bicd 100 - lecture 11

bicd 100 - lecture 11 - LECTURE - 16 1. 2. 3. 4. Sex-linked...

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Unformatted text preview: LECTURE - 16 1. 2. 3. 4. Sex-linked Characteristics Penetrance and Expressivity Sex-influenced Characteristics Sex-limited Characteristics Sex Determination • • • • XX-XO: grasshoppers XX-XY: humans ZW-ZZ: birds, fish Haplodiploidy: (no sex chromosomes) bees, wasps, ants • Environmental: slipper limpets • Genic balance: Drosophila Dosage Compensation X inactivation in Female Mammals • In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development • If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character LE 15-11 Two cell populations in adult cat: Active X Early embryo: Orange fur X chromosomes Cell division Inactive X and X chromosome Inactive X inactivation Black fur Allele for orange fur Allele for black fur Active X X chromosome inactivation Xic (X inactivation center) Xist (X inactivation specific transcript) Thomas Hunt Morgan First studied sex-linked inheritance in Drosophila melanogaster. It all started when he observed a white eyed male fly in 1910. X-linked diseases X-linked recessive alleles • • • • Hemophilia Colorblindness SCID Duchenne Muscular Dystrophy (also Becker Muscular Dystrophy) • Females can be homozygous or heterozygous • Males are hemizygous for X-linked genes X-linked dominant alleles Amelogenesis imperfecta: produces abnormal tooth enamel. About 5% of amelogenesis imperfecta cases are caused by mutations in the AMELX gene and are inherited in an X-linked pattern. At least 3 other autosomal genes known. Rett Syndrome: mental retardation, neurodegeneration CGH (Congenital Generalized Hypertrichosis) Penetrance and Expressivity • A genotype may not always produce the expected phenotype (Yikes, it get’s complicated!) • Penetrance: percentage of individuals with a particular genotype that express the expected phenotype. • Dominant allele for polydactyly shows incomplete penetrance (i.e. not 100%). • If 76 out of 84 individuals with the polydactyly allele are polydactylous, penetrance is 76/84=0.9 or 90% • Expressivity: degree to which a character is expressed. Sex limited traits May be X linked or autosomal: beard growth, breast development Sex influenced traits Male pattern baldness: autosomal allele dominant in males and recessive in females. • X-linked dominant allele Amelogenesis imperfecta: produces abnormal tooth enamel Rett Syndrome: mental retardation, neurodegeneration CGH (Congenital Generalized Hypertrichosis) Sex influenced traits Male pattern baldness: autosomal allele dominant in males and recessive in females. (Myth: man inherit’s baldness from mother) Sex limited traits May be X linked or autosomal. Extreme form of sex-influenced inheritance with zero penetrance in one sex! Examples: 1. Changes during puberty: beard growth, breast development 2. Cock feathering (autosomal recessive trait sex-limited to males) Cock feathering in male chicken hh Hen feathering in female chicken HH, Hh or hh Hen feathering in male chicken Hh or HH Precocious Puberty Male-limited precocious puberty: males with this autosomal dominant allele undergo puberty at a very early age, usually before 4 years of age. Receptor for the hormone LH (Luteinizing Hormone) is mutated and “on” even in the absence of LH. Penis enlarges, pubic hair appears, voice deepens. Fully fertile males of short stature. ...
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This note was uploaded on 01/25/2011 for the course BICD 100 taught by Professor Nehring during the Spring '08 term at UCSD.

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