Linked Genes: Pleitropy and Epistasis
Genetics is seldom a straight path from gene to protein to mustache. At every step, there can be modification and multiple effects. Pleitropy is when a single gene has an effect on multiple traits. For our mustache example, a gene regulating secondary sexual characteristics (the changes in men and women's bodies that occur during the advent of adolescence, such as facial hair) can have an effect on hair produced all over the body. Generally, the more hirsute a person is, the more likely they are to have a mustache. Since there is no actual mustache-gene, and since hairlessness isn't necessarily selected for, we will see mustaches as long as there is positive selection for hair.
Epistasis is when the expression of one gene effects the expression of another gene. For example say there is a gene at the "hipster-ness" locus that has two alleles, "hipster" and "non-hipster." When an individual has the "hipster" allele, the expression of this gene will turn on the expression of other genes such as the genes for mustaches, non-prescription thick-rimmed glasses, androgynous hair styles, and appreciation of terrible indie-rock. Any one of those downstream traits may be driving the selection for the hipster allele. Even if the mustache is selected against, it is regulated by another gene that may be positively selected for some inscrutable reason.
In both of these examples when we ask "what for?" we cannot give a satisfactory answer, because the expression of a mustache isn't the real unit of selection. The mustache is linked to other traits that are undergoing selection and influencing the expression of mustaches, despite their unpopularity.
No comments:
Post a Comment