Sex-influenced traits are autosomal traits that are influenced by sex. These traits are not coded on the sex chromosomes, but their expression is influenced by the sex of the individual. In other words, the same gene will show differential expression in each sex. For example, male pattern baldness is a sex-influenced trait. The gene that leads to baldness is dominant in men, meaning if they have the big "P" they are going to have the trait for baldness. In females, the trait is recessive, meaning it is much less likely for the female to exhibit the trait. They would need two big "P"s to exhibit the trait.
Sex-influenced traits can be distinguished from sex-limited traits, which are genes that are present in both sexes of sexually reproducing species but are expressed in only one sex and have no penetrance, or are simply turned off in the other. Sex-limited genes cause the two sexes to show different traits or phenotypes, despite having the same genotype. These traits are responsible for sexual dimorphism, which is a phenotypic difference between males and females of the same species regardless of genotype. These differences can be reflected in size, color, behavior (ex: levels of aggression), and morphology.
Sex-linked traits, on the other hand, are determined by genes located on the sex chromosomes. Most traits carried are present on only the X-chromosome. The Y-chromosome is smaller, and so, very few genes are located on this chromosome. Sex-linked traits would be considered traits like sickle cell anemia and color blindness. They are said to be linked because more males (XY) develop these traits than females (XX). This is because the females have a second X gene to counteract the recessive trait. Thus, the trait is more likely to be visible in the male.
Examples of sex-influenced traits include:
- Male pattern baldness
- Breast cancer
- Hemochromatosis
- High blood pressure
- Coronary heart disease
- Alcohol metabolism
