Download Sex Determination and Dosage Compensation

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+/–
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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
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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
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- inactivated X chosen at random early in development
- evidence:
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