Beyond Mendel Beyond Mendel Incomplete and Co-dominance there are patterns of inheritance that do not follow the same patterns that Mendel observed. • they still follow the same rules as laid out by Mendel’s laws Incomplete dominance • occurs when neither of the two alleles for the same gene can completely conceal the presence of the other. • example: Mirabilis jalapa (Four o’clock Plant) Incomplete Dominance • example: Mirabilis jalapa (Four o’clock Plant) • a cross between a true breeding red-flowered plant and a true-breeding white-flower produces offspring with pink flowers. • when representing incomplete dominance upper and lower case letters are not used. • all upper case letters are used with subscripts/superscripts to denote the alleles. R1R1 → red flower R2R2 → white flower R1R2 → pink flower Incomplete dominance Incomplete dominance • two human examples of incomplete dominance are sickle cell anemia and familial hypercholesterolemia Sickle Cell Anemia HbA HbA → normal red blood cells Hbs Hbs → sickle shaped red blood cells HbA Hbs → have the sickle trait • this is called heterozygous advantage because if you have one copy of the you don’t have the disease and you are resistant to malaria. Incomplete dominance Familial Hypercholesterolemia • a genetic condition that prevents the tissues from removing low-density lipoproteins (bad cholesterol) from the blood. • if you are homozygous for the trait you have six times the amount of cholesterol in your blood. • most have a heart attack by the age 2 • heterozygous individuals have about twice as much cholesterol in their blood and may have a heart attack by the age 35. Co-Dominance • occurs when both alleles are fully expressed. • example: Blue Roan Horses • a heterozygous animal where both the base colour and white are expressed. • both black and white hairs grow on the body creating a blue appearance Co-dominance In humans a single gene determines a person’s ABO blood type. • this gene determines the type of antigen, if any, that is attached to the cell membrane or red blood cells. • the gene is designated as I and has three common alleles: IA IB i • the different combinations of the three alleles produce four phenotypes known as A, B, AB and O Co-dominance Co-dominance the IA allele is responsible for the A antigen the IB allele is responsible for the B antigen the i allele causes there to be no antigen the IA and IB heterozygous mix are co-dominant with each other. Sometimes called “intermediate” inheritance