Beyond simple dominance

MDCAT Biology · Biology · Inheritance. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Not every gene follows Mendel's simple pattern. In incomplete dominance the heterozygote is in between the two parents, as in pink four o'clock flowers. In codominance both alleles show fully and separately, as in blood group AB.

A gene may have more than two alleles in the population (multiple alleles), although each person carries only two. The ABO gene has three: I^A, I^B and i.

Some characters are controlled by several genes together. In polygenic inheritance their effects add up and give continuous variation, such as skin colour. In epistasis one gene hides the effect of another gene.

Rules to remember

Incomplete dominance, heterozygote × heterozygote: 1 : 2 : 1 for both phenotype and genotype.
ABO: I^A and I^B are codominant and both are dominant to i. Group A = I^AI^A or I^Ai; B = I^BI^B or I^Bi; AB = I^AI^B; O = ii.
Three alleles give 6 genotypes and 4 phenotypes.
Group O has no A or B antigens (universal donor of red cells); group AB has no anti-A or anti-B antibodies (universal recipient).
Epistasis changes 9 : 3 : 3 : 1 to 9 : 3 : 4 (recessive epistasis) or 12 : 3 : 1 (dominant epistasis).
Rh factor: Rh-positive (D) is dominant to Rh-negative (d).
Haemolytic disease of the newborn: Rh-negative mother, Rh-positive foetus, usually in a second or later such pregnancy.

Common mistake

Dominance is between alleles of the same gene; epistasis is between different genes. Do not call epistasis a kind of dominance.

Worked example

A man of blood group AB marries a woman of blood group O. Which blood groups can their children have, and in what proportions?

Genotypes: man I^AI^B, woman ii.
His gametes: I^A or I^B (half each). Her gametes: all i.
Offspring: I^Ai (group A) and I^Bi (group B), in equal numbers.

Answer: Half group A and half group B. No child can be AB or O, the groups of the parents.

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