WAEC Biology 2012 Papers

Concept:Two heterozygous red plants crossed as $Rr \times Rr$ give a $3:1$ red-to-white phenotypic ratio.Explanation:Let $R$ represent the dominant red allele and $r$ represent the recessive white allele.Heterozygous red plants have the genotype $Rr$.When two heterozygous red plants are crossed, each parent produces two kinds of gametes: $R$ and $r$.The possible offspring genotypes are $RR$, $Rr$, $rR$, and $rr$.$RR$, $Rr$, and $rR$ all produce red flowers because the red allele is dominant.Only $rr$ produces white flowers.Thus, the expected phenotypic ratio is red:white $= 3:1$, which equals $75\%$ red and $25\%$ white.If two red-flowered plants produce this ratio, it shows that both plants are heterozygous.Answer:A. 75% red and 25% white