WAEC Biology 2010 Papers

Concept:Crossing a heterozygous dominant parent with a double recessive parent is a test cross, which gives a $1 : 1$ phenotypic ratio.Explanation:Let the heterozygous dominant parent be $Tt$.Let the double recessive parent be $tt$.The possible gametes from $Tt$ are $T$ and $t$.The possible gametes from $tt$ are only $t$.When these gametes combine, the offspring formed are $Tt$ and $tt$ in equal proportion.Therefore, half the offspring show the dominant trait, and half show the recessive trait.So, the phenotypic ratio is $1$ dominant : $1$ recessive, which is $1 : 1$.Answer:B. $1 : 1$