Question

Which of the following is an example of codominance?

• A. 4 o’ clock plant
• B. Height of pea plants
• C. AB blood group
• D. Skin colour in humans

Answer 1

The correct option is C

AB blood group

A gene may have more than two alleles in a population, and most genes possess several different alleles. Often, no single allele is dominant; instead, each allele has its own effect, and the alleles are considered codominant. A human gene with codominant alleles is the gene that determines ABO blood type. This gene encodes an enzyme that adds sugar molecules to lipids on the surface of red blood cells. The gene that encodes the enzyme, designated I, has three common alleles: $I^B$, whose product adds galactose; $I^A$, whose product adds galactosamine; and the recessive allele i (also designated as $I^O$), which codes for a protein that does not add any sugar. Different combinations of the three I gene alleles occur in different individuals because each person possesses two copies of the chromosome bearing the I gene and may be homozygous or heterozygous. A heterozygous individual having both $I^A$ and $I^B$ alleles produces both forms of the enzyme and adds both galactose and galactosamine to the surfaces of red blood cells. Because both alleles are expressed simultaneously in heterozygotes, the $I^A$ and $I^B$ alleles are codominant. Both $I^A$ and $I^B$ are dominant over the i allele because both $I^A$ or $I^B$ alleles lead to sugar addition and the i allele does not add sugar.