Beyond simple dominance: multiple alleles, pleiotropy and polygenes

When dominance is not complete

Mendel's crosses were tidy because each trait had one dominant allele. Real traits are more varied. This lesson shows five patterns that a student of the Indian syllabus and of A-Level, IB and AP biology must know.

Incomplete dominance

In the snapdragon (dog flower, Antirrhinum), a cross between a red-flowered plant (RR) and a white-flowered plant (rr) gives pink flowers (Rr) in the F1. The heterozygote looks intermediate. This is incomplete dominance. One copy of the red allele makes only half the amount of pigment.

When the F1 pink plants are crossed, the F2 is 1 red : 2 pink : 1 white. The phenotypic ratio is the same as the genotypic ratio, 1 : 2 : 1. The alleles still segregate, and red and white come back unchanged. So incomplete dominance does not break the law of segregation. It shows that dominance depends on how a trait is measured. In peas, the allele for round seeds is dominant over the one for wrinkled seeds. If you look at the starch grains inside the seeds, the heterozygote has grains of an intermediate size, so the dominance is incomplete.

Codominance

In codominance, the heterozygote shows both alleles at once, each in full. The standard example is the human blood group AB.

Multiple alleles and the ABO blood groups

A gene can have more than two alleles in a population. Each person still has only two of them, one from each parent. The ABO blood groups are controlled by the gene I, which has three alleles: $I^A$, $I^B$ and i.

• $I^A$ makes a sugar (antigen) called A on the surface of the red blood cells. $I^B$ makes antigen B. The allele i makes no sugar.
• $I^A$ and $I^B$ are both dominant over i. A person with $I^A i$ has group A, and one with $I^B i$ has group B.
• $I^A$ and $I^B$ are codominant. A person with $I^A I^B$ has group AB, with both sugars.

Three alleles give six genotypes and four phenotypes: A, B, AB and O. A cross of $I^A i$ with $I^B i$ gives all four groups, each with a chance of one quarter. The blood groups help to rule out a father, but they can never prove that a man is the father.

Parents of group A and group B can have a child of any group. Two parents of group O can have only a child of group O.

Pleiotropy

A single gene can affect more than one trait. This is pleiotropy. The classic example is phenylketonuria (PKU). A mutation in the gene for one enzyme, phenylalanine hydroxylase, causes mental retardation, and also reduced pigment in the hair and the skin. One gene changes brain, hair and skin. The sickle-cell gene (the next course) is another example. This is different from a trait that depends on many genes.

Polygenic inheritance

Traits such as human height and skin colour are controlled by three or more genes. Each dominant allele adds a little to the trait, and the effects add up. This is polygenic inheritance.

Take skin colour with three genes, A, B and C. A person with AABBCC has the most pigment and the darkest skin. A person with aabbcc has the least pigment. A person with AaBbCc has an intermediate colour. What matters is the number of dominant alleles, from 0 to 6, and not which ones they are.

If two AaBbCc people have children, the offspring fall into seven classes. Each of the 6 alleles is a capital or a small letter with equal chance, so the numbers follow the binomial coefficients of 6.

The classes come in the ratio 1 : 6 : 15 : 20 : 15 : 6 : 1, and the shape is a bell. Most children are medium, and few are very light or very dark. The environment also changes a polygenic trait, so a person's height depends on food and health as well as on genes.

Try it in Python

The program counts how the dominant alleles of a polygenic trait are shared in the offspring, and the blood groups of a family.

Practise with new numbers

Common mistakes

• Calling the pink snapdragon a blend. The alleles do not blend. Red and white come back unchanged in the F2.
• Mixing up codominance and incomplete dominance. In codominance both alleles show in full (group AB). In incomplete dominance the heterozygote is in between (pink).
• Saying that a person has three ABO alleles. A population has three alleles, and a person has only two.
• Confusing pleiotropy and polygenic inheritance. Pleiotropy is one gene with many effects. Polygenic inheritance is many genes for one trait.

Remember these

• Incomplete dominance: the F1 is intermediate, and the F2 ratio is 1 : 2 : 1 for the phenotype too.
• Codominance: both alleles show. The ABO gene has the alleles $I^A$, $I^B$ and i, six genotypes and four phenotypes.
• Pleiotropy: one gene, several traits. Polygenic inheritance: several genes, one trait, with a bell-shaped spread.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A red-flowered snapdragon is crossed with a pink-flowered one. What are the phenotypes of the offspring, and in what ratio?

2. A man of blood group A and a woman of blood group B have a child of group O. What are the genotypes of the parents?

3. How many genotypes and how many phenotypes are there for the ABO blood groups?

4. Name the pattern: a person with the genotype $I^A I^B$ shows both antigens A and B on the red cells.

5. A trait is controlled by two genes, and each dominant allele adds one unit. Two plants with the genotype AaBb are crossed. What is the ratio of the classes with 0, 1, 2, 3 and 4 dominant alleles?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 4, sections 4.2 and 4.3.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 12.3 (incomplete dominance, codominance, multiple alleles).