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 mature sterile lowered IQ is common 1 700 male births 45 X Turner Syndrome XO female short stature rudimentary ovaries sterile IQ typically normal 1 3000 female births 47 XYY Double Y syndrome male above average height otherwise phenotypically normal At one time it was claimed that XYY males are prone to violent or antisocial behavior based on elevated incidence of 47 XYY among incarcerated men Now thought to be due to higher incidence of moderate mental retardation than for XY males 47 XXX Trisomy X syndrome many phenotypically normal the frequency of lowered IQ is higher than among XX females
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