Download Sex determination and the Olympics

Sex Determination
In humans, females have two X chromosomes and males possess an X and a Y. The presence of a Y
chromosome in humans should normally result in a male because of the presence of the SRY (Sex
determining Region Y) gene which turns on at about 6 weeks of embryonic development and signals
other genes to turn on and release signals that result in formation of male internal reproductive
structures. Up until this point, the sex of the embryo is indeterminant.
In the absence of SRY, development proceeds as shown to the right and the Wolffian ducts
degenerate and the Mullerian ducts form the Fallopian tubes. This is why biologists say that female is
the “default sex” In the presence of SRY, the Mullerian ducts degenerate and the Wolffian ducts form
the Vas deferens. Errors in cell signalling can result in incomplete conversion and the formation of
sexual hermaphrodites which is a term currently falling out of favour. Hermaphrodites have sex organs
which are not clearly male or female. So a person can have a “genetic sex” (XX or XY), “gonadal sex”
(formation of internal sex organs or gonads) and a “phenotypic sex” appearance of external genitalia.
Sex Determination
Chromosome X
 Contains over 1400 genes
 Contains over 150 million base pairs, of which approximately 95% have been determined
Chromosome Y
 Contains over 200 genes
 Contains over 50 million base pairs, of which approximately 50% have been determined
Video about Y chromosome and SRY:
http://www.encyclopedia.com/video/DadqEUXf4JU-facts-of-life-gender-determination.aspx
Video about sex determination in humans and other organisms (looked at this in class):
http://ed.ted.com/lessons/sex-determination-more-complicated-than-you-thought
Sex determination and the Olympics:
Because females have two copies of the X chromosome one of them is randomly deactivated early in
embryogenesis (about the 32 cell stage) in each cell. Once inactivated it stays inactivated (except in
germ cells that form gametes) Either the X from the mother or the father can be deactivated. It shows
up later as a “Barr body” during interphase. This means that all females are “genetic mosaics” with
regard to genes on the X chromosome. This can result in some interesting phenotypes. (sweat gland
formation and colour blindness examples). Barr bodies were used as a method of gender determination
in the Olympics from 1966 to 1999.