|MadSci Network: Genetics|
The degree of dominance shown by an allele can indeed change. This phenomenon was most famously shown by E.B. Ford in 1940. Ford worked with the moth Abraxas grossulariata, which can be yellow or white. A single gene (we'll call it L) controls the color. In wild moths, L1 L1 moths were yellow, L2 L2 moths were white, and L1 L2 heterozygotes were a pale yellow intermediate color. In other words, this was an example of incomplete dominance. Ford performed four generations of artificial selection in two moth populations. In one, he selected heterozygotes that were lighter in color; in the other population he selected heterozygotes that were darker. After four generations, the allele L1 acted as a nearly complete recessive the first population -- that is, the heterozygotes were white just like L2 L2 homozygotes. In the other population the same allele, L1, had become completely dominant, so that the heterozygotes were yellow just like L1 L1 homozygotes. Ford concluded that he had selected for different alleles of modifier genes that code for products that affected the production of the yellow pigment, in such a way that a formerly incompletely dominant allele could become either dominant or recessive. There's a nice brief review of work on the evolution of dominance in a 1999 paper by Otto and Bourguet. Basically, you have to remember that an allele codes for a protein, often an enzyme. A single enzyme doesn't produce a phenotype by itself; it has to have substrates to act on, and the product of the reaction it catalyzes may then be the substrate for another reaction catalyzed by another enzyme. Thus there are genes that code for enzymes that affect how much substrate there is to work with, and what happens to the product after it's made. Not surprisingly, some of these genes will affect whether the heterozygote looks more like one homozygote or the other. As for the other answer, I'm not sure if it's phrased exactly the way your teacher had it; it's a bit hard to assess as written. Males are more likely to express a trait caused by a recessive allele of a gene on the X chromosome, as they only have one. Ford EB. 1940. The theory of dominance. American Naturalist 64:560–566. Otto SP, Bourguet D. 1999. Balanced Polymorphisms and the Evolution of Dominance. Am. Nat. 153: 561-574. Moderator's note: Actually, the words "recessive" and "dominant" do not describe independent properties of an allele; instead, they describe a relationship between two alleles. I.e. A1 is dominant to A2. The dominant allele in one relationship between two alleles can be the recessive allele in another relationship. for example, A1 is dominant to A2, where A2 is recessive; but A2 is dominant to A3 (A2 is dominant).
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