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I 7.5 A testcross reveals the effects of linkage. Results of a testcross for two loci in tomatoes that determine leaf type and plant height.

recombinant progeny and two types of nonrecombinant progeny in equal proportions.

Crossing Over with Linked Genes

Linkage is rarely complete — usually, there is some crossing over between linked genes (incomple linkage), producing new combinations of traits. Let's see how this occurs.

Theory The effect of crossing over on the inheritance of two linked genes is shown in 4 FIGURE 7.6. Crossing over, which takes place in prophase I of meiosis, is the exchange of genetic material between nonsister chromatids (see Figures 2.15 and 2.17). After a single crossover has taken place, the two chromatids that did not participate in crossing over are unchanged; gametes that receive these chromatids are

(a) No crossing over a

Homologous chromosomes pair In prophase I.

Homologous chromosomes pair In prophase I.

Meiosis II

.all resulting chromosomes In gametes have original allele | combinations and are nonrecombinants.

(b) Crossing over

| A crossover may occur In prophase I.

| A crossover may occur In prophase I.

Meiosis II

In this case, half of the resulting gametes will have unchanged chromosomes (nonrecombinants).

Nonrecombinant Recombinant Recombinant Nonrecombinant

I 7.6 Crossing over produces half nonrecombinant gametes and half recombinant gametes.

Nonrecombinant Recombinant Recombinant Nonrecombinant

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