genetic inheritance on sex chromosomes

Sex is determined by Chromosome pair number 23, where 2 large chromosomes will be female (XX) or one large and one small being male (XY).

Parents(p): XX x XY
Gametes (g): X, X, Y
Gen. One (F₁): XX, XY

Thus there is a 50% chance of each sex, each time.

Sex linked characteristics are coded for by a gene on the X chromosome.

For example, haemophilia is sex-linked so it is usually seen in Men with Women being carriers.

Male haemophiliac

p: XX x X^hY
g: X X^h,Y
F₁: XX^h, XY

p:X^supX x XY
g: X^h, X X,Y
F₁: X^hX, X^hY, XX, XY

A further example of sex-linked genes can be given from the scottish Higher Biology examinations. In cats, coat colour is controlled by a sex-linked gene, with two incompletely dominant alleles, ginger, and black, the hybrid being tortoise-shell.

Question: What are the possible offpsring of a ginger female and a black male?

Answer: The colour is sex-linked , so we must express it as either X or Y, with a colour, g/b, and it will be carried on the X Chromosome.

The female is therefore X^gX^g and the male is X^bY (Y can't carry colour!).

p: X^gX^g x X^bY
g: X^g, X^g X^b, Y
F₁: X^gX^b, X^bX^g, X^gY, X^gY

From this we can see that females are tortie and the males always ginger. Males can't be tortie because there is no carrier to match black or ginger.

Node your homework: 14.5.96: 15 yrs: Anthropolgy