Codominance and
Multiple Alleles
Most genetic examples that we look at involve alleles that are
dominant and recessive.
Some genes do not work this way and their alleles share
dominance of the characteristic.
We call this Codominance.
Snapdragon plants are one example of an organism with
codominant genes.
They can produce red flowers or white flowers in the
homozygous condition.
When the two homozygous plants cross pollinate the resulting
offspring produce pink flowers.
The characteristics of the two alleles have blended.
Gametes
CR
CR
CW
CRCW
CRCW
CW
CRCW
CRCW
Multiple Alleles
This is where a gene has three or more alleles instead of the
usual two.
A good example of this is human blood groups, where there
are three possible alleles:
• IA
• IB
• IO
The alleles IA and IB are codominant
The allele IO is recessive to the other two.
Genotype
Blood group
I AI A
A
I AI O
A
IBIB
B
IBIO
B
I AI B
AB
I OI O
O
If two parents have the
blood types AB and O,
what are the possible
genotypes and phenotypes
of their offspring?
If parents with heterozygous
type A and heterozygous
type B blood have children,
what are the possible
genotypes and phenotypes
of the offspring?
Questions
1. A rooster with grey feathers is mated with a hen with grey feathers. When
their offspring are examined, there are 15 chicks with grey feathers, 6
with black, and 8 with white.
a) What is the simplest explanation for the inheritance of these colours in
chickens?
b) What offspring would you predict from the mating of a grey rooster and
a black hen?
2. Mrs Brown and Mrs White had babies in the hospital at the same time. Mrs
Brown took home a baby girl and Mrs White a baby boy. However Mrs White
was sure the doctor told her she had a girl, so she made enquiries at the
hospital.
Blood tests showed that:
• Mr Brown was type O and Mrs Brown was type AB
• Mr and Mrs White were both type B
• Baby boy was type A and baby girl type O
Was Mrs White right?