Non-Mendelian Genetics

AP Biology· difficulty 3/5

Snapdragons crossed (Rr x Rr) produced 100 offspring with phenotypes: 28 red, 50 pink, 22 white. Expected ratio for incomplete dominance is 1:2:1.

Phenotype Count Red 28 Pink 50 White 22

The data are consistent with which inheritance pattern?

  • A

    Sex-linked inheritance.

  • B

    Codominance: each allele contributes a distinct phenotype.

  • C

    Complete dominance with a 3:1 ratio.

  • D

    Incomplete dominance: Rr heterozygotes show an intermediate (pink) phenotype.

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Explanation

The 1:2:1 phenotypic ratio (and the appearance of an intermediate phenotype in heterozygotes) is the signature of incomplete dominance.

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