CODOMINANCE
Some genes have alleles that are
both expressed in the heterozygote
individuals
© 2007 Paul Billiet ODWS
Codominant alleles have
three phenotypes
Sickle cell anaemia is a genetic disease
the heamoglobin of the red blood cells
 Haemoglobin is normally a ball-shaped
molecule
 The sickle cell allele makes it form a long
strands
 The red blood cell carrying these
molecules distorts into characteristic long
shape

© 2007 Paul Billiet ODWS
Blood cells
Sickle cell anemia
http://www.netwellness.org/ency/imagepages/1223.htm
Blood smear (normal)
http://137.222.110.150/calnet/cellbio/cellbio.htm
© 2007 Paul Billiet ODWS
The genetics of sickle cell
anaemia
The shape of the haemoglobin molecule is controlled by two alleles

Normal Haemoglobin allele

Sickle Cell Haemoglobin allele
There are three phenotypes
Normal
Normal individuals have two normal haemoglobin alleles
Sickle cell anaemia, a severe form where all the red blood cells are
affected.
Sickle cell anaemia patients have two sickle cell alleles in their genotype
Sickle cell trait, a mild condition where 50% of the red blood cells are
affected.
Sickle cell trait individuals are heterozygotes, having one of each allele
© 2007 Paul Billiet ODWS
Symbols for codominant
alleles
Both alleles are expressed in the
heterozygote both take a CAPITAL CASE
letter
 An index letter identifies the allele
Therefore:
 Normal haemoglobin allele is HbN
 Sickle cell allele is HbS

© 2007 Paul Billiet ODWS
Codominant genotypes
Genotypes
HbNHbN
HbNHbS
HbSHbS
© 2007 Paul Billiet ODWS
Phenotypes
Normal haemoglobin
Sickle cell trait
Sickle cell anaemia
Unusual proportions


In codominance, heterozygotes have their
own phenotype
This gives rise to different proportions
amongst to offspring of some genetic crosses
Phenotypes
Sickle cell
trait
Genotypes
HbNHbS
Gametes
Offspring
Proportions
HbN
x
Sickle cell trait
HbNHbS
HbS
HbN
HbN
HbS
HbN
HbNHbN
HbNHbS
HbS
HbNHbS
HbSHbS
HbS
Normal
Sickle cell trait
Sickle cell anaemia
25%
50%
25%