CODOMINANCE Some genes have alleles that are both expressed in the heterozygote individuals © 2007 Paul Billiet ODWSODWS

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 ODWSODWS

Blood cells Blood smear (normal) Sickle cell anemia © 2007 Paul Billiet ODWSODWS

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 ODWSODWS

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 ODWSODWS

Codominant genotypes GenotypesPhenotypes Hb N Normal haemoglobin Hb N Hb S Sickle cell trait Hb S Sickle cell anaemia © 2007 Paul Billiet ODWSODWS

PhenotypesSickle cell trait x GenotypesHb N Hb S GametesHb N Hb S Hb N Hb S Hb N Hb S Hb N Hb N Hb S Hb S Hb N Hb S Hb S OffspringNormalSickle cell traitSickle cell anaemia Proportions25%50%25% Unusual proportions In codominance, heterozygotes have their own phenotype This gives rise to different proportions amongst to offspring of some genetic crosses