Genetics, Continued

1. Genetics, Continued Week 10

2. Meiosis • 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 • Haploid- cells that contain only one set of chromosomes

3. 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 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 • Crossing-over- exchanging portions of chromatids while forming tetrads

4. Animation

5. Meiosis • Meiosis II- results in four haploid daughter cells • Sperm- male 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.

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

8. 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. • 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.

9. Gene Maps • 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. • 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).

10. Sex-linked Genes • 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. • 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.

11. Nondisjunction • 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.

12. Natural Selection • 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. • 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. • Disruptive selection- individuals at the upper and lower ends of the curve have higher fitness that the individuals in the middle.

13. 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.

14. 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. • Genetic equilibrium- allele frequencies do not change • Five conditions are required to maintain genetic equilibrium from generation to generation: • random mating • very large population • no movement into or out of the population • no mutations • no natural selection