Meiosis and Reproduction Biology 1-2

Chromosomes Chromosomal theory of inheritance- genes have specific locations on chromosomes. Chromosomal theory of inheritance- genes have specific locations on chromosomes. The movement and separation of chromosomes accounts for inheritance patterns. The movement and separation of chromosomes accounts for inheritance patterns.

Chromosomes cont’d Chromosome-a long strand of DNA, made of up of many genes. Chromosome-a long strand of DNA, made of up of many genes. There are hundreds of genes per chromosome. There are hundreds of genes per chromosome. In humans, there are 46 chromosomes (23 different sizes; 2 of each size) in every cell. In humans, there are 46 chromosomes (23 different sizes; 2 of each size) in every cell.

Meiosis The law of segregation says each parent passes on one copy of each gene. The law of segregation says each parent passes on one copy of each gene. The offspring receives two copies, one from each parent. The offspring receives two copies, one from each parent. Each parent donates 23 chromosomes (one of each size). Each parent donates 23 chromosomes (one of each size).

Meiosis cont’d To make sure the offspring gets one of each gene (one of each size chromosome), the parent will create gametes. To make sure the offspring gets one of each gene (one of each size chromosome), the parent will create gametes. Gamete-specialized cell used for reproduction carrying half the usual number of chromosomes (one of each size). Gamete-specialized cell used for reproduction carrying half the usual number of chromosomes (one of each size). Gametes are made in specialized tissues (testes and ovaries). Gametes are made in specialized tissues (testes and ovaries).

Meiosis cont’d Reproductive tissues go through meiosis to produce gametes. Reproductive tissues go through meiosis to produce gametes. Meiosis-a two-stage cell division that produces four genetically different gametes. Meiosis-a two-stage cell division that produces four genetically different gametes.

Meiosis cont’d Meiosis I Meiosis I The cell copies the DNA. The cell copies the DNA. Chromosomes pair up by size (homologous pairs). Chromosomes pair up by size (homologous pairs). Homologous pairs separate. Homologous pairs separate. Cell divides once. Cell divides once. Each cell has 23 chromosomes, two identical copies of each. Each cell has 23 chromosomes, two identical copies of each.

Meiosis cont’d Meiosis II Meiosis II Each cell divides a second time. Each cell divides a second time. The copies of each chromosome are split. The copies of each chromosome are split. There are now 4 cells, each with 23 chromosomes. There are now 4 cells, each with 23 chromosomes.

Meiosis cont’d Nondisjunction Nondisjunction Homologous pairs fail to separate. Homologous pairs fail to separate. One gamete has too many chromosomes One gamete has too many chromosomes One has too few. One has too few. When these gametes produce an offspring, there is an incorrect number of chromosomes. When these gametes produce an offspring, there is an incorrect number of chromosomes.

Sexual Reproduction Sexual reproduction-the creation of genetically unique offspring by fusing two gametes from different parents. Sexual reproduction-the creation of genetically unique offspring by fusing two gametes from different parents. Fertilization-the union of a sperm and egg, producing a zygote. Fertilization-the union of a sperm and egg, producing a zygote. Zygote-a fertilized egg. Zygote-a fertilized egg. The zygote goes through mitosis to grow up. The zygote goes through mitosis to grow up.

Sexual Reproduction The zygote now has 46 chromosomes. The zygote now has 46 chromosomes. 23 pairs, one of each size. 23 pairs, one of each size. The 23 rd pair of chromosomes are the sex chromosomes. The 23 rd pair of chromosomes are the sex chromosomes. They create the sex of the offspring. They create the sex of the offspring. XX = female XX = female XY = male XY = male