Incomplete Dominance and Co-Dominance Taking it a step beyond Mendel 1

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? 2

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 F 1 Generation 3

The genes of snapdragons The gene that codes for flower colour is called C It has two possible alleles, Red – C R, and White – C W GenotypePhenotype CRCRCRCR Red CRCWCRCW Pink CWCWCWCW White Note the style of writing genes! The C represents the Gene, the superscript represents the allele 4

The ratio stays the same 5

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. 6

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 7

Codominance Note how we also use superscripts here to show that both genes are equally dominant! 8

Comparing the two Incomplete Dominance Resulting offspring looks like a HOMOGENEOUS BLEND of both parents Neither allele is dominant, both have some influence on the phenotype 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 x x Incomplete dominance: Co-dominance: 9

Blood Types Human blood is both a dominant and codominant trait Possible Blood Types (Phenotypes): A, B, AB, O Possible Alleles: I A, I B, i Everyone still only inherits two alleles (one from each parent) 10

Human Blood Types GenotypePhenotype (Blood Type) I A A I A i A I B B I B i B I A I B AB ii O Codominance Dominance 11

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 12

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 13

Type O Blood – Universal Donor Type AB Blood – Universal Recipient DONATE BLOOD!!! 14

You think you got it? Let’s see… 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… Solution: Mom’s genotype: I A I B Dad’s genotype: I A i or I A I A Consider the following crosses: I A I B x I A i and I A I B x I A I A 15

Solutions Continued Cross #1 (homozygous dad): I A I B x I A I A Cross #2 (heterozygous dad): I A I B x I A i IAIA IBIB IAIA i IAIA IAIA IAIA IBIB IAIA i IBIB i All children from these two parents must be type A, type B or type AB IAIA IBIB IAIA IAIA IAIA IAIA IAIA IAIA IAIA IAIA IBIB IBIB All children from these two parents must be type A or type AB 16