1. 11.4 Meiosis

2. Chromosome Number Chromosomes carry genes.
Genes are located in specific positions on chromosomes.

3. Diploid Cells 2 1 1 2 Fruit flies have eight chromosomes.
Four come from the father, four come from the mother.
Homologous pair – The two sets of chromosomes that match.
Always one from each parent. 3 3
Why might chromosome 4 be different? X and Y chromosome (Larger X, smaller Y) 4 4

4. Diploid Cells
Diploid cell – Cell containing both sets of homologous chromosomes. 
Represented by 2N.
For the fruit fly, the diploid number is 8, which can be written as 2N = 8
All cells are diploid except the sperm and egg.

5. Haploid Cells
Haploid (N) - cells containing a single set of chromosomes, and therefore a single set of genes.
Gametes (sperm and egg) are haploid.
Fruit flies are 2N with 8 chromosomes. How many chromosomes do the gametes have?
Four

6. Phases of Meiosis
Meiosis - process in which the number of chromosomes in a diploid cell is cut in half.
Made up of meiosis I and meiosis II.
By the end of meiosis II, the diploid cell becomes four haploid cells.
Interphase
Prophase I
Metaphase I
Anaphase I
Telophase I
Cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
Meiosis II
Meiosis I

7. Interphase Meiosis I - Prophase I Chromosomes replicate
Homologous chromosomes pair up.
Form tetrads – four chromatids
Go through crossing-over.
Example cell: 2N (4 chromosomes) 

8. Prophase I – Crossing-Over
First, homologous chromosomes cross over one another.
Crossed sections of the chromatids are exchanged.
Important because it produces new combinations of alleles (genes).

9. Metaphase I
Anaphase I
Homologous chromosomes line up across the middle of the cell.
Homologous chromosomes are pulled apart and move towards opposite ends of the cell.

10. Cytokinesis Telophase I
Nuclear membrane reforms around each cluster of chromosomes.
Cells splits, forming two new cells.

11. Meiosis I Concludes Results in two haploid (N) daughter cells
Each has 2 chromosomes, 4 chromatids.
Each cell has different chromosomes than when it started because of crossing-over.

12. Meiosis II After meiosis I comes meiosis II. No interphase
The final four phases of meiosis II are similar to those in meiosis I. However, the result is four haploid daughter cells.

13. Metaphase II Prophase II Chromosomes become visible.
No tetrads
Chromosomes line up in the middle of each cell.

14. Telophase II, and Cytokinesis Anaphase II
Paired chromatids separate.
Splits into four haploid (N) daughter cells
Each has 2 chromosomes, 2 chromatids.

15. Interphase Prophase I Prophase II Metaphase I Metaphase II Meiosis II
Anaphase I
Telophase I
Cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
Meiosis II
Meiosis I
4 cells
2 chromosomes
2 chromatids
2 cells
2 chromosomes
4 chromatids

16. Gametes to Zygotes
The haploid cells produced by meiosis II are gametes.
In males, gametes are called sperm.
In females, gametes are called eggs.

17. Gametes to Zygotes
Fertilization — the fusion of the male and female gametes
Makes a zygote.
The zygote undergoes mitosis and eventually forms a new organism.
Zygote has a new combinations of alleles

18. Comparing Meiosis and Mitosis
One division
Two divisions
Daughter cells have the same number of chromosomes as first cell
Daughter cells have half the number of chromosomes as first cell
Results in two genetically identical diploid cells
Results in four genetically different haploid cells

20. Meiosis I 23
Meiosis II

21. Gene Linkage and Gene Maps
How can two alleles from different genes be inherited together?
Alleles from different genes tend to be inherited together when those genes are located on the same chromosome.

22. Gene Linkage
A scientist used a fly with reddish-orange eyes and miniature wings in a series of test crosses.
His results showed that the genes for those two traits were almost always inherited together.
Morgan discovered that many of them appeared to be “linked” together in ways that seemed to violate the principle of independent assortment.

23. Gene Linkage Findings led to two conclusions:
First, each chromosome has groups of linked genes.
Second, it is the chromosomes that assort independently, not individual genes.
Alleles of different genes tend to be inherited together when those genes are located on the same chromosome.

24. Gene Mapping
Sturtevant wondered could gene linkage be a clue to the genes’ locations?
Sturtevant reasoned if two genes are close together, then crossovers between them should be rare.
If two genes are far apart, then crossovers between them should be common.

25. Gene Mapping
Used the frequency of cross-overs between genes to determine their distances from each other.
Method still used today.