D

m d and half have mottled leaves and are dwarf: m d m

These progeny display the original combinations of traits present in the P generation and are nonrecombinant progeny, or parental progeny. No new combinations of the two traits, such as normal leaves with dwarf or mottled leaves with tall, appear in the offspring, because the genes affecting the two characteristics are completely linked and are inherited together. New combinations of traits could arise only if the linkage between M and D or between m and d were broken.

These results are distinctly different from the results that are expected when genes assort independently ( FIGURE 7.5b). With independent assortment, the heterozygous plant (MmDd) would produce four types of gametes: two nonrecombinant gametes containing the original combinations of alleles (MD and md) and two gametes containing new combinations of alleles (Md and mD). Gametes with new combinations of alleles are called recombinant gametes. With independent assortment, nonrecombinant and recombinant gametes are produced in equal proportions. These four types of gametes join with the single type of gamete produced by the homozygous parent of the test-cross to produce four kinds of progeny in equal proportions (see Figure 7.5b). The progeny with new combinations of traits formed from recombinant gametes are termed recombinant progeny.

In summary, a testcross in which one of the plants is heterozygous for two completely linked genes yields two types of progeny, each type displaying one of the original combinations of traits present in the P generation. Independent assortment, in contrast, produces two types of

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