Question

In Antirrhinum (dog flower), phenotypic ratio in $F_2$ generation for the inheritance of flower colour would be

• A. $3 : 1$
• B. $1 : 2: 1$
• C. $1 : 1$
• D. $2 : 1$

Answer 1

Answer: (B)

$1 : 2: 1$

Answer 2

The inheritance of flower colour in the Antirrhinum majus (snapdragon or dog flower) is an example of incomplete or partial dominance. Incomplete dominance is the phenomenon in which neither of the two alleles of a gene is completely dominant over the other. In a cross between true-breeding redflowered $(RR)$ and true-breeding white-flowered plants $(rr)$, the $F_1$ plants obtained were pink $(Rr)$ coloured. When the $F_1$ plants were self-pollinated, the $F_2$ generation resulted in the ratio, $1 \,(RR)$ Red : $2\, (Rr)$ Pink : $1 \,(rr)$ White. The phenotypic ratios had changed from the normal $3 : 1$ dominant : recessive ratio to $1 :2 : 1$. $R$ was not completely dominant over $r$ and this made it possible to distinguish $Rr$ (pink) from $RR$ (red) and $rr$ (white).