Co- Dominance In most situations (and all of Mendel’s experiments) one allele is completely dominant over the other, so there are just two phenotypes. But in some cases there are three phenotypes, because neither allele is completely dominant over the other, so the heterozygous genotype has its own phenotype. This situation is called co-dominance or incomplete dominance. Since there is no dominance we can no longer use capital and small letters to indicate the alleles, so a more formal system is used. The gene is represented by a letter and the different alleles by superscripts to the gene letter.

Monohybrid Inheritance of Co- Dominant Alleles Occasionally, alleles show co-dominance... Where both alleles are expressed in the phenotype, and neither one is recessive. Heterozygous genotype has a separate phenotype. One example in humans is the allele for sickly-cell anaemia, a genetic disorder caused by a mutation in the haemoglobin gene. It causes red blood cells to be sickle-shaped.

What about pink with pink?

GenotypePhenotype Codominant alleles Alleles are codominant if they are both expressed in the phenotype of a heterozygote. They can be represented by two capital letters superscript to the letter representing the gene. For example, flower colour in snapdragons Antirrhinum majus. C R = red flowers C W = white flowers CRCRCRCR homozygousred flowers CRCWCRCW heterozygouspink flowers CWCWCWCW homozygouswhite flowers

Sickle Cell Anaemia 1.People who are homozygous for normal haemoglobin (H N H N ) don’t have the disease. 2.People who are homozygous for sickle haemoglobin (H S H S ) have sickle-cell anaemia- all their blood cells have are sickle shape. 3.People who are heterozygous (H N H S ) have an in-between phenotype, called the sickle-cell trait. They have some normal haemoglobin and some sickle. The two alleles are co dominant because they’re both expressed in the phenotype.

Example H N H S HN HSHN HS H N H S H N H S HN HSHN HS HN HSHN HS Parents Genotype Gametes Alleles Possible genotypes of offspring NormalSickle- Cell Trait Sickle Cell Trait Sickle Cell Anaemia