Introduction

Cytoplasmic male sterility (CMS) is a maternally inherited trait caused by mutations in the mitochondrial genome, resulting in plants that produce non-functional pollen. This sterility can be reversed by nuclear “restorer of fertility” (Rf) genes, usually dominant, which suppress the sterility phenotype. This interaction between mitochondrial CMS genes and nuclear Rf genes leads to distinctive inheritance patterns crucial for plant breeding and hybrid seed production.

Explanation of Each Option

Option 1: All Male Sterile Progeny

This would mean no progeny restore fertility. However, since the fertile pollen parent has a dominant restorer gene (Rf) in heterozygous form, some progeny should inherit the Rf allele and restore fertility. Therefore, this option is incorrect.

Option 2: All Fertile Progeny

For all progeny to be fertile, every offspring must inherit at least one Rf allele from the pollen parent. Since the pollen parent is Rf/rf heterozygous, only about half the gametes carry Rf, so this is unlikely. Also, CMS is maternally inherited; all progeny inherit the sterile cytoplasm, so without the restorer gene, sterility would persist. Thus, not all will be fertile.

Option 3: 50% Fertile and 50% Male Sterile Progeny

This is correct. The female contributes sterile cytoplasm causing CMS in all progenies. The pollen parent is heterozygous for the restorer gene (Rf/rf) and transmits either Rf or rf with equal probability. Progeny with Rf will restore fertility (dominant trait), and those with rf remain male sterile. Hence, 50% fertile and 50% sterile.

Option 4: 75% Fertile and 25% Male Sterile Progeny

This ratio applies to dominant nuclear traits inherited from both parents but here, the CMS trait is cytoplasmic and only maternally inherited. The pollen parent provides the Rf allele heterozygously. Thus, Mendelian 3:1 ratios do not apply; fertililty restoration depends solely on Rf allele presence from one parent. Therefore, 75:25 segregation does not fit here.

Conclusion

The correct progeny ratio after a male sterile pea plant (with CMS cytoplasm) is pollinated by a heterozygous fertile plant with the restorer gene Rf is 50% fertile and 50% male sterile progeny.

Correct answer: (3) 50% of the progeny fertile and 50% male sterile.