Brassica juncea has bisexual flowers. A mutation in the mitochondria leads to cytoplasmic male sterility (CMS). CMS can be restored by a restorer of fertility gene (Rf) which is a nuclear gene. Fertility restoration is a dominant phenotype. A CMS line is crossed to a homozygous Rf line. The obtained F1 progeny is self-pollinated. What percentage of F2 progeny will be male sterile?

Correct option is B

CMS is caused by a mitochondrial mutation, and the restoration of fertility is controlled by a dominant nuclear gene Rf. When a CMS line (which is homozygous for the sterility trait, and let’s assume its genotype is CMS CMS) is crossed with a homozygous Rf Rf line, the F1 progeny will inherit one copy of the Rf gene from the father (since the Rf gene is nuclear) and will show restored fertility. The F1 generation will all be CMS Rf and fertile because Rf is dominant.

Now, when these CMS Rf F1 plants are self-pollinated, the resulting F2 progeny will inherit one of two possible combinations: CMS or Rf. The ratio will be a Mendelian 1:1, with half of the progeny inheriting the Rf gene (fertile) and half inheriting the mitochondrial CMS trait without the dominant Rf allele (male sterile).

Hence, 25% of the F2 progeny will be male sterile.