Biology 30 Genetics Practice Test

What is incomplete dominance? Give the genotype-phenotype example.

One allele completely masks the other in heterozygotes. Heterozygotes show an intermediate phenotype; example: RR red, rr white, Rr pink. Alleles do not affect phenotype. Heterozygotes show both traits simultaneously; example AB blood type (IA IB).

Correct Answer:

Heterozygotes show an intermediate phenotype; example: RR red, rr white, Rr pink.

Explanation:

Incomplete dominance is when the heterozygous genotype produces a phenotype that is intermediate between the two homozygous phenotypes because neither allele fully masks the other. An example uses color: red is produced by one allele, white by the other, so RR plants are red, rr plants are white, and Rr plants are pink. This shows a blending effect where the heterozygote sits between the two extremes. This differs from complete dominance, where the heterozygote looks like one of the homozygotes, and from codominance, where the heterozygote displays both traits at once (as with AB blood type having both A and B antigens).

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