BSC 219
8/30/12
Sex determination and sex-linked traits
Sex Is Determined by a Number of
Different Mechanisms
Sex-Linked Characteristics Are
Determined by Genes on the Sex Chromosomes
Sex
Determination Mechanisms
Hermaphroditism: both
sexes in the same organism
Monoecious: both
male and female reproductive structures in the same organism
Dioecious: either
male or female reproductive structures in one organism
Chromosomal
Sex-Determination Systems:
Sex chromosomes and non-sex chromosomes (autosomes)
XX-XO system:
XX – female
XO – male
grasshoppers
XX-XY system:
XX – female
XY – male
mammals
Chromosomal
Sex-Determination Systems
ZZ-ZW system:
ZZ – male
ZW – female
Birds,
snakes, butterflies, some amphibians, and fishes
Haplodiploidy system:
Haploid
set – male
Diploid
set – female
Bees,
wasps, and ants
Genic Sex-Determining System
No sex chromosomes, only the
sex-determining genes
Environmental
Sex Determination
Environmental factors
LimpetÕs position in the stack
Temperature
in turtles
Sex
Determination in Drosophila melanogaster
genic balance
system
X : A ratio
(X, number of X chromosomes;
A, number of haploid sets of autosomes)
Sex
Determination in Humans
XX-XY
SRY gene on the Y chromosome
determines maleness
Encodes
transcription regulator that turns on male specific genes during development
Turner syndrome: XO;
1/3000 female births
Klinefelter syndrome: XXY, or
XXXY, or XXXXY, or XXYY; 1/1000 male births
Poly-X females: 1/1000
female births
The Role of
Sex Chromosomes
The X chromosome contains genetic information
essential for both sexes; at least one copy of an X is required.
The male-determining gene is
located on the Y chromosome. A single Y, even in the presence of several X,
still produces a male phenotype.
The absence of Y results in a
female phenotype.
The
male-determining gene in humans
Sex-determining region Y (SRY) gene
Androgen-insensitivity syndrome
Caused
by the defective androgen receptor
Sex-Linked
Characteristics Are Determined by Genes on the Sex Chromosomes
X-Linked
Characteristics
X-linked white eyes in Drosophila
X-Linked
Characteristics
X-linked color blindness in humans
X-Linked
Characteristics
Dosage compensation: the amount of
protein produced by X-linked genes and randomly inactivated in two sexes
Having
two copies of genes in females and one copy in males could produce problems
One
copy of X chromosome is inactivated in each cell- called Barr body
Female
is mosaic with respect to which X is inactive
Sex-Linked
Characteristics Are Determined by Genes on the Sex Chromosomes
Z-linked characteristics
Y-linked characteristics
Hairy
ears
Characteristics
of the Y and Y-linked genetic markers