When Genetics is Not Simple

Other patterns of inheritance

It’s not so Simple

• Not all traits are controlled by single genes with simple dominant - recessive alleles:– Multiple alleles – Polygenic traits– Incomplete dominance– Codominance

Multiple Alleles

• More than two alleles exist for a particular trait (multiple flavors)– Labeled using a single letter

and superscripts– Results in many possible

phenotypes

Ex: Human blood types IA, IB, or i

Polygenic Traits

• More than one gene controls a particular More than one gene controls a particular traittrait– Very complicated hereditary patterns Very complicated hereditary patterns – Results in an vast array of possible Results in an vast array of possible

phenotypesphenotypes

Ex:Ex: human skin, eye, and hair color human skin, eye, and hair color

Incomplete dominance

• One allele is not completely dominant over the other– Heterozygous individuals show a trait that is

an intermediate or blend of the two homozygous trait

– Neither homozygous trait shows

Incomplete dominance

– Example: Four o’clock flowers• RR = red flowers• WW = white flowers• RW =

– Pink is a blend of red and white

CodominanceCodominance

– Both alleles are dominantBoth alleles are dominant– Both homozygous traits show in heterozygous Both homozygous traits show in heterozygous

individuals individuals – There is There is no blending of traitsno blending of traits– Results in blotches of colorsResults in blotches of colors

CodominanceCodominance

– Example: Feather color in chickens• BB = black feathers• WW = white feathers• BW =

– No gray!Both colors“Checkered

Chicken”

CodominanceCodominance

– Example: Feather color in chickens• BB = black feathers• WW = white feathers• BW =

– No gray!Both colors“Checkered

Chicken”