7.17: Red hair and freckles: genes on the same chromosome are sometimes inherited together.

Most redheads have pale skin and freckles. This simple observation is problematic for the law of independent assortment as Mendel imagined it. After all, in his law, he asserted that the inheritance of one trait does not influence the inheritance of another. But clearly having a trait, such as red hair, seems to influence the presence of another trait, pale skin (FIGURE 7-29). Strictly speaking, Mendel’s second law is not true for every pair of traits. Sometimes the alleles for two genes are inherited together and expressed almost as a package.

Figure 7.29: Violating the law of independent assortment. Red hair and freckles are often inherited as a package deal.

In the case of humans, for example, there are about 21,000 genes in our genome. Yet we have only 23 unique chromosomes (two copies of each). Thus, genes influencing different traits must be on the same chromosome, maybe even right next to each other. When they are close together, we say that they are linked genes. One 2008 study, for example, demonstrated a link between human genes that influence hair color and skin pigmentation (including freckles).

Q

Question 7.10

Why do most redheads have pale skin?

Why are linked genes inherited together? To answer this, we must revisit the behavior of chromosomes during the production of gametes, discussed in Chapter 6. When you made that sperm or egg by meiosis, only one of the two copies of each of your chromosomes ended up in the gamete. It may have been the one from your mother or it may have been the one from your father. In either case, all of the alleles that were on the chromosome from that one parent were passed on as a group to the child that resulted from the fertilization involving that gamete. This process continues generation after generation. The linked alleles never get split up unless, during meiosis, recombination occurs between them, moving one or more to the other chromosome in the pair so that they now become linked with the alleles on that chromosome (FIGURE 7-30).

Figure 7.30: Gene linkage. Alleles that are closely linked on the same chromosome will be passed on to offspring in one bundle.

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When alleles are linked closely on the same chromosome, Mendel’s second law doesn’t hold true. It is very surprising—and was fortunate for Mendel—that of the seven pea plant traits he chose for his studies, none of them were close together on the same chromosome. For this reason, they all behaved as if they weren’t linked.

TAKE-HOME MESSAGE 7.17

Sometimes, having one trait influences the presence of another trait. This is because the alleles for two genes are inherited and expressed almost as a package deal when the genes are located close together on the same chromosome.

Under what conditions do traits fail to assort independent of each other?

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