Genetics and Gene Technology

Lessons

Senior 3 · Biology

Genetics and Gene Technology

5 lessons

Lesson 3 of 5

Incomplete dominance, co-dominance and blood groups

25 min

Neither red nor white

The four o'clock plant, Mirabilis jalapa, is a garden flower that opens in the late afternoon. When a pure-breeding red-flowered plant is crossed with a pure-breeding white one, the F1 flowers are neither red nor white: they are all pink. It looks as if the two colours have mixed like paint. But cross two pink plants and the F2 contains red, pink and white flowers. Paint, once mixed, cannot be unmixed. So the red and white alleles were never blended; they were passed on whole, and only their effect in the heterozygote was in between.

In Lesson 2 one allele completely masked the other. In this lesson neither allele is completely dominant, and the heterozygote has a phenotype of its own.

Definition

Incomplete dominance (partial dominance): neither allele is completely dominant, so the heterozygote has a phenotype in between the two homozygotes. Example: pink four o'clock flowers from red and white.

Co-dominance: both alleles are fully expressed in the heterozygote, so both phenotypes appear together, side by side. Example: roan cattle, with red hairs and white hairs mixed on one coat.

Multiple alleles: three or more alleles of one gene exist in a population, although each diploid organism carries only two of them. Example: the alleles $\mathrm{I^A}$ , $\mathrm{I^B}$ and $\mathrm{i}$ of the ABO blood group gene.

Writing the alleles

When neither allele is dominant, a capital and small letter would be misleading. Instead, use one capital letter for the gene and a superscript for each allele. For flower colour, use C for the gene, $\mathrm{C^R}$ for the red allele and $\mathrm{C^W}$ for the white allele.

Genotype	Phenotype (incomplete dominance)
$\mathrm{C^R C^R}$	red
$\mathrm{C^R C^W}$	pink
$\mathrm{C^W C^W}$	white

Each genotype has its own phenotype, so here you can read the genotype from the flower.

Worked example

Worked example 1: incomplete dominance. A red four o'clock ( $\mathrm{C^R C^R}$ ) is crossed with a white one ( $\mathrm{C^W C^W}$ ). Give the F1, then the F2 from crossing two F1 plants.

P gametes: all $\mathrm{C^R}$ from the red parent and all $\mathrm{C^W}$ from the white parent. F1: all $\mathrm{C^R C^W}$ , all pink.
F1 × F1: $\mathrm{C^R C^W} \times \mathrm{C^R C^W}$ . Each parent makes $\mathrm{C^R}$ and $\mathrm{C^W}$ gametes in equal numbers.

F2: 1 red : 2 pink : 1 white. With incomplete dominance the phenotype ratio is the same as the genotype ratio, 1 : 2 : 1, instead of 3 : 1.

Co-dominance: both at once

In Shorthorn cattle, a coat-colour gene has an allele for red hair, $\mathrm{C^R}$ , and an allele for white hair, $\mathrm{C^W}$ . A $\mathrm{C^R C^W}$ animal is not pink. Its coat has red hairs and white hairs growing side by side, which looks reddish-grey from a distance and is called roan. Both alleles are fully expressed, each in its own hairs: that is co-dominance.

	Incomplete dominance	Co-dominance
Heterozygote	one phenotype in between the two	both phenotypes, side by side
Example	pink four o'clock flower	roan coat: red and white hairs; blood group AB
F2 ratio from heterozygote × heterozygote	1 : 2 : 1	1 : 2 : 1

The ABO blood groups: co-dominance and multiple alleles

In Unit 7 you met the four blood groups, A, B, AB and O, set by antigens A and B on the red blood cells. One gene, called I, decides them, and it has three alleles:

$\mathrm{I^A}$ makes antigen A; $\mathrm{I^B}$ makes antigen B. These two are co-dominant: a person with both makes both antigens and is group AB.
$\mathrm{i}$ makes neither antigen. It is recessive to both $\mathrm{I^A}$ and $\mathrm{I^B}$ .

Blood group (phenotype)	Possible genotypes	Antigens on red cells
A	$\mathrm{I^A I^A}$ or $\mathrm{I^A i}$	A
B	$\mathrm{I^B I^B}$ or $\mathrm{I^B i}$	B
AB	$\mathrm{I^A I^B}$	A and B
O	$\mathrm{i\,i}$	neither

Three alleles give six genotypes but only four phenotypes, because $\mathrm{i}$ is hidden whenever $\mathrm{I^A}$ or $\mathrm{I^B}$ is present.

Worked example

Worked example 2: all four groups in one family. A father of group A and a mother of group B have a child of group O. Find the parents' genotypes and the chance of each blood group in their children.

The group O child is $\mathrm{i\,i}$ , so each parent carries $\mathrm{i}$ . The father is $\mathrm{I^A i}$ and the mother is $\mathrm{I^B i}$ .
Father's gametes: $\mathrm{I^A}$ and $\mathrm{i}$ . Mother's gametes: $\mathrm{I^B}$ and $\mathrm{i}$ .

Each child has a $\frac{1}{4}$ chance of each group: AB, A, B or O. Parents of groups A and B can have a child of any group.

Worked example

Worked example 3: a question of parentage. Two babies are born on the same night in a hospital, and their name tags are found loose. Couple 1 are both group O. Couple 2 are group AB and group O. Baby X is group O and baby Y is group A. Which baby belongs to which couple?

Couple 1: $\mathrm{i\,i} \times \mathrm{i\,i}$ . Every gamete carries $\mathrm{i}$ , so all their children are group O.
Couple 2: $\mathrm{I^A I^B} \times \mathrm{i\,i}$ . The children are $\mathrm{I^A i}$ (group A) or $\mathrm{I^B i}$ (group B); they cannot be group O, because the AB parent always passes on $\mathrm{I^A}$ or $\mathrm{I^B}$ .
So baby Y (group A) belongs to couple 2, and baby X (group O) to couple 1.

Blood groups can rule a parent out, but they cannot prove who a parent is: many people share each group.

Warning

Two common mistakes. (1) Calling a pink flower "co-dominant". Pink is a blend of effects, so it is incomplete dominance; co-dominance shows both phenotypes separately, like red and white hairs. (2) Writing blood group genotypes as "AO" or "AB" with no gene letter. Examiners expect $\mathrm{I^A i}$ and $\mathrm{I^A I^B}$ , so the reader can see which alleles are co-dominant and which is recessive.

Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!, from the Amoeba Sisters. For this lesson, watch the first two parts, on incomplete dominance and co-dominance; polygenic traits come up again in Unit 14.

Tip

Blend or both? Ask what the heterozygote looks like. Something new in between (pink) means incomplete dominance. Both parents' features showing at once (red and white hairs; antigens A and B) means co-dominance. Either way, the F2 ratio from two heterozygotes is 1 : 2 : 1.

✦

Practice: when neither allele dominates

Distinguish between incomplete dominance and co-dominance, giving one example of each.
Pink four o'clock plants are crossed with each other and 200 seeds are grown. How many plants of each colour are expected?
A pink four o'clock is crossed with a white one. Using $\mathrm{C^R}$ and $\mathrm{C^W}$ , give the gametes and the expected offspring ratio.
Why can the genotype of every four o'clock plant be read from its flower colour, when the genotype of a tall pea plant cannot?
A roan bull is mated with red cows. What proportion of the calves are expected to be roan?
Give every possible genotype of a person of (a) blood group A, (b) blood group O.
A mother of group A has a child of group O. Which blood groups could the father have? Explain.
A man of group AB says he is the father of a child of group O. Use genetics to evaluate his claim.

Show answersHide answers

In incomplete dominance the heterozygote has an intermediate phenotype (red × white four o'clock gives pink). In co-dominance both alleles are fully expressed together (roan cattle have red and white hairs; group AB has both antigens).
1 : 2 : 1 of 200: 50 red, 100 pink, 50 white.
Gametes: $\mathrm{C^R}$ and $\mathrm{C^W}$ from the pink parent, $\mathrm{C^W}$ only from the white parent. Offspring: $\mathrm{C^R C^W}$ and $\mathrm{C^W C^W}$ , 1 pink : 1 white.
With incomplete dominance each genotype has a different phenotype. In peas, TT and Tt both look tall.
$\mathrm{C^R C^W} \times \mathrm{C^R C^R}$ gives $\mathrm{C^R C^R}$ and $\mathrm{C^R C^W}$ equally: half are roan.
(a) $\mathrm{I^A I^A}$ or $\mathrm{I^A i}$ ; (b) $\mathrm{i\,i}$ only.
The child is $\mathrm{i\,i}$ , so the father must carry $\mathrm{i}$ : he could be group A ( $\mathrm{I^A i}$ ), group B ( $\mathrm{I^B i}$ ) or group O ( $\mathrm{i\,i}$ ), but not group AB.
The claim is false. A group AB man ( $\mathrm{I^A I^B}$ ) passes on $\mathrm{I^A}$ or $\mathrm{I^B}$ to every child, so none of his children can be group O ( $\mathrm{i\,i}$ ).

Key points

Incomplete dominance: the heterozygote is intermediate (pink). Co-dominance: both alleles are expressed side by side (roan; group AB).
In both, heterozygote × heterozygote gives 1 : 2 : 1, and every genotype can be read from the phenotype.
The ABO gene has three alleles: $\mathrm{I^A}$ and $\mathrm{I^B}$ are co-dominant, and both are dominant to $\mathrm{i}$ .
Alleles do not blend: red and white return in the F2.

Sources and credits

Photo: Light Roan Shorthorn Heifer, Robert Scarth, via Wikimedia Commons, licensed under CC BY-SA 2.0.
Video: Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!, Amoeba Sisters, embedded from YouTube.

Written by Ganzaa: the text, the tables, the Punnett squares, the worked examples and the practice questions.

1Neither red nor white

2Writing the alleles

3Co-dominance: both at once

4The ABO blood groups: co-dominance and multiple alleles

Neither red nor white

Writing the alleles

Co-dominance: both at once

The ABO blood groups: co-dominance and multiple alleles