Non-Mendelian Genetics: Incomplete Dominance and Codominance

Non-Mendelian inheritance covers patterns in which traits do not follow simple dominant and recessive rules, such as incomplete dominance (a blended phenotype), codominance (both alleles fully expressed) and multiple alleles (more than two alleles in a population). In brief: in snapdragons, red × white flowers give pink in incomplete dominance. In human ABO blood groups, A and B are codominant, so a person with genotype I^AI^B has blood group AB. The ABO gene has three alleles, but each person has only two.

These patterns extend Mendel’s laws rather than contradicting them: alleles are still inherited as separate units, but the way they are expressed differs. This guide explains incomplete dominance, codominance and multiple alleles, works through the ABO blood group system in detail, compares the patterns in a table and gives worked crosses.

Why do some traits not follow simple dominance?

Mendel’s pea traits had two alleles, one completely dominant over the other. Many genes behave differently. The protein made by the allele can have partial activity, two different alleles can each make a working protein, or a population can have many different alleles. The underlying inheritance is still particulate: alleles do not blend or change in the offspring. They can reappear unchanged in later generations.

What is incomplete dominance?

In incomplete dominance, the heterozygote has a phenotype that is intermediate between the two homozygous phenotypes. Neither allele is fully dominant.

The classic example is flower color in snapdragons (Antirrhinum) and four-o’clock plants. Plants with two alleles for red (C^RC^R) have red flowers, those with two alleles for white (C^WC^W) have white flowers, and heterozygotes (C^RC^W) have pink flowers. The red allele makes a pigment, and one copy makes half as much as two copies, so the flower is lighter. Another example in humans is hair texture: a person who has one allele for curly hair and one for straight hair may have wavy hair.

For incomplete dominance, the genotypic and phenotypic ratios are the same. A cross of two pink plants gives 1 red : 2 pink : 1 white, and the 1:2:1 ratio replaces Mendel’s 3:1.

What is codominance?

In codominance, both alleles are fully expressed in the heterozygote, and the phenotype shows both traits at the same time, not a blend. For example, a cow with a red allele and a white allele in some breeds has a roan coat with both red and white hairs. In humans, the AB blood group and the sickle-cell trait (in some definitions) are examples. A person with sickle-cell trait makes both normal and sickle hemoglobin in their red blood cells.

The difference from incomplete dominance is that in codominance the two products are both present and separate. In incomplete dominance, the result is an intermediate level of one product.

What are multiple alleles?

A gene can have more than two alleles in the population, even though each diploid individual carries only two. The number of possible genotypes then grows: with n alleles, there are n(n + 1) ÷ 2 genotypes. For three alleles, that is 3 × 4 ÷ 2 = 6. Examples include the ABO blood group genes in humans, coat color genes in rabbits and the many alleles of the genes that determine self-incompatibility in plants.

How does the ABO blood group system work?

The ABO gene codes for an enzyme that adds sugar groups to proteins on the surface of red blood cells. It has three main alleles:

• I^A: codes for the enzyme that adds sugar A. Cells carry antigen A.
• I^B: codes for the enzyme that adds sugar B. Cells carry antigen B.
• i: codes for a non-functioning enzyme. Cells carry no A or B antigen.

I^A and I^B are codominant with each other, and both are dominant over i. This produces four blood groups from six genotypes.

Antibodies matter in transfusions. A person’s plasma contains antibodies against the antigens they lack, so transfusing blood with the wrong antigens causes the donated red cells to clump (agglutinate), which can be fatal. Group O red cells have no A or B antigens and are safe for most patients, so O is called the universal red-cell donor, and AB patients can receive red cells from all ABO groups. The separate Rhesus (Rh) system, with a dominant Rh-positive allele and a recessive Rh-negative allele, is inherited independently.

Patterns of dominance compared

Worked example: crosses with pink flowers and blood groups

Example 1: pink snapdragons

Cross a red snapdragon (C^RC^R) with a white one (C^WC^W), then cross two of the pink offspring.

1. Red × white: all offspring are C^RC^W, so all are pink.
2. Pink × pink: the Punnett square gives 1 C^RC^R : 2 C^RC^W : 1 C^WC^W. The phenotypes are 25 percent red, 50 percent pink and 25 percent white.

Example 2: blood groups of a child

A mother has blood group A, and the father has blood group B. Their first child has blood group O. What are the parents’ genotypes, and what other blood groups could their children have?

1. The child is ii, so each parent must have given an i allele.
2. The mother is group A and has an i, so she is I^Ai. The father is group B and has an i, so he is I^Bi.
3. Cross I^Ai × I^Bi: the gametes are I^A or i from the mother and I^B or i from the father.
4. The offspring are I^AI^B (group AB), I^Ai (A), I^Bi (B) and ii (O), each with a 25 percent probability.

All four blood groups are therefore possible, each with a 1 in 4 chance for every child. This type of reasoning is used in genetics teaching and, together with DNA tests, in some paternity cases, where blood groups can show that a man cannot be the father but never prove that he is.

How was blood group inheritance discovered?

Karl Landsteiner discovered the ABO groups in 1901 by mixing blood from different people and noting which samples clumped, and he received the Nobel Prize in 1930. Felix Bernstein showed in 1924 that the groups are controlled by one gene with three alleles. The Rh factor was identified in 1940 by Landsteiner and Alexander Wiener in work with rhesus monkeys. Incomplete dominance was first described by Carl Correns around 1900 in four-o’clock plants.

Which mistakes are common?

Mistake 1: Describing incomplete dominance as blending of genes

The alleles stay intact and separate. A red and a white flower can reappear in the next generation.

Mistake 2: Mixing up incomplete dominance and codominance

In incomplete dominance, the phenotype is a mix (pink). In codominance, both phenotypes appear together (red and white patches).

Mistake 3: Saying a person can carry three ABO alleles

A population has three alleles, but a diploid person has only two.

Mistake 4: Assuming that O is dominant

The O allele (i) is recessive. A person with group O is ii.