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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%
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