Sex Determination Systems Flashcards
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
XX females
Homogametic
XY males
Heterogametic
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.
XY females
Cytogenic; has small pieces of Y chromosome attached to X.
XX males
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
Turner Syndrome
Klinefelter syndrome
XYY male
XO female
Usually embryonic lethal
Secondary sexual characteristics poorly developed
short stature
Turner syndrome
XXY (male)
Breast development
Above average height
Klinefelter syndrome
XYY (male)
Taller than average
antisocial?
XYY male
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.
Barr Bodies
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.
Lyonization
All mitotic descendents of the cell inherit that inactivation pattern.
Epigenetic phenomena
Femalse are mosaics
Lyonization
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
