The Dihyb rid Cross

So far, our examples have involved single genes. However, it is often desirable to study a pair of genes using a dihybrid cross. This is especially valuable in gene mapping studies. That topic is discussed later in this chapter. Mendel's law of independent assortment states that the factors (genes) controlling two or more traits segregate independently of each other. This is one law that, while true for the traits Mendel studied, often breaks down. That is because the two genes under study may be physically close to each other on a chromosome. If this is the case, then the genes are basically attached together and will follow each other through meio-sis. They do not segregate independently. In this situation, the genes are said to be linked. In the dihybrid example that follows, however, we will assume that the pair of genes is unlinked. That is, they are on different chromosomes or are far apart on a single chromosome.

Let's consider the genes for plant height and pod color. Recall that tallness in peas is dominant and dwarfness is recessive, while green pod is dominant over yellow pod. The homozygous dominant (GGTT) parent will have a green-podded, tall phenotype, while the recessive parent (ggtt) will be yellow-podded and dwarf. All the gametes from the dominant parent will be GT, and those of the recessive parent will be gt. The F1 generation will be composed of dihybrid (GgTt) plants that are green-podded and tall (Fig. 13.12).

When intercrossed to produce the F2 generation, these dihybrid members of the F1 generation will produce four types of gametes in equal proportions. The gamete genotypes are GT, Gt, gT, and gt. Remember that each gamete will carry only one allele of each gene. Since any one of the four kinds of gametes can unite randomly with any of the four kinds of gametes of the other parent, 16 combinations are possible.

In order to avoid confusion when trying to make all possible combinations of gametes, a diagram called a Punnett square is used to determine the genotypes of the

Genetics 245

zygotes. The Punnett square diagram looks somewhat like a checkerboard, with one set of gametes across the top and the other set down the left side. To fill in each square on the checkerboard, look at the gamete designation above the square and to the side of it. Multiply the gamete proportions (e.g. 1/4 x 1/4) and add together the gamete letters (e.g., GT + GT). In Figure 13.11, the first square would therefore be 1/16 GGTT.

Nine different zygote genotypes are possible in the 16-box Punnett square. They are produced in a ratio of 1 GGTT: 2 GGTt: 1 GGtt: 2 GgTT: 4 GgTt: 2 Ggtt: 1 ggTT: 2 ggTt: 1 ggtt. Four phenotypes are possible in a ratio of 9 green-podded, tall: 3 green-podded, dwarf: 3 yellow-podded, tall: 1 yellow-podded, dwarf. A geneticist observing a 9:3:3:1 ratio from a dihybrid cross can deduce that the genes controlling the two traits are unlinked and are exhibiting dominance. Genetic ratios such as the ones previously outlined are expected when large numbers of individuals are studied. Note in Mendel's data (Table 13.1) that he looked at over 8,000 F2 offspring following a monohybrid cross involving seed color. With small numbers, chance alone may not produce the expected ratios. Human populations, for example, are divided relatively equally into males and females (a genetically controlled trait), but in smaller groups, such as families, the general population ratio of one male to one female may not be evident.

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