100% Red-eyed insects

By inbreeding F1 individuals, red-eyed insects and white-eyed insects appear in the ratio 3:1, where the white colour of eyes appears in males only as follows:

Red-eyed female Red-eyed male

P2 R r X R

XX XY

G2

R r R R r r

F2 XX XY XX XY

Red-eyed Red-eyed Red-eyed White-eyed

Female male female male

Red-eyed insects White-eyed insects

3 : 1

Uses of Morgan assumption:

• Morgan assumption in which he determined the location of the white-eyed recessive gene on the sex chromosome X has explained the results of his experiments.

• Morgan has predicted from his assumption the possibility of obtaining white-eyed females by crossing red-eyed females (whose male parents were white-eyed) with white-eyed males. When Morgan performed this cross he obtained insects with red eyes and others with white eyes in the ratio 1:1. However, 50% of the white-eyed insects were females. Thus, Morgan was able to prove the validity of his prediction according to the above mentioned assumption.

• Morgan's chromosome theory i.e. genes are carried on chromosomes, became valid.

Heterozygous red-eyed White-eyed

female male

P1 R r X r

XX XY

G1

R r R r r r

F1 XX XY XX XY

Red-eyed Red-eyed White-eyed White-eyed

female male female male

25% 25% 25% 25%

Sex-linked characteristics in Man:

The linkage of some characteristics with sex is common among livings. In Man, both Colour blindness (the inability to distinguish red from green) and Haemophilia (lack of blood clotting factors) are sex-linked diseases.

Pedigree studies indicated that these two characteristics are recessive and rarely occur in females. Sex linkage follows the same system as that seen in case of colour of eyes in Drosophila:

• The female passes these traits to her sons.

• The sick male passes these traits to his grandsons through his daughters.

• Sons inherit the sex linked gene from their mothers.

• Sex-linked trait (as Colour blindness and Haemophilia) appears in daughters when they gain this gene from both parents (this requires that the mother should be a carrier, whereas the father would have colour blindness).

Healthy mother (Carrier) Healthy father

P1 C c X C

XX XY

G1

C C C C c c

F1 XX XY XX XY

Healthy Healthy Healthy Colour blind

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:

Normal-haired mother Bald-headed father

P1 + +

B B X B B

G1

+ + + +

B B B B B B B B

F1 Bald-headed male Bald-headed male and Healthy male

and hair fall female normal hair female and female

1 : 2 : 1