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
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.
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