Sex limited genes:
Sex limited genes express characters in only one sex. The sex limited genes may be located on any chromosome(sex linked genes are located only on the sex chromosome). Their expression in the vertebrates is governed by the sex-hormones. Sex limited genes are responsible for the secondary sexual characteristics as well as primary sexual characters.
Examples:
a) In man, the beard is produced by sex limited genes. A women, normally, does not have a beard, yet she surely carries all the genes necessary to produce beard. But the expression of that particular gene in ladies is prevented by the absence of a particular hormone. In rare cases, abnormalities in hormone secretion may occur in a woman which allow these genes to express themselves and the result is a bearded lady.
b) Breast development is normally limited to women but hormone unbalance may cause breast development in a man.
c) The genes for the deep masculine voice and masculline musculature in man will express themselves only when the male hormone is present. Genes for the feminine musculature, express themselves in the absence of male hormone. They do not require the presence of the female hormone.
d) In cattle's, the milk production is controlled by sex limited genes. The bulls also carry the gene for milk production. But it is not expressed in the male sex. In the cows, the genes for milk production makes its expression in the presence of female hormones. The brilliant plumage of peacock is also due to some sex limited genes.
e) An excellent illustration of sex limited inheritance is provided by the plumage pattern in birds. In domestic fowl of Leghorn, males have long curved, fringed feathers on tail and neck, but feathers of female are shorter, straighter and without fringe. Thus the males are cock feathered and females are hen feathered.
Results of various crosses show that hen feather is due to a dominant gene H and cock feathering is due to it's recessive allele "h". It has been found that the expression of the genes H and h, depends upon the sex hormone. Any male that receives at least one dominant gene will be hen feathered, but those males which are homozygous for the recessive gene(hh), will show the cock feathered condition.
Female are all hen feathered regardless of genotype. It shows that a particular type of feathering depends upon specific combination of genotype and sex hormones. The dominant gene H produces hen feathering in the presence of male or female sex hormone. The recessive gene, "h" produces cock feathering in the absence of female hormone and hen feathering in the presence of female hormone.
Genotype Phenotype
Male Female
HH Hen feathered Hen feathered
Hh Hen feathered Hen feathered
hh Cock feathered Hen feathered