In some organisms, traits are influenced by sex of the organism. In other words, male and female individuals, which are genotypically similar for a particular trait, give different expressions of the same trait, thus demonstrating that the trait is influenced by sex. Such traits are called sex-influenced traits. This can be illustrated with the help of example of horns in sheep, which are effectively controlled by a single gene, since other genes controlling this trait are always homozygous. Among Dorset
sheep, both sexes are horned and homozygous (h+h+
sheep, both sexes are hornless and homozygous recessive (hh
(horned) are crossed to Suffolk
(hornless), the F1
)are found to be horned males and hornless females. When F1
)are intercrossed, in the resulting F2
progeny, males segregated as 3 (horned) to 1 (hornless), while females segregated as 1 (horned) to 3 (hornless). It is thus obvious that expression of only heterozygote (h+h
)is influenced by sex, h+h
being horned in male and hornless in female (Table 16.1). In other words, while horned character is dominant in male, it is recessive in female. This influence is believed to be mainly due to male and female hormones.
Certain characters in human beings which are not located on sex chromosomes, are also believed to be sex influenced. For instance, white forelock (areas of skin completely without pigment), absence of upper lateral incisor teeth, a type of enlargement of terminal joint fingers, harelip (a fissure in upper lip), cleft palate (a fissure in roof of mouth) and stuttering (involuntary repeats of letters or words) are more common and more severely influenced in males.