Beyond Dominant and Recessive Alleles

Beyond Dominant and Recessive Alleles

• There are important exceptions to Mendel’s discoveries

• Not all genes show simple patterns of dominant and recessive alleles

• Many traits are controlled by multiple alleles or multiple genes

Incomplete Dominance

• Cases in which one allele is NOT completely dominant over another

• Heterozygous phenotype is somewhere in between the two homozygous phenotypes– Get a mixture/blending of the parents’ traits– Ex. four o’ clock plants and flower colors

Incomplete Dominance – Four O’ Clock Flowers

R = Red

W = White

RW = Pink

Incomplete Dominance – Four O’ Clock Flowers

R = Red

W = White

RW = Pink

Incomplete Dominance - What About This?

What happens when you cross a Pink Four O’ Clock (RW) with another Pink Four O’Clock (RW)?

RW x RW

R W

R

W

R R

R W

R W

W W

R = Red

W = White

RW = Pink

Codominance

• Both alleles contribute to the phenotype of organism– See both parents’ traits in offspring– Ex. chickens and feather color

• Allele for black feathers is codominant with allele for white feathers. Chickens with both alleles are black and white speckled

– Ex. cattle and hair color• Allele for red hair is codominant with allele for

white hair. Cattle with both alleles are roan, or pinkish brown in color because the coat is a mixture of both red and white hairs

X

=

Codominance – Hair Color in CattleCross a Homozygous Red Hair Cow (RR) with a

Homozygous White Hair Bull (WW)

RR x WW

R = red hair

W = white hair

RW = roan

R R

W

W

R R

R R

W W

W W

Codominance - What About This?What happens when you cross a Roan Cow (RW)

with a Roan Bull (RW)?

RW x RW

R = red hair

W = white hair

RW = roan

R W

R

W

R R

R W

R W

W W

Multiple Alleles

• Many genes have more than two alleles and are therefore said to have multiple alleles

• An organism doesn’t have more than two alleles, but more than two possible alleles exist

Human Blood Type – A Multiple Allele Trait

There are three different alleles for human blood type:

Blood types For simplicity, we call these

IA A

IB B

i O

Each of us has two ABO blood type alleles, because we each inherit one blood type allele from our biological mother and one from our biological father. A description of the pair

of alleles in our DNA is called the genotype.

Since there are three different alleles, there are a total of six different genotypes at the human ABO genetic locus.

Allele from Parent 1

Allele from Parent 2

Genotype of offspring

Blood types of offspring

A A AA A

A B AB AB

A O AO A

B A AB AB

B B BB B

B O BO B

O O OO O

Blood Type O is universal donor – Why?

Blood Type AB is universal recipient – Why?

IA IA IA IB IA iIA IB IB IB IB Ii i

Technical Genotype

Polygenic Traits

• Many traits are produced by the interaction of several genes

• Polygenic traits = many, many different phenotypes possible!

• Traits controlled by two or more genes = polygenic traits– Ex. 3 genes involved in making reddish-brown

pigment in eyes of fruit-flies. Different combinations of these 3 genes produce different eye colors

– Ex. Human skin color possibly controlled by 4 genes