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Gender and Genes

download (2)You may be thinking that both males and females have the same probability of inheriting any specific gene, however you would be wrong. Gender plays a large role in the probability of an individual’s chances of inheriting specific traits. There are three main variations of how gender can influence the appearance of certain traits: sex-linked traits, sex-influenced traits, and sex-limiting traits

Sex-linked traits are characterized by the difference in male and female sex chromosomes. Females have two large X chromosomes whereas males have only one X chromosome and one smaller Y chromosome. If a trait is only carried on the X chromosome, then it has drastic affects on the phenotype of the individual. With two X chromosomes, females need the presence of two recessive alleles in order for a recessive trait to be expressed.  Males, however, only need the presence of one recessive allele for a recessive trait to be expressed. This is because there is only room for one allele on the singular male X chromosome. Whether the allele is dominant or recessive, it will be expressed. One example of a sex-linked trait is red-green colorblindness. Red-green colorblindness is a recessive sex-linked trait and since males have only one X chromosome, more males are affected than females.

Sex-influenced traits, unlike sex-linked traits, are autosomal. This means that these traits are not carried on the sex chromosomes. Sex-influenced traits are unique because of the way gender affects how the trait is expressed phenotypically. One example is the trait pattern baldness. Pattern baldness has two alleles: a “bald” allele and a “non-bald” allele. The expression of the pattern baldness gene is highly affected by hormones like testosterone. In the presence of high levels testosterone, the baldness allele has a very powerful influence. In the presence of low levels of testosterone, the baldness allele has a less-powerful influence. All humans have testosterone, but males have a lot more than females. This makes the pattern baldness trait act like a dominant allele in men and a recessive allele in women. Even if a female has two “baldness” alleles, there will most likely only a small presence of thinning hair.

Sex-limited traits are also autosomal traits. The genes for sex-limited traits act the same way that any other autosomal gene acts, but the expression of the gene varies in the phenotype of each individual. Sex-limited traits are only expressed in a particular gender. For example, both males and females carry the gene that controls milk lactation, but only females express this gene.

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