Genetics, Continued Week 10

Meiosis Homologous- each chromosome from the male parent has a corresponding chromosome from the female parent Homologous- each chromosome from the male parent has a corresponding chromosome from the female parent Diploid- a cell that contains both sets of homologous chromosomes Diploid- a cell that contains both sets of homologous chromosomes Haploid- cells that contain only one set of chromosomes Haploid- cells that contain only one set of chromosomes

Meiosis Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Meiosis I- results in two diploid daughter cells, each with the same number of chromosomes as the original cell. Meiosis I- results in two diploid daughter cells, each with the same number of chromosomes as the original cell. Tetrad- structure formed by the pairing of homologous chromosomes Tetrad- structure formed by the pairing of homologous chromosomes Crossing-over- exchanging portions of chromatids while forming tetrads Crossing-over- exchanging portions of chromatids while forming tetrads

Animation

Meiosis Meiosis II- results in four haploid daughter cells Meiosis II- results in four haploid daughter cells Sperm- male haploid gametes Sperm- male haploid gametes Egg- female haploid gametes Egg- female haploid gametes Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells. Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells.

Gene Maps Chromosomes assort independently, not individual genes. Chromosomes assort independently, not individual genes. A gene map shows the relative locations of each known gene on a chromosome. A gene map shows the relative locations of each known gene on a chromosome. A karyotype is a picture of arranged chromosomes. A karyotype is a picture of arranged chromosomes.

Gene Maps Humans have 46 chromosomes. Two of the 46 are known as sex chromosomes. Females have two copies of X, and males have 1 X and 1 Y. Humans have 46 chromosomes. Two of the 46 are known as sex chromosomes. Females have two copies of X, and males have 1 X and 1 Y. The remaining 44 are autosomes. The remaining 44 are autosomes. All human egg cells carry a single X chromosome. Half of all sperm cells carry an X chromosome and half carry a Y chromosome. All human egg cells carry a single X chromosome. Half of all sperm cells carry an X chromosome and half carry a Y chromosome.

Gene Maps A pedigree chart shows the relationships within a family. A pedigree chart shows the relationships within a family. The average human gene has about 3,000 base pairs. The largest gene in the human genome has more than 2 million base pairs. The average human gene has about 3,000 base pairs. The largest gene in the human genome has more than 2 million base pairs. Chromosome 21 and 22 are the smallest. Chromosome 22 problems can cause leukemia and tumors. Chromosome 21 problems can result in Lou Gehrig’s disease (ALS). Chromosome 21 and 22 are the smallest. Chromosome 22 problems can cause leukemia and tumors. Chromosome 21 problems can result in Lou Gehrig’s disease (ALS).

Sex-linked Genes Sex-linked genes are found on the X or Y chromosome. Sex-linked genes are found on the X or Y chromosome. Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive. Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive. Females randomly switch off one of the X chromosomes. Females randomly switch off one of the X chromosomes. The most common error in meiosis occurs when homologous chromosomes fail to separate. This is called nondisjunction. The most common error in meiosis occurs when homologous chromosomes fail to separate. This is called nondisjunction.

Nondisjunction If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result. If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result. Down syndrome- involves having 3 copies of chromosome 21. Down syndrome- involves having 3 copies of chromosome 21.

Natural Selection Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Natural selection can affect the distributions of phenotypes in any of three ways. Natural selection can affect the distributions of phenotypes in any of three ways. Directional selection- individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. Directional selection- individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. Stabilizing selection- individuals near the center of the curve have higher fitness that individuals at either end. Stabilizing selection- individuals near the center of the curve have higher fitness that individuals at either end. Disruptive selection- individuals at the upper and lower ends of the curve have higher fitness that the individuals in the middle. Disruptive selection- individuals at the upper and lower ends of the curve have higher fitness that the individuals in the middle.

Genetic Drift A random change in allele frequency is called genetic drift. A random change in allele frequency is called genetic drift. In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

Hardy-Weinberg Principle The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. Genetic equilibrium- allele frequencies do not change Genetic equilibrium- allele frequencies do not change Five conditions are required to maintain genetic equilibrium from generation to generation: Five conditions are required to maintain genetic equilibrium from generation to generation: random mating random mating very large population very large population no movement into or out of the population no movement into or out of the population no mutations no mutations no natural selection no natural selection