1. Section 11.4

2. Chromosome Number An organism with 2 parents must inherit a single copy of every gene from each parent. When gametes are made, the 2 sets of genes must be separated so that each gamete has 1 set of genes.

3. Chromosome Number Diploid Cells (2) – When a cell has both sets of homologous chromosomes. Body cells Haploid Cells (1) – When a cell has one set of chromosomes Gametes

4. Phases of Meiosis Meiosis is when the number of chromosomes is cut in half. Has two divisions.

5. Meiosis I Chromosomes replicate prior to meiosis I. Each chromosome has 2 sister chromatids, joined at the center by a centromere.

6. Meiosis I ***Prophase I – Each replicated chromosome pairs with its homologous chromosome. – Pair forms a tetrad – Crossing-over occurs between tetrads. Makes new combos of alleles.

7. Meiosis I Metaphase I: paired homologous chromosomes line up across the middle. Anaphase I: chromosomes pulled to opposite ends of the cell. Telophase I:nuclear envelope reforms Cytokinesis I: cytoplasm divides, forms 2 diploid cells.

8. Meiosis I

9. Meiosis II Each daughter cell undergoes a second meiotic division. Chromosomes are NOT replicated before this.

10. Meiosis II Prophase II – Chromosomes become visible – DO NOT form tetrads Homologous pairs were separated during meiosis I.

11. Meiosis II Metaphase II, Anaphase II, Telophase II, Cytokinesis II. Stages just like meiosis I. Results in 4 haploid cells

13. Gametes to Zygotes 4 haploid cells are gametes – Sperm in males – Eggs in females When an egg is fertilized (by a sperm), it is called a zygote. – Zygote forms a new organism

14. Comparing Meiosis and Mitosis Replication and Separation of Genetic Material – Mitosis: each daughter cell has one complete set of chromosomes. (Body Cells) – Meiosis: daughter cells contain 1/2 the number of chromososmes. (Gametes)

15. Comparing Meiosis and Mitosis Changes in Chromosome Number – Mitosis: Normally doesn’t change chromosome # Diploid – Meiosis: Chromosome number is halved Haploid

16. Comparing Meiosis and Mitosis Number of Cell Divisions: – Mitosis 2 cells; genetically identical; diploid – Meiosis 4 cells; genetically different; haploid

17. Gene Linkage Thomas Morgan used fruit flies. He determined that genes located on the same chromosome segregate together.

18. Gene Maps If genes are farther apart, the more likely it is that they will cross-over. If genes are close together, cross-overs are rare. Use frequency of crossing- over to determine distance.