1. Meiosis
Chapter 10

2. Chromosomes Genes are located on chromosomes inside the cell nucleus
When offspring are formed, 1 set of chromosomes from each parent is passed on

3. Chromosome Number
Homologous: chromosomes that are passed on from parents
1 from mom/1 from dad
Diploid: cell that contains both sets of homologous chromosomes
2 complete sets of genes
# chromosomes in diploid cells written as “2N” (Human 2N = 46)

4. Haploid: cell that contains only 1 set of chromosomes (gamete)
Written as “N” (Human N=23)
Gametes: reproductive cells (sperm & eggs)
When gametes are formed, contain only 1 set of genes

5. Meiosis
Def: The process by which the # chromosomes in a cell is cut in half, and homologous chromosomes are separated
2 Stages:
Meiosis I
Meiosis II
Creates 4 haploid cells (gametes), all genetically different

6. Meiosis I
Interphase I: chromosomes replicate , making 2 new chromatids (copies) connected by a centromere
Occurs in 4 steps:
1) Prophase I: Each chromosome pairs with homologous chromosome & forms a tetrad

7. Meiosis I Sometimes when tetrads form, crossing-over can occur
Crossing Over: when homologous chromosomes exchange portions of themselves
Results in new combinations of genes

8. Meiosis I
2) Metaphase I: Spindle fibers attach and chromosomes line up
3) Anaphase I: fibers pull homologous chromosomes apart
4) Telophase I: cytokinesis occurs; results in 2 new diploid (2N) “daughter” cells

9. Meiosis II Chromosomes DO NOT DUPLICATE again (no Interphase)
Occurs in 4 steps:
1) Prophase II: From Meiosis I, have 2 diploid (2N) daughter cells
2) Metaphase II: Chromosomes line up in center of cell (just like Metaphase I)

10. 3) Anaphase II: fibers pull chromatids apart
4) Telophase II: cytokinesis occurs; results in 4 haploid (N) daughter cells

11. Gamete Formation Male Animals: 4 haploid gametes all become sperm
Female Animals: 4 haploid gametes become 1 egg and 3 polar bodies (not used)

12. Mitosis vs. Meiosis
Mitosis → 2 genetically identical diploid (2N) cells
Used for cell growth and replacement
Meiosis → 4 genetically different haploid (N) cells
Used for reproduction & formation on gametes

13. Linkage and Gene Maps
Thomas Morgan (1910): discovered many traits inherited together
i.e.: fruit fly’s red eyes and mini wings
Genes can be placed in “linkage groups”
“Linkage Group” = chromosome
Mendel just happened to study genes on different chromosomes

14. T. Morgan: genes on same chromosome should be inherited together
Chromosomes = group of linked genes
Chromosomes assort independently; not individual genes

15. However, crossing-over may “unlink” some genes
Farther apart = more likely to separate
This action creates new combinations of alleles → more genetic diversity

16. Gene Mapping
Rate which genes become separated and recombine is measured
Describes the relative location of each known gene on a chromosome
Created by “mapping” genes relative locations