Flashcards in Sex Determination Systems Deck (32):
Single allele at a gene locus determines the sex of that individual.
ex. yeast exist as 2 different haploid sexes: a or alpha.
Genic sex determination
Combination of sex chromosomes determine the sex of that individual
ex. ZW female and ZZ Male in birds or XX female and XO in insects
Genotypic sex determination
Presence of Y chromosome dictates Male or Female
Y determination mechanism
Aligns with X chromosome during meiotic synapsis.
Pseudoautosomal regions (PARs)
Euchromatin has about 50-60 genes involved in sex determination.
Male-specific region of the Y (MSY)
-Becomes active at about 6-8 weeks during development.
-Region encodes protein called Testes Determining Factor (TDF) which directly controls testes development
Sex-determining Region of Y (SRY)
Absence of TDF results in:
Development of gonads into ovaries.
How was SRY first identified?
Observation: sex reversal in humans
Hypothesis: SRY is essential for male sex determination
Cytogenic; has deletion in the Y chromosome.
Cytogenic; has small pieces of Y chromosome attached to X.
SRY first identified, cont...
experiment in mice: SRY gene ortholog cloned and introduced into XX mouse embryos
Result: Introduced trans gene caused normal male development from the XX mice.
Conclusion: SRY causes male testes development
Evidence for Y chromosome mechanism
Usually embryonic lethal
Secondary sexual characteristics poorly developed
Above average height
Taller than average
the inactive X chromosome in a female somatic cell, rendered inactive in a process called lyonization, in those species in which sex is determined by the presence of the Y (including humans) or W chromosome rather than the diploidy of the X or Z.
Silencing of one X chromosome in female allows for the same level of expression for genes on the X chromosome in males and females.
Gene dosage compensation
total # if X chromosomes - 1 =
# of Barr bodies in somatic cells
Only X chromosome is changed from euchromatin to heterochromatin about 16 days after fertilization
-deactivation occurs randomly between 2 X chromosomes in every cell.
All mitotic descendents of the cell inherit that inactivation pattern.
Femalse are mosaics
ex. calico cat: only some genes are coded for orange and some for black. Determined randomly.
Genes on X or Y chromosome
Sex linked genes
Many linked genes are "innocent passengers", meaning;
they don't have anything to do with sex determination
Sex linked genes in humans:
more than 100 traits known, examples:
blood clotting factors
dystrofen: large protein for muscle cell structure
Sex-linked traits in humans:
1. X-linked recessive inheritance
2. X-linked dominant inheritance
-Trait occurs more frequently in males because males are hemizygous for genes on the chromosome.
-Females express the trait when they are homozygous for the allele. Heterozygotes are carriers.
X-linked Recessive Inheritance
What proportion of the sons of an affected mother would show the trait?
What proportion of the sons of a carrier mother would show the trait?
A homozygous normal mother marries an affected male, what proportion of their daughters would show the trait?