Question

The dominance pattern of a gene can be determined from the phenotypes of the parents and offspring. In the examples below, assume that each parent is homozygous for the specific allele and that the progeny are heterozygous. Classify each example as either complete dominance, incomplete dominance, or codominance.

a) A pea plant with all purple flowers and a pea plant with all white flowers produce a pea plant with all purple flowers.
b) A mother with type A blood and a father with type B blood have a daughter with type AB blood.
c) A red snapdragon and a white snapdragon produce a pink snapdragon.
d) A green fish and a yellow fish have mottled green and yellow offspring.
e) A mother with straight hair and a father with curly hair have a son with wavy hair.

Answer 1

a) Complete dominance (purple is dominant over white)

b) Codominance (both alleles A and B are expressed at the same time)

c) Incomplete dominance (blended heterozygous)

d) Codominance (both expressed)

e) Incomplete dominance (blended heterozygous)

Complete dominance refers to Mendelian inheritance; when two different alleles are in heterozygous only one hat is dominant to another will be expressed in phenotype.

Incomplete dominance is when the phenotype of a heterozygous organism is a blend between the phenotypes of two homozygous (heterozygous is an intermediate).

Codominance is another example of non-Mendelian inheritance: both alleles are simultaneously expressed in the heterozygote.