Meiosis Section 3.3

Vocabulary Homologous chromosomes: Chromosomes that have the same sequence of genes and the same structure. 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. Sex chromosome: One of the pair of chromosomes that determine the sex of an individual. Pedigree: A diagram that shows the occurrence of a genetic trait in several generations of a family.

Asexual vs. Sexual Reproduction In asexual reproduction, only one parent cell is needed. The structures inside the cell are copied and the parent cell divides into two identical cells. This is called MITOSIS. In sexual reproduction, two parent cells join together to form offspring that are different from both parent cells. The parent cells are called sex cells. Sex cells are different from ordinary body cells. Human body cells have 46 chromosomes (or 23 homologous pairs). Sex cells only have 23 chromosomes.

Genes & Chromosomes A graduate student named Walter Sutton observed that cells’ chromosomes are located in the nucleus and proposed that genes are located on chromosomes.

Meiosis & Mendel

Sex Chromosomes Sex chromosomes carry genes that determine sex. In humans, females have two X chromosomes and males have one X and one Y chromosome. During meiosis one of each of the chromosome pairs ends up in a sex cell. When meiosis produces egg cells, each egg contains one X chromosome. When meiosis produces sperm, they contain either an X or Y chromosome. An egg fertilized by a sperm with an X will be female and an egg fertilized by a sperm with a Y will be a male.

Sex-Linked Disorders Genes for certain disorders, such as colorblindness and hemophilia, are carried on the X chromosome. These disorders are called sex- linked disorders Because the Y chromosome does not carry all the genes of an X chromosome and females have two X chromosomes (two copies of each gene), AND the gene for sex-linked disorders is recessive . . . men are more likely to have sex-linked disorders.

Genetic Counseling Genetic disorders can be traced through a family tree. Genetic counselors use a diagram, known as a pedigree, to predict whether a person is a carrier of a heredity disease.

Selective Breeding In selective breeding, organisms with desirable characteris tics are mated. This is common in pets and plants.