Co-dominance, Incomplete Dominance, Polygenic Traits, and Multiple Alleles More about Mendel:: The Principle of Independent Assortment: genes for different traits can segregate independently during the formation of gametes. This means that the alleles separate randomly during meiosis. Independent assortment helps account for the many genetic variations observed in plants, animals and other organisms. It adds to genetic diversity.

Co-dominance:: We have discussed the existence of dominant and recessive alleles. If a dominant allele is present, the dominant trait is expressed. Sometimes alleles can be co-dominant. Co-dominance: both dominant alleles are expressed at the same time (you see both traits equally).

EX) In chickens, both black and red coloring are dominant to white coloring. Cross a homozygous black chicken with a homozygous red chicken Black = B Red = R Parents: BB x RR Genotypic Ratio: Phenotypic Ratio: The chickens will be black and red checkered. They will be equally black and equally red.

Incomplete Dominance:: Incomplete dominance: there is no completely dominant allele. This results in blending. Their will be a mixture of the two traits.

EX) In the four o’clock plant, red and white flowers exhibit incomplete dominance. Cross a red flower with a white flower. R = red W = white RW = pink Parents: RR x WW Genotypic Ratio: Phenotypic Ratio:

EX) Now cross two of the F1 generation plants. R = red W = white RW = pink (the blend of red and white) Parents: RW x RW Genotypic Ratio: Phenotypic Ratio:

Multiple Alleles:: Most traits are affected by many different allele combinations. Every combination would give you a different appearance. EX) In rabbits, there are four different coat colors: brown, grey, white with black points, and albino Possible alleles: A = Brown. Brown is the most dominant. If the bunny has an A it will be brown. A g = Grey. Grey is recessive to brown but dominant to white with black points and albino. A w = White with black points. This is recessive to grey and brown but dominant to albino aa = Albino. aa is the recessive condition.

A = Brown A g = Grey A w = White with black points aa = Albino EX) Possible bunny colors: Bunny 1) AA g =Bunny 2) AA = Bunny 3) aa =Bunny 4) A w a = Bunny 5) A g A w =Bunny 6) AA w = ** Multiple alleles are involved in determining eye color and hair color in many organisms

Polygenic Traits:: A trait produced by the interaction of many different genes. Polygenic trait: a trait controlled by two or more genes. EX) skin pigment and hair color in humans is a polygenic trait.

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