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Transcript
Lecture 37 – Sex determination and dosage compensation I. Introduction A. What is it? In early development, animal embryo must specify gender B. How is it studied? Mutations that transform individuals into inappropriate gender. II. Sex determination - mechanism varies between species A. C. elegans - acts at level of individual cell ratio chromo. gender 1.0 2X:2A hermaphrodite 0.5 1X:2A male 0.67 2X:3A male 0.75 3X:4A hermaphrodite 1. genetic pathway regulating sex in C. elegans 2. what about mutations? - eg. her-1, tra-1 B. Interactions between genes in the same pathway 1. either mutation causes same phenotype - what does this do to the dihybrid ratio? c+/c–; p+/p– X c+/c–; p+/p– c+/–; p+/– 1 c+/–; p–/p– c–/c–; p+/– c–/c–;p–/p– C. Drosophila - acts at level of individual cell ratio 1.0 0.5 0.67 0.33 chrom. 2X:2A 1X:2A 2X:3A 1X:3A 1. genetic pathway 2. mutations, eg. tra, dsx 2 gender female male intersex male D. How do cells count chromosomes? 1. transcription factors on autosomes and X chromosome 2. XX has two copies of gene for protein in X, XO or XY has one copy of gene for protein on X E. human and other mammals 3 1. gender determined by male promoting gene on Y X:A chrom. gender 0.5 XY male 1.0 XX female 0.5 X female 1.0 XXY male 2. sex determined at level of individual, not at level of cell a. chromosomal complement determines if primordial gonad cells are testis or ovary - if testis, secrete testosterone male - if ovary, no testosterone female b. gender determined by sry gene, on Y chromosome - identified using sex reversal in mice and human - XX + small piece of Y translocated to X - develop as male even though XX - how to domonstrate sry is right gene III. Dosage compensation A. human X chromosome aneuploidies XX normal XXX often normal, mild retardation XXXX, XXXXX – similar to XXX, but somewhat more pronounced XY normal XXY sterile, mild retardation XXXY, XXXXY – similar to XXY, but somewhat more pronounced - why aren’t 3X, 4X, 5X etc. more severely defective? B. Mutations that disrupt dosage compensation - in worms, hermaphrodite that can’t dosage compensate die - males that dosage compensate die C. very different mechansims used by different species 1. C. elegans 2. Drosophila 3. mammals - one copy of X is almost completely shut down 4 - inactivated X chosen at random early in development - evidence: 5