Incomplete dominanceCodominanceMultiple alleles

Ch. 10.3 (p. 215-216)

Incomplete dominance• This is a case where neither of the 2 alleles

is totally dominant over the other

• So instead of having 2 phenotypes…we see an appearance of a 3rd phenotype

– In dominant/recessive cases, AA and Aa showed the dominant phenotype

– In incomplete dominance, Aa will show an intermediate of the other 2 phenotypes

Example of incomplete dominance

• Snapdragons

Red x White (P1)

All Pink (F1)

Example con’t

• All F1 offspring were heterozygous

• Let’s take 2 F1 individuals to make an F2 generation

Pink x Pink

Red, Pink, and White

Using alleles (letters) for incomplete dominance

• Since there isn’t a recessive gene here, we WILL NOT use a lower case letter

• Instead we use…

R = Red R’ = White

• So our possible genotypes and phenotypes are:

RR = Red RR’ = Pink R’R’ = White

The example written in alleles (letters)

RR x R’R’ (P1)

All R R’ (F1)

RR’ x RR’

1RR: 2RR’: 1R’R’

*Also our phenotypic ratio is:

1 Red: 2 Pink: 1 White

Codominance

• This is a case where neither of the 2 alleles is totally dominant over the other, but we can still see each phenotype product individually (they’re not blended)

• So again, we’ll see 3 phenotypes instead of 2…the heterozygous will show a combination of the other 2

Example of codominance

• Chickens with checkered feathers:

Black x White(P1)

All checkered (F1)

Written in alleles as:

BB x B’B’(P1)

All BB’ (F1)

Example con’t

• Taking it to the F2:

BB’ x BB’

1BB: 2BB’: 1B’B’

Phenotypic ratio:

1 Black: 2 Checkered: 1White

Multiple Alleles

• For some traits, more than 2 alleles exist in the population (each individual still only has 2 alleles for that trait)

• Having more than 2 alleles increases the possible genotypes and phenotypes for the trait

Example of multiple alleles

• Blood type for humans– 3 alleles:

IA = A IB = B i = O– IA and IB are codominant to each other– IA and IB are both dominant over i

Blood Types

Genotype Phenotype

IA IA Type A Blood

IA i Type A Blood

IB IB Type B Blood

IB i Type B Blood

IA IB Type AB Blood

i i Type O Blood

Example multiple allele problem

• A person homozygous for Type A marries a person with Type O. What are the possible outcomes of bloodtypes for their children?