Written Assignment 2

Sometimes you can be right about something for the wrong reason. This happened to Gregor Mendel. He didn’t know that genes were carried on chromosomes. He believed that the units of heredity were just free-floating entities within cells. From this perspective, it made sense that the inheritance pattern of one trait wouldn’t influence the inheritance of any other trait. Let’s consider an example. Earlier in this chapter we saw that all cats with completely white fur have at least one parent that also has completely white fur. This is because white fur is coded for by a single dominant gene. Imagine that you had a true-breeding population of cats with white fur (i.e., all offspring manifest the trait and, in this case, have the genotype WW). Now suppose that an individual in this population mated with a cat from a true-breeding population of cats that all had some colored fur (all individuals have the genotype ww). All of their offspring would have white fur, but they would be heterozygous (Ww). If two heterozygotes had offspring together, though, they would produce three-quarters white- furred and one-quarter non-white-furred offspring, with genotypes in the ratio of 1/4 WW, 1/2 Ww, and 1/4 ww. That is just what Mendel observed for traits in pea plants. But what if we concurrently observed another characteristic of these cats? Suppose the original true-breeding population of white-furred cats (WW) was also true-breeding for long hair (all ll), a condition caused by carrying two recessive alleles for a single gene. And suppose that individuals in the colored-fur population (ww) were also true- breeding for short hair (all LL). The question is, do the alleles an individual inherits for the white-fur trait influence which alleles that individual inherits for fur length? The answer is that they do not (Figure 9-27). Rather, the first cross of a long-haired, all-white cat with a cat having short, colored fur would result in offspring heterozygous for both traits— referred to as dihybrid—and expressing each of the dominant traits.

51

Phenotypically, all of the offspring from this cross would have short, completely white fur. In a mating between two of these doubly heterozygous individuals—referred to as a dihybrid cross—three-quarters of the offspring would have the dominant trait and one-quarter would have the recessive trait, regardless of which trait you are tallying. In other words, neither trait influences the inheritance pattern for the other trait; all traits are inherited independently of each other. This is known as Mendel’s law of independent assortment. In the next section, we’ll see that, despite Mendel’s correct understanding that separate traits are inherited independently, his belief that this happened because all genes just float freely around in the cell was not correct. The genes, as we now know, are carried on chromosomes. And this sometimes leads to situations in which independent assortment does not occur.

52

Most redheads have pale skin and freckles. This simple observation is problematic for the law of independent assortment as Mendel imagined it. After all, he asserted that the inheritance of one trait does not influence the inheritance of another. But clearly, having red hair seems to influence the presence of another trait, pale skin. 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. We have about 21,000 genes in our genome. Yet we have only 23 unique chromosomes (two copies of each). Thus, genes influencing different traits must sometimes be on the same chromosome, maybe even right next to each other. When they are close together, we say that they are linked genes. A 2008 study demonstrated a link between human genes that influence hair color and skin pigmentation (including freckles).

54

Why are linked genes inherited together? To answer this, we must revisit the behavior of chromosomes during the production of gametes, discussed in Chapter 8. When producing a sperm or egg by meiosis, only one of the two copies of each of your chromosomes ends up in the gamete. It may have been from your mother or it may have been from your father. All of the alleles on the chromosome from one parent get passed on as a group to the child at fertilization. This process continues generation after generation. The linked alleles remain together unless, during meiosis, recombination occurs, exchanging one or more alleles with the other chromosome in the pair so that they now become linked with the alleles on that chromosome. When alleles are linked closely on the same chromosome, Mendel’s second law doesn’t hold true. It is surprising—and was fortunate for Mendel—that of the seven pea plant traits he chose for his studies, none were close together on the same chromosome. For this reason, they all behaved as if they weren’t linked.

55