2Chromosome Number Chromosomes carry genes. Genes are located in specific positions on chromosomes.
3Diploid 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.33Why might chromosome 4 be different? X and Y chromosome (Larger X, smaller Y)44
4Diploid CellsDiploid 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 = 8All cells are diploid except the sperm and egg.
5Haploid CellsHaploid (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
6Phases of MeiosisMeiosis - 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.InterphaseProphase IMetaphase IAnaphase ITelophase ICytokinesisProphase IIMetaphase IIAnaphase IITelophase IICytokinesisMeiosis IIMeiosis I
7Interphase Meiosis I - Prophase I Chromosomes replicate Homologous chromosomes pair up.Form tetrads – four chromatidsGo through crossing-over.Example cell: 2N (4 chromosomes)
8Prophase 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).
9Metaphase IAnaphase IHomologous chromosomes line up across the middle of the cell.Homologous chromosomes are pulled apart and move towards opposite ends of the cell.
10Cytokinesis Telophase I Nuclear membrane reforms around each cluster of chromosomes.Cells splits, forming two new cells.
11Meiosis 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.
12Meiosis 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.
13Metaphase II Prophase II Chromosomes become visible. No tetradsChromosomes line up in the middle of each cell.
14Telophase II, and Cytokinesis Anaphase II Paired chromatids separate.Splits into four haploid (N) daughter cellsEach has 2 chromosomes, 2 chromatids.
15Interphase Prophase I Prophase II Metaphase I Metaphase II Meiosis II Anaphase ITelophase ICytokinesisProphase IIMetaphase IIAnaphase IITelophase IICytokinesisMeiosis IIMeiosis I4 cells2 chromosomes2 chromatids2 cells2 chromosomes4 chromatids
16Gametes to ZygotesThe haploid cells produced by meiosis II are gametes.In males, gametes are called sperm.In females, gametes are called eggs.
17Gametes to ZygotesFertilization — the fusion of the male and female gametesMakes a zygote.The zygote undergoes mitosis and eventually forms a new organism.Zygote has a new combinations of alleles
18Comparing Meiosis and Mitosis One divisionTwo divisionsDaughter cells have the same number of chromosomes as first cellDaughter cells have half the number of chromosomes as first cellResults in two genetically identical diploid cellsResults in four genetically different haploid cells
21Gene 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.
22Gene LinkageA 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.
23Gene 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.
24Gene MappingSturtevant 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.
25Gene MappingUsed the frequency of cross-overs between genes to determine their distances from each other.Method still used today.