Female male female (Carrier) male

25% 25% 25% 25%

N.B.:

Haemophilia and colour blindness do not appear with the same degree in all diseased persons. This led scientists to assume that this is due to the interference of a number of different genes; most of them are located at different positions on the sex chromosome X.

Sex-influenced Inheritance

The sex of living organisms sometimes influences the dominance of some traits where the action of some genes is influenced by sex hormones secreted by the gonads of adult males and females in some animals and in Man:

Premature baldness in Man:

Premature baldness in Man is an example of sex-influenced characteristics in Man. It spreads among males of some families more than in females. This characteristic is controlled by a dominant gene which is influenced by male hormone. The baldness appears in the male in the presence of one gene only, whereas in females it requires the presence of two dominant genes where the hair of the female head falls down. When a bald man marries a normal woman, both of them heterozygous, the results will be as follows: