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More Genetics Chromosomes and Human Genetics A cool frog Outline Key concepts Gene linkage Sex determination in human Sex linkage Errors in chromosome number Key Concepts: Each gene has its own position in sequence on a chromosome Crossing over allows alleles in sequence to swap places Allele recombination contributes to variations in phenotypes Changes in chromosomes can give rise to genetic abnormalities or disorders Gene linkage I. II. Linkage – if genes for 2 traits are located on the same chromosome, they are linked. (genes on the same chromosome tend to be inherited together) Drosophila (Fruit fly) P: Tan body & Long wings - TTLL Black body & short wings – ttll F1: Tt Ll – all Tan body & Long wings By mating F1 with each other F2: ¾ Tan body & Long wings (T-L-) ¼ black body & short wings (ttll),Why? Gene linkage [Punnett Square] ♂\♀ TL tl TL TTLL TtLl tl TtLl ttll some: Tan bodies & short wings (T-ll) black bodies & long wings (ttL-) Crossing Over and Genetic Recombination Homologous Chromosomes Crossover Genetic Recombination Sex Determination in Humans Females Males XX XY Human Embryo and Sex Organ Development XY 8 Embryo Weeks old Gene on Y chromosome governs development of testes Early Human Embryo Duct system in early human embryo Develop into male or female reproductive organs Sex linkage The X chromosome has some genes that Y chromosome does not have. Only one recessive gene can get expressed. A son receives his X chromosome from his mother and can pass it only to his daughters. Thus, sex-linked diseases often have a unique pattern – skip generations. For example: red-green color blind & hemophilia Hemophilia is a disease in which the blood does not clot normally. The disease is recessively inherited and the gene is carried on the X chromosome. Sex linkage Suppose that a normal man marries a woman who is a carrier of the disease gene of hemophilia, what are the genotypic and phenotypic ratios of their children? XH = normal gene, Xh = hemophilia gene: P: XHY X XHXh F1: ♂\♀ XH Xh XH XHXH XHXh Y XH Y XhY H ♂: geno-ratio: ½ X Y, ½ XhY pheno-ratio: ½ normal, ½ disease ♀ : geno-ratio: ½ XHXH, ½XHXh pheno-ratio: all normal (½ carrier) Patterns of X-Linked Inheritance X-Linked Recessive Inheritance Hemophilia Color Blindness Duchenne Muscular Dystrophy Changes in Chromosome Structure Changes in Chromosome Number 1. Chromosomes: vary in nature from 2n = 2 to 2n = 1264 human 2n = 46: ♂ 22 autosomes and XY ♀ 22 autosomes and XX Barr bodies – every “extra” X produces a Barr body in the nucleus (a ♀ has 1 Barr body; a ♂, none.) 2. Abnormal: Klinefelter syndrome –22II + XXY, 2n = 47(♂), 1/1000 Turner syndrome – 22II + X, 2n = 45(♀), 1/5000 Speck syndrome – 22II + XYY, 2n = 47(♂), 1/1000 Trisomy: Down syndrome – 3 #21, 2n = 47(♂), 2n = 47(♀), 1/750 Trisomy X – 22II + XXX, 2n = 47(♀), 1/1000 Down Down Changes in the Number of Autosomes Syndrome Changes in the Number of Autosomes Syndrome Turner Changes in the Number of Chromosomes Syndrome Changes in the Number of Chromosomes Klinfelter Syndrome Changes in the Number of Chromosomes Klinfelter Syndrome Changes in the Number of Chromosomes Speck Syndrome Changes in the Number of Chromosomes XXY Triploidy Changes in the Number of Chromosomes XXX Triploidy Changes in the Number of Chromosomes Barr Bodies of XXXXY Abnormal Chromosomes In Conclusion Genes are arranged in sequence on a chromosome Humans have 23 pairs of homologous chromosomes Females have two X chromosomes whereby males have an X and a Y All other chromosomes are autosomes In Conclusion The parental chromosome number may be changed Crossing over and random segregation adds to adaptive variation in traits among members of a population