Sex influenced genes
The sex influenced genes are influenced by the sex of the bearer. They are located on the autosomes. The sex influenced genes express more frequently in one sex than in the other.
I) Baldness in man:
The baldness in man is a very good example for sex influenced character. This particular character is dominant in men and recessive in women. This is because the gene for baldness(B) in heterozygous condition(Bb) expresses itself in male but the heterozygous females are normal even though they carry the gene for baldness. It shows that only one gene is enough to produce a baldman, whereas a women must require two such genes to be bald. In man, the single gene for baldness can operate only in the presence of male hormone.
ii) Index Finger:
There is an another interesting sex influenced gene in man which affect the length of the index finger. The short index finger is due to a gene which is dominant in the male and recessive in the female.
iii) Horns in Sheep:
In sheep, the Dorset breed have horns while Suffolk breeds are hornless. The horned condition is dominant(HH) and the hornless condition is recessive(hh).
A pure horned Dorset breed is crossed with Suffolk hornless breed(hh). The F1 offspring are heterozygous(Hh). The males are horned and females are hornless.
HH geneotype produces horns in both sexes.
Hh genotype produces horns in male but not in female.
hh genotype produces hornless condition in both sexes.
In males, one dominant gene, in cooperation with male hormones, develops horns. But in females. one dominant gene cannot develop horns because of the absence of male hormones.
When the F1 hybrids are crossed, In the F2 the males having one or two dominant alleles, develop horns. The females having two dominant alleles alone develop horns. Thus horn in sheep is influenced by male sex.
