IMAT Biology · Genetics and Inheritance

Incomplete Dominance, Codominance, Blood Groups and Polygenes

Not every gene has one dominant and one recessive allele: alleles can blend in the phenotype, both show, come in sets of three or more, or add up across many genes.

All Genetics and Inheritance notes

Why this matters

No past IMAT question has yet been set on this page, but all of it is on the syllabus and in every school course. Blood groups in particular are a natural target for a short cross or a fact about antigens and antibodies.

Concept 1 of 3: Incomplete dominance and codominance

With complete dominance, the heterozygote looks like the dominant homozygote. Sometimes it does not. If one dose of the pigment allele makes only half the pigment, the heterozygote is an intermediate colour. If both alleles make a product and both products show, the heterozygote shows both traits at once. Either way the heterozygote has its own phenotype, so the genotype ratio and the phenotype ratio become the same.

Definition

Incomplete dominance: the heterozygote has an intermediate phenotype. Red snapdragons $C^R C^R$ crossed with white $C^W C^W$ give pink $C^R C^W$ .
Codominance: both alleles are fully expressed side by side. Roan cattle have red and white hairs mixed; blood group AB has both A and B antigens.
In both cases a cross of two heterozygotes gives 1 : 2 : 1 for genotypes and phenotypes alike.
The alleles do not blend permanently: crossing two pink plants brings back red and white. Inheritance is still by separate units.

Heterozygote cross without dominance

C^R C^W \times C^R C^W \rightarrow 1\ \text{red} : 2\ \text{intermediate} : 1\ \text{white}

$C^R, C^W$ two alleles, neither dominant (superscripts are used instead of capital and small letters)

Worked example

In snapdragons, flower colour shows incomplete dominance:

C^R C^R

is red,

C^R C^W

pink,

C^W C^W

white. Two pink plants are crossed and 80 seedlings flower. How many of each colour are expected? What ratio would a pink plant crossed with a white plant give?

Practice this conceptself-check · 3 quick reps

Incomplete dominance is not blending inheritance

A pink snapdragon looks like a blend, but its alleles stay separate: two pink parents produce red and white offspring again. Blending inheritance (the idea that parental traits mix irreversibly) was disproved by Mendel's work.

Concept 2 of 3: Multiple alleles: the ABO and Rh blood groups

A gene can have more than two alleles in a population, even though each person carries only two. The ABO gene has three:

I^A

and

I^B

each add a different sugar to a surface molecule on red cells (antigen A or B), and

i

adds nothing.

I^A

and

I^B

are codominant, and both are dominant to

i

. The plasma carries antibodies against whichever antigen the body lacks.

Definition

Multiple alleles: three or more alleles of one gene exist in the population; any one person has at most two.
ABO: $I^A$ and $I^B$ are codominant; both are dominant to $i$ . Four phenotypes, six genotypes.
A person makes antibodies against the ABO antigens they lack. Group O red cells carry no A or B antigen (useful for transfusion); group AB plasma carries no anti-A or anti-B.
Rh (rhesus): the D antigen; the allele for it ( $D$ ) is dominant to $d$ . Rh negative ( $dd$ ) people make anti-D only after exposure to Rh positive blood.
Haemolytic disease of the newborn: an Rh negative mother carrying an Rh positive fetus can be sensitised at the first birth; in a later Rh positive pregnancy her anti-D crosses the placenta and destroys fetal red cells. An anti-D injection after birth prevents sensitisation.

Blood group	Genotypes	Antigen on red cells	Antibodies in plasma
A	$I^A I^A$ or $I^A i$	A	Anti-B
B	$I^B I^B$ or $I^B i$	B	Anti-A
AB	$I^A I^B$	A and B	Neither anti-A nor anti-B
O	$i\,i$	Neither A nor B	Anti-A and anti-B
Rh negative	$dd$	No D antigen	Anti-D only after exposure to D

Rh positive is

DD

Dd

, with the D antigen and no anti-D.

Practice this conceptself-check · 4 quick reps

An AB parent cannot have a group O child

A group AB parent passes on either

I^A

I^B

, never

i

, so every child has at least one antigen. A family with an AB parent and an O child points to a wrong parentage claim (or a rare mutation), not to normal inheritance.

Antibodies are against the antigens you lack

Group A people have anti-B, not anti-A. Mixing these up reverses every transfusion answer. Group O red cells can go to all ABO groups because they carry no A or B antigen; group AB people can receive all groups because their plasma has neither antibody.

Concept 3 of 3: Polygenic inheritance and continuous variation

Height and skin colour do not fall into a few neat classes. They are controlled by many genes, each adding a small amount, plus the environment (food, sunlight). With many small steps the classes run into each other and the population forms a bell-shaped curve.

Definition

Polygenic inheritance: one trait controlled by several genes, often each with an additive allele that adds the same small amount.
It gives continuous variation (height, mass, skin colour), usually a normal distribution, and the environment adds further spread.
A single gene with few alleles gives discontinuous variation: distinct classes, such as ABO blood groups.
With $n$ genes of two additive alleles each, there are $2n + 1$ phenotype classes (from 0 to $2n$ contributing alleles).

Phenotype classes for n additive genes

\text{classes} = 2n + 1

$n$ number of genes, each with one contributing and one non-contributing allele

Worked example

A simple model of skin colour uses three genes (A, B, C), each with one allele that adds pigment (capital letter) and one that adds none. Two

AaBbCc

parents have children. How many shades are possible, and what fraction of the children have all six pigment alleles?

Practice this conceptself-check · 4 quick reps

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Incomplete dominance and codominance
Heterozygote cross without dominance
$C^R C^W \times C^R C^W \rightarrow 1\ \text{red} : 2\ \text{intermediate} : 1\ \text{white}$
Polygenic inheritance and continuous variation
Phenotype classes for n additive genes
$\text{classes} = 2n + 1$

Reference tables (1)

Multiple alleles: the ABO and Rh blood groups5 rows

Blood group	Genotypes	Antigen on red cells	Antibodies in plasma
A	$I^A I^A$ or $I^A i$	A	Anti-B
B	$I^B I^B$ or $I^B i$	B	Anti-A
AB	$I^A I^B$	A and B	Neither anti-A nor anti-B
O	$i\,i$	Neither A nor B	Anti-A and anti-B
Rh negative	$dd$	No D antigen	Anti-D only after exposure to D

Rh positive is

DD

Dd

, with the D antigen and no anti-D.

Watch out for (3)

Incomplete dominance is not blending inheritance
→ Incomplete dominance and codominance
An AB parent cannot have a group O child
→ Multiple alleles: the ABO and Rh blood groups
Antibodies are against the antigens you lack
→ Multiple alleles: the ABO and Rh blood groups

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