Beyond Dominant and Recessive Alleles Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

Codominance Heterozygous genotype has its own phenotype both alleles contribute to the phenotype a mixture of the two alleles Example: In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.

Co-dominant vs Incomplete Dominant

Multiple Alleles Genes that are controlled by more than two alleles An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population. Example: Rabbit fur color Human blood type Example A rabbit's coat color is determined by a single gene that has at least four different alleles, so is human blood type and eye color

Different combinations of alleles result in the colors shown here. KEY C = full color; dominant to all other alleles cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles ch = Himalayan; color in certain parts of the body; dominant to c allele c = albino; no color; recessive to all other alleles Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here. photo credits: 1. ©John Gerlach/Visuals Unlimited 2.Animals Animals/©Richard Kolar 3. ©Jane Burton/Bruce Coleman, Inc. 4. ©Hans Reinhard/Bruce Coleman, Inc. Example A rabbit's coat color is determined by a single gene that has at least four different alleles, so is human blood type and eye color Full color: CC, Ccch, Cch, or Cc Chinchilla: cchch, cchcch, or cchc Himalayan: chc, or chch AIbino: cc

Human Blood Types There are four possible human blood types: type A = IAIA or IAi type B = IBIB or IBi 3. type AB = IAIB 4. type O = ii

What are the possible blood types of the children? Homozygous Male Type B (IBIB) x Heterozygous Female Type A (IAi) IA IB i IAIB IBi 1/2 = IAIB 1/2 = IBi

What are the possible blood types of the children? male Type O (ii) x female type AB (IAIB) i IA IB IAi IBi 1/2 = IAi 1/2 = IBi

Incomplete Dominance  When one allele is not completely dominant over another heterozygous genotype has its own phenotype a blend of the two homozygous phenotypes

RR A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). RR = Red WW = White RW = Pink WW Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring.

Polygenic Traits Traits controlled by two or more genes Example Skin color in humans is controlled by more than eight different genes

Genetics and the Environment Phenotypes can also be affected by the environment Example: During the summer the arctic fox’s fur is reddish brown to blend into the summer landscape. In the winter the coat is white to blend into the snowy background.