypes resulting from a cross. The axes of the grid represent the possible gamete genotypes of each parent. The grid boxes show all of the possible genotypes of offspring from those two parents. Because segregation and fertilization are random events, each combination of alleles is as likely to be produced as any other. By counting the number of squares with each genetic combination, we can find the ratio of genotypes in that generation. If we also know how the genotype corresponds to the phenotype, we can find the ratio of phenotypes in that generation as well.

Autosome

Any chromosome that is not a sex chromosome. Collectively, chromosome pairs 1 through 22 make up your autosomes, chromosomes that contain genes for characteristics not directly related to the sex of an organism.

recombination

Because crossing over results in new combinations of genes, it is also called recombination. any mixing of parental alleles, including recombination events, other than crossing over

Alleles and Phenotype

Because some alleles are dominant over others, two genotypes can produce the dominant phenotype. Most alleles are not simply dominant or recessive; some are codominant.

Egg

Female gamete. The formation begins before birth, inside the developing body of a female embryo, and is not finished until that egg is fertilized by a sperm many years later. The process includes periods of active development and long periods of inactivity. An egg gives DNA to an embryo and organelles, molecular building blocks, and other materials an embryo needs to begin life. Only one of the four cells produced by each round of meiosis actually makes an egg. One cell—the egg—receives most of the organelles, cytoplasm, and nutrients. The other cells become polar bodies, cells with little more than DNA that are eventually broken down.

Heterozygous-Homozygous

From the heterozygous parent, half the offspring receive a dominant allele, F, and half receive a recessive allele, f. Half the offspring have a heterozygous genotype, Ff. Half have a homozygous recessive genotype, ff. The resulting genotypic ratio is 1:1 of heterozygous:homozygous recessive.

Cross

Mating of two organisms.

Mendel conclusions

1. traits are inherited as discrete units 2. Organisms inherit two copies of each gene, one from each parent. 3. Organisms donate only one copy of each gene in their gametes.

Homozygous-Homozygous

100 percent of the offspring have the heterozygous genotype, Ff. because they all have a copy of the dominant allele

Law of Segregation

Mendel's first law, stating that 1. organisms inherit two copies of genes, one from each parent, 2. organisms donate only one copy of each gene in their gametes because the genes separate during gamete formation.

Allele

One of the alternative forms of a gene that governs a characteristic, such as hair color. is any of the alternative forms of a gene that may occur at a specific locus. Your cells have two alleles for each gene, one on each of the homologous chromosomes on which the locus for that gene is found. Each parent gives one allele. The two alleles may be the same, or they may be different.

Purebred

Type of organism whose ancestors are genetically uniform. If a line of plants has self-pollinated for long enough, that line becomes genetically uniform, or purebred. As a result, the offspring of purebred parents inherit all of the parent organisms' characteristics. Mendel was able to mate plants with specific traits by interrupting the self-pollination process.

Recessive

allele is the allele that is expressed only when two copies are present. allele that is expressed only when no dominant allele is present in an individual.

Heterozygous

alleles are different from each other. Describes an individual that carries two different alleles of a gene.

Homozygous

alleles are identical to each other Describes an individual that has identical alleles for a trait on both homologous chromosomes.

Germ cells

are cells in your reproductive organs, the ovaries or the testes, that develop into gametes

Trait

are distinguishing characteristics that are inherited, such as eye color, leaf shape, and tail length. traits are hereditary

Gamete

are sex cells—ova, or eggs, in the female, and spermatozoa, or sperm cells, in the male. DNA in your gametes can be passed on to your children

Homologous chromosome

are two chromosomes—one inherited from the mother, one from the father—that have the same length and general appearance. More importantly, these chromosomes have copies of the same genes, although the two copies may differ. homologous chromosomes are ordered from largest to smallest

Somatic cell

body cells, make up most of your body tissues and organs except gametes. DNA in your body cells is not passed on to your children

Polar body

cells with little more than DNA that are eventually broken down.

body cells

contains a set of 46 chromosomes, which come in 23 pairs. These cells are genetically identical to each other unless mutations have occurred 46 that can be divided into two sets: 23 from your mother and 23 from your father

Dihybrid cross

crosses that examine the inheritance of two different traits. A cross between individuals that have different alleles for the same gene.

Heterozygous-Heterozygous

one-fourth of the offspring have a homozygous dominant genotype, FF; half have a heterozygous genotype, Ff; and one- fourth have a homozygous recessive genotype, ff. the genotypic ratio is 1:2:1 of homozygous dominant:heterozygous:homozygous recessive

Haploid

represented as n. a cell has only one copy of each chromosome. Each human egg or sperm cell has 22 autosomes and 1 sex chromosome. In the egg, the sex chromosome is always an X chromosome. In the sperm cell, the sex chromosome can be an X chromosome or a Y chromosome.

Law of Independent Assortment

states that genes separate independently of one another in meiosis. states that allele pairs separate independently of each other during gamete formation, or meiosis. That is, different traits appear to be inherited separately.

Gregor Mendel

the father of genetics" for discovering hereditary units 1800s an Austrian monk named Gregor Mendel bred thousands of plants, carefully counting and recording his results, predicted the results of meiosis long before chromosomes were discovered. organisms inherit two copies of each discrete unit, what we now call genes, Mendel also described how traits were passed between generations. Mendel studied plant variation in a monastery garden. He made three key choices about his experiments that played an important role in the development of his laws of inheritance: 1. control over breeding, 2. use of purebred plants, 3. observation of "either-or" traits that appeared in only two alternate forms

Probability

the likelihood that a particular event will happen. It predicts the average number of occurrences, not the exact number of occurrences. the mathematical ratio of the number of times one outcome of any event is likely to occur to the number of possible outcomes of the event. number of ways a specific event can occur probability= ------------------------------------------------ number of total possible outcomes

Phenotype

the physical characteristics, or traits, of an individual organism An organism's appearance or other detectable characteristic that results from the organism's genotype and the environment.

meiosis and mitosis. differences

• Meiosis has two cell divisions. Mitosis has only one cell division. • During meiosis, homologous chromosomes pair up along the cell equator. During mitosis, homologous chromosomes never pair up. • In anaphase I of meiosis, sister chromatids remain together. In anaphase of mitosis, sister chromatids separate. • Meiosis results in haploid cells. Mitosis results in diploid cells

Meiosis

A process in cell division during which the number of chromosomes decreases to half the original number by two divisions of the nucleus, which results in the production of sex cells (gametes or spores). occurs in germ cells to produce gametes is a form of nuclear division that divides a diploid cell into haploid cells. This process is essential for sexual reproduc ion. occurs in germ cells to produce gametes. This process is sometimes called a "reduction division" because it reduces a cell's chromosome number by half. In cells undergoing meiosis, DNA is copied once but divided twice. Meiosis makes genetically unique haploid cells from a diploid cell. These haploid cells then undergo more processing in the ovaries or testes, finally forming mature gametes.

alleles are represented with letters

Alleles are often represented on paper with individual letters. An organ- ism's genotype for a trait can be shown with two letters—one per allele. Uppercase letters are used for dominant alleles, and lowercase letters are used for recessive alleles. For example, the dominant allele for height in pea plants is written as T, for tall. The recessive allele for short plants is written as t. A plant's genotype might be homozygous dominant (TT ), heterozygous (Tt), or homozygous recessive(tt)

Meiosis I

Before meiosis begins, DNA has already been copied. divides homologous chromosomes, producing two haploid cells with duplicated chromosomes. 1. Prophase I the nuclear membrane breaks down, the centrosomes and centrioles move to opposite sides of the cell, and spindle fibers start to assemble. The duplicated chromosomes condense, and homologous chromosomes pair up. 2. Metaphase I Spindle fibers align the homologous chromosomes along the cell equator. Each side of the equator has chromosomes from both parents. The result is that 23 chromosomes This arrangement mixes up the chromosomal combinations and helps create and maintain genetic diversity. Since human cells have 23 pairs of chromosomes, meiosis may result in 2 23 , or 8,388,608, possible combinations of chromosomes. 3. Anaphase I the paired homologous chromosomes separate from each other and move toward opposite sides of the cell. The sister chromatids remain together during this step and throughout meiosis I. 4. Telophase I The nuclear membrane forms again, the spindle fibers disassemble, and the cell undergoes cytokinesis. The end result is two cells that each have a unique combination of 23 duplicated chromosomes coming from both parents

Testcross

Cross between an organism with an unknown genotype and an organism with a recessive phenotype. The organism with the recessive phenotype must be homozygous recessive. The offspring will show whether the organism with the unknown genotype is heterozygous or homozygous dominant.

Sex chromosome

One of the pair of chromosomes that determine the sex of an individual. 23rd pair sex chromosomes that directly control the development of sexual characteristics. Humans have two very different sex chromosomes, X and Y. an organism's sex is determined by the XY system. An organism with two X chromo- somes is female. An organism with one X and one Y chromosome is male. Sex chromosomes make up your 23rd pair of chromosomes. Although the X and Y chromosomes pair with each other, they are not homologous. The X chromosome is the larger sex chromosome and contains numerous genes, including many that are unrelated to sexual characteristics. The Y chromosome is the sex chromosome that contains genes that direct the development of the testes and other male traits. It is the smallest chromosome and carries the fewest genes

Genetic linkage

Tendency for genes located close together on the same chromosome to be inherited together. Linked genes will be inherited in the same predicted ratios as would a single gene. genes that are far apart are more likely to assort independently.

Fertilization

The actual fusion of an egg and a sperm cell to form a zygote.

Sexual reproduction creates unique gene combinations

The major advantage of sexual reproduction is that it gives rise to a great deal of genetic variation within a species This variation results largely from (1) the independent assortment of chromosomes during meiosis (2) the random fertilization of gametes. Recall that homologous chromosomes pair up randomly along the cell equator during meiosis I. In other words, it's a matter of chance which of the two chromosomes from any homologous pair ends up on a given side of the cell equator. As you've learned, human cells have 23 pairs of chromosomes, and each pair lines up independently. As a result, gametes with 223, or about 8 million, different combinations of chromosomes can be produced through meiosis from one human cell. Now think about the fact that sexual reproduction produces offspring through the random combination of gametes. In humans, for example, a sperm cell with one of 223 (about 8 million) chromosome combinations fertilizes an egg cell, which also has one out of 2 23 chromosome combinations. Since any sperm cell can fertilize any egg, the total number of possible combinations is the product of 2 23 X 2 23 , or more than 70 trillion. In other words, any human couple can produce a child with one of about 70 trillion different combinations of chromosomes.

Sperm

The male gamete (sex cell) main contribution to an embryo is DNA. Yet it must swim to an egg to fertilize it, so the ability to move is critical. Sperm formation starts with a round cell and ends by making a streamlined cell that can move rapidly. DNA is tightly packed and much of the cytoplasm is lost, resulting in a compact head. The sperm cell develops a whiplike flagellum and connecting neck region packed with mitochondria that drive the cell.

Genetics

The science of heredity and of the mechanisms by which traits are passed from parents to offspring.

Meiosis II

divides sister chromatids, and results in undoubled chromosomes. DNA is not copied again between these two stages. 1. Prophase II The nuclear membrane breaks down, centrosomes and centrioles move to opposite sides of the cell, and spindle fibers assemble. 2. Metaphase II Spindle fibers align the 23 chromosomes at the cell equator. Each chromosome still has two sister chromatids at this stage. 3. Anaphase II the sister chromatids are pulled apart from each other and move to opposite sides of the cell. 4. Telophase II nuclear membranes form around each set of chromosomes at opposite ends of the cell, the spindle fibers break apart, and the cell undergoes cytokinesis. The end result is four haploid cells with a combination of chromosomes from both the mother and father.

Process of Meiosis

form of nuclear division that creates four haploid cells from one diploid cell. This process involves two rounds of cell division—meiosis I and meiosis II. Each round of cell division has four phases, which are similar to those in mitosis. meiosis reduces chromosome number and creates genetic diversity. homologous chromosomes are two separate chromosomes: one from your mother, one from your father. Homologous chromosomes are very similar to each other, since they have the same length and carry the same genes. But they are not copies of each other. In contrast, each half of a duplicated chromosome is called a chromatid. Together, the two chromatids are called sister chromatids. Thus, sister chromatids refers to the duplicated chromosomes that remain attached (by the centromere). Homologous chromosomes are divided in meiosis I. Sister chromatids are not divided until meiosis II

Genotype

genetic makeup of a specific set of genes

Sexual reproduction

involves the fusion of two gametes, resulting in offspring that are a genetic mixture of both parents Reproduction in which gametes from two parents unite.

Genome

is all of an organism's genetic material.

Dominant

is the allele that is expressed when two different alleles or two dominant alleles are present allele that is fully expressed whenever the allele is present in an individual.

Crossing over

is the exchange of chromosome segments between homologous chromosomes during prophase I of meiosis I. When homologous chromosomes are in this position, some of the chromatids are very close to each other. Part of one chromatid from each chromosome breaks off and reattaches to the other chromosome

Gametogenesis

is the production of gametes. includes both meiosis and other changes that produce a mature cell. The final stages of gametogenesis differ between the sexes

Gene

most basic physical unit of heredity. a segment of nucleic acids that codes for a functional unit of RNA and/or a protein. piece of DNA that provides a set of instructions to a cell to make a certain protein.