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Announcements Homework due 9:00 Thursday – only your first grade will count! Review Session Sunday 3:00 to 4:00 PM 124 Burrill Hall Exam Tuesday 9:00 AM, in Lecture room Sex Chromosomes and Sex Determination X and Y are clearly not homologous chromsomes. How do they segregate normally at Meiosis? During what part of Meiosis do they pair and then segregate? Human Y chromosome • Yellow: pseuoautosomal regions • Red: genes with homologs on the X chromosome • Blue: genes without homologus on the X Genetically, what determines gender in mammals? Presence of a single gene (SRY) that usually, but not always, occurs on the Y chromosome. If the Y chromosome is missing (this gene deleted) or has a non-functional mutation in the gene, an XY individual can be a perfectly normal female. If the SRY gene becomes translocated to another chromosome, an XX individual can be a phenotypically normal (but sterile) male. XX (+SRY) mice Dosage Compensation Do males have half as much of the products of genes on the X as do females? X Inactivation Barr Body: Inactive X Interphase: Chromomes can’t be stained, but a dark-staining body is visible in the nuclei of cells of female mammals Which X gets inactivated? Mary Lyon & Lianne Russell (1961) proposed that one or other of X becomes inactivated at a particular time in early development. Within each cell,which X becomes inactivated is ____________. As development proceeds, all cells arising by cell division after than time have _______ _________________. In 64-cell embryos Adult female mammals are ___________ for genes on the X chromosome. Genetic Mosaics Calico cats are almost always ____________. One X chromosome carries the allele for black coat color The other X chromosome carries the allele for orange coat color In 64-cell embryos, one of each pair of X chromosomes and its genes are _____________ ________________. Daughter cells inherit active or inactive X chromosomes, creating a cat with patches of coat color XOXo Tortoiseshell (calico) coat color allele allele Tortoiseshell (calico) coat color Tortoiseshell (calico) coat color Carbon Copy X inactivation is different in marsupials kangaroos, wombats, bandicoots, opossums Sex chromosome aberrations 47,XXY Klinefelter syndrome male tall stature testes do not ma ture steril e lowered IQ is comm on 1/700 male bir ths 45,X Turner Syndrom e (XO) female sho rt stature rudimentary ova ries steril e IQ typicall y nor mal 1/3000 female births 47,XYY Doubl e-Y syndrome male above ave rage height, otherwise pheno typicall y normal. At one time , it was claim ed that XYY males are prone to violent or antisocial behav ior, based on eleva ted incidenc e of 47,XYY a mong inca rcerated men. Now though t to be due to higher incidence of moderate mental retardation than for XY males. 47,XXX Trisomy-X syndrome many pheno typicall y no rmal the frequency of lowered IQ is higher than among XX females.