CODOMINANCE Some genes have alleles that are both

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:

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 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 Hb. N ¢ Sickle cell allele is Hb. S ¢ © 2007 Paul Billiet ODWS

Codominant genotypes Genotypes Hb. NHb. S © 2007 Paul Billiet ODWS Phenotypes Normal haemoglobin

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 Hb. NHb. S Gametes Offspring Proportions Hb. N x Sickle cell trait Hb. NHb. S Hb. NHb. SHb. S Normal Sickle cell trait Sickle cell anaemia 25% 50% 25%