Q.64 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)
The test cross of an F1 female plant with Rr genotype to a male-fertile rr male plant produces 50% male-fertile progeny due to segregation of the dominant restorer allele R. Thus, the percentage of the male-fertile population is 50%. This aligns with standard cytoplasmic male sterility (CMS) restoration genetics where fertility requires at least one R allele.
Cross Analysis
A cytoplasmic male-sterile female (CMS cytoplasm, nuclear rr) crossed to male-fertile RR male yields F1 progeny with CMS cytoplasm (maternally inherited) and nuclear Rr genotype. All F1 plants are male-fertile since R is dominant over r for restoration.
The F1 female (Rr, fertile) test-crossed to rr male (fertile, as CMS cytoplasm is absent) produces progeny with CMS cytoplasm from the female parent. Nuclear genotypes segregate 1:1 (Rr : rr) via meiosis in the Rr female.
Fertility Determination
-
Rr progeny: Male-fertile (presence of dominant R restores pollen fertility despite CMS cytoplasm).
-
rr progeny: Male-sterile (no restorer, CMS cytoplasm causes sterility).
This yields 50% male-fertile population, as confirmed by testcross ratios in CMS systems.
Punnett Square
| Female Gametes (Rr) | Male Gametes (rr) |
|---|---|
| R (50%) | Rr (fertile) |
| r (50%) | rr (sterile) |
Answer
50
Cytoplasmic male sterility (CMS) restorer genetics problems test understanding of nuclear-cytoplasmic interactions in plant breeding, key for CSIR NET Life Sciences. This detailed breakdown solves the cytoplasmic male sterile rr x RR cross, F1 Rr female testcross to rr, revealing exactly 50% male fertile population through dominant R allele segregation.
