Beyond Dominant and Recessive Alleles

There are important exceptions to Mendel’s discoveries Not all genes show simple patterns of dominant and recessive alleles Many traits are controlled by multiple alleles or multiple genes

Incomplete Dominance Cases in which one allele is NOT completely dominant over another Heterozygous phenotype is somewhere in between the two homozygous phenotypes –Get a mixture/blending of the parents’ traits –Ex. four o’ clock plants and flower colors

Incomplete Dominance – Four O’ Clock Flowers R = Red W = White RW = Pink

Incomplete Dominance – Four O’ Clock Flowers R = Red W = White RW = Pink

Incomplete Dominance - What About This? What happens when you cross a Pink Four O’ Clock (RW) with another Pink Four O’Clock (RW)? RW x RW RW R W RR R W RW W W R = Red W = White RW = Pink

Incomplete Dominance In people, hypercholesterolemia – dangerous amounts of cholesterol in blood – is an example of incomplete dominance –HH = normal amounts of cholesterol –hh = about 5 times as much cholesterol in blood –Hh = about 2 times as much cholesterol in blood

Codominance Both alleles contribute to the phenotype of organism –See both parents’ traits in offspring –Ex. chickens and feather color Allele for black feathers is codominant with allele for white feathers. Chickens with both alleles are black and white speckled –Ex. cattle and hair color Allele for red hair is codominant with allele for white hair. Cattle with both alleles are roan, or pinkish brown in color because the coat is a mixture of both red and white hairs X =

Codominance – Hair Color in Cattle Cross a Homozygous Red Hair Cow (RR) with a Homozygous White Hair Bull (WW) RR x WW R = red hair W = white hair RW = roan RR W W RR RR WW WW

Codominance - What About This? What happens when you cross a Roan Cow (RW) with a Roan Bull (RW)? RW x RW R = red hair W = white hair RW = roan RW R W RR R W RW W W

Multiple Alleles Many genes have more than two alleles and are therefore said to have multiple alleles An organism doesn’t have more than two alleles, but more than two possible alleles exist

Human Blood Type – A Multiple Allele Trait Blood Type O is universal donor – Why? Blood Type AB is universal recipient – Why? I A I A I B I A i I A I B I B I B I i Technical Genotype

Polygenic Traits Many traits are produced by the interaction of several genes Polygenic traits = many, many different phenotypes possible! Traits controlled by two or more genes = polygenic traits –Ex. 3 genes involved in making reddish-brown pigment in eyes of fruit-flies. Different combinations of these 3 genes produce different eye colors –Ex. Human skin color possibly controlled by 4 genes