Question

A gene showing codominance has
A. Both alleles are independently expressed in the heterozygote
B. One allele dominant on the other
C. Alleles tightly linked on one another
D. Alleles that are recessive to each other

Answer 1

Hint: When one allele is completely dominant over the other, it is called dominant allele and the other that is suppressed is called recessive allele. Blood group types are the best example to study codominance in humans.

Complete answer: Co-dominance is a type of expression in which two alleles independently and simultaneously express themselves without one being dominant or recessive to the other in heterozygous condition. This can be seen in ABO blood grouping. Alleles for ABO blood group are I^A, I^B and i. When allele I^A and I^B are together, both express themselves leading to not A, not B but AB blood group. In the case of codominance, two alleles of a gene pair independently express themselves in a heterozygote. Codominance is very easy to spot in plants and animals that have more than one pigment colour. Spotted cows and flowers with petals of two different colours are examples of codominance. Another example of codominance can be seen in Rhododendron, where the crossing of red flowers with white flowers give rise to flowers having both white and red patches.
Hence, the correct answer is option A.

Note: Codominance is different from incomplete dominance. In case of incomplete dominance, both the alleles are not able to express themselves fully and intermediate is formed. Incomplete dominance can be well studied in snapdragon; where a homozygous white flower and a homozygous red flower plant produces a plant with pink flowers.