WAEC Biology 2020 Papers

Concept:This is a dihybrid cross between two pure-breeding parents that differ in two traits, so the offspring inherit a heterozygous genotype for both traits.Explanation:Plant A is homozygous dominant, with genotype $\mathrm{YYRR}$.Plant B is homozygous recessive, with genotype $\mathrm{yyrr}$.During gamete formation, $\mathrm{YYRR}$ produces only one type of gamete, carrying $\mathrm{YR}$.Similarly, $\mathrm{yyrr}$ produces only one type of gamete, carrying $\mathrm{yr}$.When these gametes fuse, each offspring receives one dominant allele and one recessive allele for each trait.Therefore, the genotype of every offspring is $\mathrm{YyRr}$.All the offspring are uniform because the dominant alleles mask the expression of the recessive alleles.Answer:B. $\mathrm{YyRr}$