WAEC Biology 2012 Papers

Concept:A monohybrid cross between two heterozygotes under complete dominance produces a $3:1$ phenotypic ratio.Explanation:The parent cross is $Bb \times Bb$.Each parent produces gametes $B$ and $b$.The Punnett square gives the genotypes: $BB$, $Bb$, $Bb$, and $bb$.Since $B$ is completely dominant, both $BB$ and $Bb$ show the black phenotype.Only the homozygous recessive genotype $bb$ shows the white phenotype.Therefore, the offspring ratio is 3 black : 1 white.Answer:D. 3 blacks : 1 white