Question

A cytoplasmic male-sterile female plant with the restorer (nuclear) genotype rr is crossed to a male-fertile male plant with the genotype RR. Both RR and Rr can restore the fertility, whereas rr cannot. When an F1 female plant with Rr genotype was test-crossed to a male-fertile male plant with the rr genotype, the percentage of the population that is male fertile would be _______. (Answer in integer)

Hint:

In test crosses, the progeny ratio is always 1:1 if the parent is heterozygous. Fertility here depends directly on presence of at least one dominant R allele.

Answer 1

Step 1: Initial cross. Cytoplasmic male-sterile female (rr) $\times$ male (RR).
All F$_1$ = Rr (male fertile, since R allele restores fertility).
Step 2: Test cross. F$_1$ female (Rr) $\times$ male (rr).
Progeny genotypes: 50% Rr + 50% rr.
Step 3: Fertility. Rr (fertile), rr (sterile).
Thus, percentage of male fertile = 50%.