In a cross between two individuals heterozygous for three autosomal, independently assorting loci (i. e., A/a; B/b; E/e), what is the probability of getting an offspring whose phenotype differs from both parents?

1. A plant of genotype

is testcrossed to

If the two loci are 10 m. u. apart, what proportion of progeny will be A B/a b?

45 percent of the progeny will be A B/a b

2. The A locus and the D locus are so tightly linked that no recombination is ever observed between them. If A d/A d is crossed to a D/a D, and the F1 is intercrossed, what phenotypes will be seen in the F2 and in what proportions?

P A d/A d * a D/a D

F1 A d/a D

F2 1 A d/A d

2 A d/a D

1 a D/a D

3. The R and S loci are 35 m. u. apart. If a plant of genotype

is selfed, what progeny phenotypes will be seen and in what proportions?

4. The cross E/E · F/F * e/e · f/f is made, and the F1 is then backcrossed to the recessive parent. The progeny genotypes are inferred from the phenotypes. The progeny genotypes, written as the gametic contributions of the heterozygous parent, are in the following proportions:

Explain these results.

The two genes are 33.3 map units (m. u.) apart.

5. A strain of Neurospora with the genotype H · I is crossed with a strain with the genotype h · i. Half the progeny are H · I, and half are h · i. Explain how this is possible.

Because only parental types were recovered, the two genes must be quite close to each other, making recombination quite rare.