Question

In snapdragon (Antirrhinum majus) a plant with red flowers was crossed with a plant with white flowers. Work out all the possible genotypes and phenotypes of F₁ and F₂ generations. Comment on the pattern of inheritance in this case.

Answer 1

In snapdragon (Antirrhinum majus), a cross between a plant with red flowers (genotype RR) and a plant with white flowers (genotype rr) results in the F₁ generation having all pink flowers with the genotype Rr. This shows that neither allele is completely dominant; instead, the heterozygous condition produces an intermediate phenotype (pink), which is a case of incomplete dominance.

For the F₂ generation, when the F₁ plants (Rr) are self-crossed, the possible genotypes and phenotypes are:

• Genotypes: RR, Rr, rr
• Phenotypes: Red flower (RR), pink flower (Rr), white flower (rr)

The phenotypic ratio in the F₂ generation is 1 red : 2 pink : 1 white.

Comment on inheritance pattern: This pattern does not follow Mendel's classical dominance where one allele is completely dominant over the other, resulting in a 3:1 ratio of dominant to recessive phenotypes. Instead, incomplete dominance occurs where the heterozygotes show an intermediate phenotype. The alleles show partial expression, and both contribute to the phenotype.