Determination of Sex and Disorders of Sex Chromosomes Flashcards
Why is sexual reproduction advantageous?
speeds evolution and contributes to increased survival among species by increasing genetic variability
what is X chromosomal inactivation?
one X chromosome in females is inactivated randomly. only one X chromosome is active in all people males and females
what does X inactivation in females cause?
females are mosaic for which X is expressed (maternal or paternal)
what determines which X chromosome will be inactivated?
expression of the XIST gene on chromosome that will be inactivated
where is the XIST gene located?
on chromosome that will be inactivated, not present on active chromosome
what does the XIST gene produce?
non-coding RNA product that binds to inactive X chromosome
what is a Barr body?
inactive X chromosome
what does expression of XIST allow for?
allows for DNA methylation and histone modification to deactivate chromosome
when does X inactivation occur?
1st week of embryogenesis
how many genes on the X chromosome escape inactivation?
10-15%
where are the genes located on the X chromosome that escape inactivation?
the p arm
why are the genes that “escape” inactivation important?
because they could have greater clinical significance than those on Xq
what is nonrandom X inactivation?
if there is structural abnormality in X chromosome, that chromosome will be selected for inactivation
what happens to an X chromosome that has a balanced translocation with an autosome?
it will remain activated even though structural abnormality because doesn’t want to inactivate the autosome
what are the 3 steps of sex differentiation?
- genetic sex determination
- gonad formation
- internal and external reproductive organs
how is the genetic sex determined?
XX female
XY male
absence of some genes from Y chromosome initiates female phenotype
what gene is responsible for inducing development of the testes?
SRY gene (sex-determining region on Y)
what happens if the SRY gene is absent or not expressed?
female gonads form
where is the WT gene located and what does it do?
chromosome 11
directs differentiation of the mesonephros and genital ridge
where is the SOX9 gene located and what does it do?
chromosome 17
codes for transcription factors that induce differentiation of Sertoli and Leydig cells
what other genes does SOX9 work with?
SF1 and SRY
where is steroidogenic factor 1 (SF1) located and what does it do?
chromosome 9
induces differentiation of sertoli and leydig cells
what does SRY do to SF1?
upregulates expression
what X chromosome gene induces ovarian development?
DAX1
