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

Incomplete Dominance One allele is not completely dominant over another. –The offspring has an intermediate (blend) phenotype –Ex: 4 o’clock flowers RR (red) X rr (white) Rr genotype produces a pink flower

Incomplete Dominance

Codominane Codominance – a condition in which both alleles contribute to the phenotype of the organism. –Ex: Roan cattle RR (red) X WW (white) RW genotype produces a roan cow, which has both red and white hair.

Codominance Sickle-cell anemia follows codominant inheritance.

Polygenic Inheritance Polygenic Inheritance – some traits are determined by the interaction of many pairs of genes. –Ex: Height, weight and skin color in humans Skin tone depends on the amount of melanin pigment in skin cells. Very dark-skinned people have alleles that code for the production of melanin at many gene positions for skin color. Whereas, light-skinned people have alleles that code for the production of melanin at fewer gene positions for skin color.

Pedigrees A pedigree is a diagrammed family history that is used to study inheritance patterns of a trait through several generations. They can also be used to predict disorders in future offspring.

Pedigrees A pedigree uses symbols to illustrate inheritance of the trait. –Males = squares –Females = circles –Expresses the trait = completely shaded shape –Carries the trait = half shaded shape –Does not express the trait = unfilled shape

Pedigrees