NOTES 22 – Complex Inheritance Patterns Most human traits have more than 2 or 3 forms. Types of inheritance that are more complex than simple dominance, incomplete dominance or codominance control these traits.

Multiple Allelic Inheritance (Multiple Alleles) Traits are controlled by 1 gene with more than 2 alleles Ex. Blood Type in humans The gene that controls blood type has 3 alleles A = Type A blood B = Type B blood o = Type O Blood Alleles A and B are codominant and allele o is recessive

There are 6 possible genotypes and 4 possible phenotypes: A A or A o = Type A Blood B B or B o = Type B Blood AB = Type AB Blood o o = Type O Blood

Blood Cells

There are four basic Blood types: Type A with A antigens on the red cells and anti B antibodies in the plasma. Type B with B antigens on the red cells and anti A antibodies in the plasma. Type AB with both A and B antigens on the red cells and no blood type antibodies in the plasma. Type O with no antigens on the red cells and both anti A and anti B antibodies in the plasma

Polygenic Inheritance Traits are controlled by more than 1 gene There are many possible phenotypes Ex. Skin color is controlled by at least 3 genes A = dark skin a = light skin B = dark skin b = light skin C = dark skin c = light skin

Differences in skin color are largely due to differences in the amount of melanin, a dark pigment produced by skin cells. Each “dark skin” allele (A, B, C) codes for high melanin production Each “light skin” allele (a, b, c) codes for low melanin production Each dark skin allele (A, B, C) in the genotype adds a small but equal amount of pigment to the skin

Some possible genotypes and phenotypes: aabbcc = very light skin AABBCC = very dark skin AaBbCc = medium skin

What would a Punnett Square for skin color look like? Cross two medium skin tone parents AaBbCc X AaBbCc Each parent could produce 8 different possible combination to pass on: ABC, ABc, AbC, Abc, aBC, aBc, abC, or abc