The human genome (and the genomes of most sexually reproducing organisms) consists of several pieces of DNA called chromosomes. Parents make copies of their chromosomes and pass one copy of each chromosome to their offspring (refer to Chapter 3 for more info on chromosomes). How the genome is put together sometimes has consequences for how natural selection results in changes in gene frequencies over generations.
Imagine that loci are on the same chromosome. One codes for hair color (its alleles are A and a); the other codes for eye color (its alleles are B and b). Suppose the parent has the AaBb genotype. This parent could have the allele combination Ab and aB (or the combination ab and AB) on each chromosome. Because the loci are linked, the transmission of one allele determines the transmission of the other.
If the hair color locus and the eye color locus are on different chromosomes, their alleles sort independently; if they're on the same chromosome, they may be transmitted together. Note that I say may. Here's where things get more complicated.
The chromosomes can break and rejoin, so even if two genes are on the same chromosome, they aren't necessarily inherited together. What that means is that
^ Even if the parent had the AB and ab allele combinations on its chromosomes, there's still some possibility of producing aB and Ab gametes — and this possibility is greater the farther apart the two loci are on the chromosome.
^ The closer two genes are, the more likely they are to be inherited together. If the eye color gene is right next to the hair color gene, it's more likely that when you passed on these traits to your children, they got whatever combination of alleles were found together on one of your chromosomes.
Now suppose that a beneficial mutation occurs at the eye color locus, and selection acts on that allele. Because the mutation is beneficial, the person who carries it will be more likely to leave descendants; hence, this particular allele will increase through time. As a result of the close link between the eye
color gene and the hair color gene, whatever hair color allele happens to be next to it on the chromosome will also increase in frequency, even though it may not have any significance.
Why is this example important? Because if you don't know that a ho-hum allele can increase in frequency simply because it's next to a wham-bang allele, you may misinterpret the increasing frequency of the ho-hum allele as being evidence of strong selection for that allele. In this case, you may think that something about the hair color gene itself is important evolution-wise, when what really happened is that it hitchhiked a ride because of its proximity to the eye color.
In hitchhiking, changes through time in the allele frequencies at one locus can be caused by selection acting on a different locus.
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