The results of this testcross are diagrammed in I FIGURE 7.5a. During gamete formation, the heterozygote produces two types of gametes: some with the M_chromosome crossing over occurs, these gametes are the only types produced by the heterozygote. Notice that these gametes contain only combinations of alleles that were present in the original parents: either the allele for normal leaves together with the allele for tall height (M and D) or the allele for mottled leaves together with the allele for dwarf height (m and d). Gametes that contain only original combinations of alleles present in the parents are nonrecombinant gametes, or parental gametes.

The homozygous parent in the testcross produces only one type of gamete; it contains chromosome _m_d_

and pairs with one of the two gametes generated by the heterozygous parent (see Figure 7.5a). Two types of progeny result: half have normal leaves and are tall:

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