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Lecture 2a: Sex: Chromosomes Sutton-Boveri hypothesis (1902): Walter Sutton (US grad student), Theodor Boveri (German Biologist) suggested that chromosomes contain the units of heredity. Sutton was motivated by observations that sex seems to be determined by the presence/absence of a chromosome in the great lubber grasshopper (Brachystola magna). Why? Because Mendel's particles parallel the behavior of chromosomes at meiosis. Support for Sutton-Boveri hypothesis: 1913 Elinor Carothers - showed independent assortment of chromosomes using observations of a heteromorphic chromosome pair (from a grasshopper) at anaphase Sex chromosomes: 1916 Calvin Blackwell Bridges (as opposed to autosomes) Used red and white-eyed mutants in Drosophila: XR = red, Xr = white; Red dominant In females: XR XR , XR Xr = red-eye female and Xr X r = white-eyed females In males: XR Y = red-eye male and Xr Y = white-eyed male Homogametic sex - that sex containing two like sex chromosomes. In most animals species these are females (XX); in butterflies and birds, ZZ males. Heterogametic sex - that sex containing two different sex chromosomes. In most animal species these are XY males. In butterflies and birds, ZW females; grasshoppers have XO males. Pairing of sex chromosomes: In the homogametic sex: pairing like normal autosomal chromosomes; in the heterogametic sex, the two sex chromosomes are very different, and have special pairing regions to insure proper pairing at meiosis. Dosage Compensation: Problem: gene difference in males vs females. Mammals inactivate one X (X inactivation - in each female cell in mammals , one X is picked at random and inactivated); Drosophila overexpress X in male (hyperactivation of X) . Imprinting: Differential behavior of identical genes, depending on which parent contributed them. In extreme cases, some genes are transmitted from one sex in an active form and in the other sex in an inactive form, e.g., Prader-Willi syndrome, located on an autosome (Chromosome 15). The maternal copy is inactivated. Individuals with defected paternal copy have PWS, and individuals with two maternal copies have PWS. Sex linkage: XA = Locus on X chromosome XX females, ZZ males "normal" genetics XA XA, XaXa = AA & aa homozygotes XA Xa = Aa heterozygote XY males, WZ females XA Y, XaY = hemizygous for A, a