This cross will produce different combinations of traits in the progeny, but the only way that colorless and waxy progeny can arise is through crossing over in the doubly heterozygous parent:

Crossing over

Crossing over

Colored, starchy progeny

Colorless, waxy progeny

Notice that, if crossing over entails physical exchange between the chromosomes, then the colorless, waxy progeny resulting from recombination should have a chromosome with an extra piece, but not a knob. Furthermore, some of the colored, starchy progeny should possess a knob but not the extra piece. This outcome is precisely what Creighton and McClintock observed, confirming the chromosomal theory of inheritance. Curt Stern provided a similar demonstration by using chromosomal markers in Drosophila at about the same time. We will examine the molecular basis of recombination in more detail in Chapter 12.

Predicting the Outcomes of Crosses with Linked Genes

Knowing the arrangement of alleles on a chromosome allows us to predict the types of progeny that will result from a cross entailing linked genes and to determine which of these types will be the most numerous. Determining the proportions of the types of offspring requires an additional piece of information — the recombination frequency. The recombination frequency provides us with information about how often the alleles in the gametes appear in new combinations and allows us to predict the proportions of offspring phenotypes that will result from a specific cross entailing linked genes.

In cucumbers, smooth fruit (t) is recessive to warty fruit (T) and glossy fruit (d) is recessive to dull fruit (D). Geneticists have determined that these two genes exhibit a recombination frequency of 16%. Suppose we cross a plant homozygous for warty and dull fruit with a plant homozygous for smooth and glossy fruit and then carry out a testcross by using the F1

What types and proportions of progeny will result from this testcross?

Colored, starchy progeny

Colorless, waxy progeny

Four types of gametes will be produced by the heterozygous parent, as shown in ( FIGURE 7.10): two types of nonrecombinant gametes ( T_D^ and _t_dj and two types of recombinant gametes ( T_^ and

_t_D_). The recombination frequency tells us that

16% of the gametes produced by the heterozygous parent will be recombinants. Because there are two types of recombinant gametes, each should arise with a frequency of 16%/2 = 8%. All the other gametes will be nonrecombi-nants; so they should arise with a frequency of 100% — 16% = 84%. Because there are two types of nonrecom-binant gametes, each should arise with a frequency of 84%/2 = 42%. The other parent in the testcross is homozy-gous and therefore produces only a single type of gamete (t_d) with a probability of 1.00.

The progeny of the cross result from the union of two gametes, producing four types of progeny (see Figure 7.10). The expected proportion of each type can be determined by using the multiplication rule, multiplying together the probability of each uniting gamete. Testcross progeny with warty and dull fruit

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