Mendel’s second law states that copies of different genes assort independently

Now let’s consider inheritance patterns involving two different genes. Consider an organism that is heterozygous for genes for seed color (yellow or green) and seed shape (round or wrinkled). In our example, the dominant R and Y alleles came from one parent, and the recessive r and y alleles came from the other parent. When this organism produces gametes, do the R and Y alleles always go together in one gamete, and the r and y alleles in another? Or can a single gamete receive one recessive and one dominant allele (R and y or r and Y)?

Mendel performed another series of experiments to answer these questions. He began with peas that differed in two characters: seed shape and seed color. One parental variety produced only round, yellow seeds (RRYY), and the other produced only wrinkled, green ones (rryy). A cross between these two varieties produced an F1 generation in which all the plants were RrYy. Because the R and Y alleles were dominant, the F1 seeds were all round and yellow.

Mendel continued this experiment into the F2 generation by performing a dihybrid cross—a cross between individuals that are identical double heterozygotes. In this case, he simply allowed the F1 plants, which were all double heterozygotes, to self-pollinate. Depending on whether the alleles of the two genes are inherited together or separately, there are two possible outcomes, as Mendel saw:

  1. The alleles could maintain the associations they had in the parental generation. If this were the case, the F1 plants would produce two types of gametes (RY and ry). The F2 progeny resulting from self-pollination of these F1 plants would consist of two phenotypes: round yellow and wrinkled green in the ratio of 3:1, just as in the monohybrid cross.

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  2. The segregation of R from r could be independent of the segregation of Y from y—the two genes could be unlinked. In this case, four kinds of gametes would be produced in equal numbers: RY, Ry, rY, and ry. When these gametes combine at random, they should produce an F2 generation with four phenotypes (round yellow, round green, wrinkled yellow, wrinkled green). Putting these possibilities into a Punnett square, we can predict that these four phenotypes would occur in a ratio of 9:3:3:1.

The results supported the second prediction: four different phenotypes appeared in the F2 generation in a ratio of about 9:3:3:1 (Figure 12.4). On the basis of such experiments, Mendel proposed his second law—the law of independent assortment: alleles of different genes assort independently of one another during gamete formation. In the example above, the segregation of the R and r alleles is independent of the segregation of the Y and y alleles. Mendel’s second law is now understood in the context of meiosis (see Figure 11.15): chromosomes segregate independently during the formation of gametes, and so do any two genes located on separate chromosome pairs (Focus: Key Figure 12.5).

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Figure 12.4 Independent Assortment The 16 possible combinations of gametes in this dihybrid cross result in nine different genotypes. Because R and Y are dominant over r and y, respectively, the nine genotypes result in four phenotypes in a ratio of 9:3:3:1. These results show that the two genes segregate independently.

focus: key figure

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Figure 12.5 Meiosis Accounts for Independent Assortment of Alleles We now know that copies of genes on different chromosomes are segregated independently during metaphase I of meiosis. Thus a parent of genotype RrYy can form gametes with four different genotypes.

Question

Q: A diploid heterozygote with a single gene can produce haploid gametes of only two genetically distinct types. A heterozygote with two genes can produce four genetically distinct types of gametes, as illustrated here. How many genetically distinct gametes could be produced by a heterozygote with four genes assorting independently? (Answer this without drawing a diagram!)

NEED Answer for Question in Figure 12.5.

Animation 12.1 Independent Assortment of Alleles

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Media Clip 12.1 Mendel’s Discoveries

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