Incomplete Dominance and Co-Dominance

Incomplete Dominance and Co-Dominance Taking it a step beyond Mendel

Complete Dominance • Before Mendel, the prevailing theory of inheritance was blending inheritance – that children appear as a combination of their parents • We learned that Mendel disproved this theory. The cross of purple and white only yielded purple or white flowers, no other colours. • Complete dominance occurs when only one allele is expressed, regardless of the presence of another • Is this always the case in nature?

Blending is Common in Nature! • There are exceptions to the dominance rule • In snapdragons, a homozygous red flower and a homozygous white flower will produce pink flowers! P Generation F1 Generation

The genes of snapdragons • The gene that codes for flower colour is called C • It has two possible alleles, Red – CR , and White – CW Note the style of writing genes! The C represents the Gene, the superscript represents the allele

The ratio stays the same

Incomplete Dominance • In the case of snapdragons, neither of the alleles is dominant. Both red and white pigments are expressed and neither masks each other. • Heterozygote looks like a blend of the homozygotes • Incomplete dominance is a situation where neither allele dominates the other and both exercise an influence on phenotype, resulting in partial expression of BOTH traits.

Codominance • Sometimes two alleles will both be equally dominant in the offspring • Codominance is the situation where both alleles are expressed fully to produce offspring with a third genotype

Codominance Note how we also use superscripts here to show that both genes are equally dominant!

Co-dominance: Incomplete dominance: x x Comparing the two Incomplete Dominance Complete Dominance Resulting offspring looks like a MECHANICAL MIXTURE of both parents Both alleles are dominant, both are completely expressed, but on different parts of the organism • Resulting offspring looks like a HOMOGENEOUSBLEND of both parents • Neither allele is dominant, both have some influence on the phenotype

Blood Types • Human blood is both a dominant and codominant trait • Possible Blood Types (Phenotypes): • A, B, AB, O • Possible Alleles: • IA , IB , i • Everyone still only inherits two alleles (one from each parent)

Human Blood Types Dominance Codominance

Human Blood Types Antigen – a molecule recognized by the immune system Antibody – fight any foreign material in the body; they link the foreign substances together to create a clump of cells

Blood Donation • When you receive a blood transfusion the concern is that your body’s antigens will attack the foreign red blood cells leading to an immune response • The appropriate donor blood cells will not have a an antigen that can be recognized by an antibody in the recipients blood

Type O Blood – Universal Donor • Type AB Blood – Universal Recipient • DONATE BLOOD!!!

Solution: Mom’s genotype: IAIB Dad’s genotype: IAior IAIA You think you got it? Let’s see… Consider the following crosses: IAIB x IAi and IAIB x IAIA Q: If a woman has blood type AB and a man has blood type A, what possible blood types will their children have? Hint: There is only one possible genotype for mom but two for dad…

IA i IA IA IA IA i IB i IB IA IB IA IA IA IA IA IA IA IA IB IB IB IA Solutions Continued Cross #2 (heterozygous dad): IAIB x IAi All children from these two parents must be type A, type B or type AB All children from these two parents must be type A or type AB Cross #1 (homozygous dad): IAIB x IAIA

Incomplete Dominance and Co-Dominance