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Genetics Part 2:

Classic/Complete Dominance Classic Dominance: (aka “complete

dominance”) the heterozygote displays the dominant phenotype.

It doesn’t always work this way, though …

Observation: Observation: A Litter of PuppiesA Litter of Puppies

Dad 2?Dad 2?

Dad 1?Dad 1?

MomMom

Dad 2 is the sire of the litter!

Which is Dad?Which is Dad?

How is that possible??How is that possible?? Many traits have complex inheritance.

They can involve more than two alleles! (Example: blood types)

May not be as simple as complete dominance

Sometimes involves more than 1 gene! (Examples: eye color, height, skin color)

In the case of the dogs shown in the previous two slides, there are 2 separate gene locations that control coat color.

Incomplete DominanceIncomplete Dominance Incomplete Dominance:Incomplete Dominance: occurs when occurs when

offspring have a phenotype that is in offspring have a phenotype that is in between the two parent phenotypes.between the two parent phenotypes.

Heterozyote displays a blended phenotype Heterozyote displays a blended phenotype (dominant no longer masks recessive!)(dominant no longer masks recessive!) Ex) Four o’clock flowersEx) Four o’clock flowers Get ready to write inGet ready to write in

the answers to the the answers to the example and Square 1!example and Square 1!

Incomplete Dominance Example

Four o’clock flowers (This is square 3 on your notes sheet)

Show a cross between two heterozygous pink flowers.

Genotypic ratio:1 RR : 2 Rr : 1 rr

Phenotypic ratio:1 Red : 2 Pink: 1 white

R

r

RR

rrRr

Rr

R r

There is only 1 genotype per phenotype!

Genotype Phenotype

RR Red Flowers

rr White Flowers

Rr Pink Flowers Rr X Rr

Now, you try it!Now, you try it!Complete squares 2 and 3 Complete squares 2 and 3

on your fill-in notes sheet on your fill-in notes sheet using the “rules” from our using the “rules” from our Four ‘O Clock Flowers Four ‘O Clock Flowers example.example.

CodominanceCodominance

Codominance:Codominance: both alleles are expressed in a both alleles are expressed in a heterozygote.heterozygote.

NeitherNeither allele is recessive! allele is recessive! BothBoth show up in the phenotype – there is no show up in the phenotype – there is no

“in between” like with incomplete dominance.“in between” like with incomplete dominance. Examples: Examples:

blood types, blood types, spotted cows,spotted cows, spotted flower color spotted flower color

SPECIAL CASE: How to write SPECIAL CASE: How to write genotypes for codominant alleles genotypes for codominant alleles

Notation:Notation: USE WITH CODOMINANCE ONLY!USE WITH CODOMINANCE ONLY! Example:Example:

Codominance in cow coat colorsCodominance in cow coat colors

CRepresents the gene for coat color

R

The allele for the color

(in this case, it is red)

CR

CRGenotype written

as:Phenotype written as: Red

CR

CRCW

Genotypic Ratio:

Genotype Phenotype

CR CR Red Coat

CWCW White Coat

CR CW Roan Coat

Codominance ExampleRoan Coat Color in Cattle

Show a cross between a red bull and a roan cow.

CRCR X CRCW

CR

CW

CR CRCR CRCR

CRCW

2 CRCR : 2 CRCW

Phenotypic Ratio:2 Red Coat : 2 Roan Coat

Blood type = Codominance and Multiple AllelesBlood type = Codominance and Multiple Alleles There are 3 alleles for blood genotypes: IA, IB, i There are 4 blood phenotypes: A, B, O, AB IA and IB are codominant; both are expressed for blood

type AB. Type O, which has genotype ii, is recessive to both A and

B.

The only one The only one that is that is Codominant!Codominant!

GenotypesGenotypes PhenotypesPhenotypes

IA IA or IA i Type A

IB IB or IBi Type B

i i Type O

IA IB Type AB

Multiple alleles!Multiple alleles!

Blood Type Example Blood Type Example (write this down!)

A woman has Type A blood and her genotype is IA woman has Type A blood and her genotype is IAAi i She marries a man who is Type B with genotype IShe marries a man who is Type B with genotype IBBi. i. What are the possible blood types of their children?What are the possible blood types of their children?

IA

i

i

IAIB IAi

IBi ii

Type AB Type A

Type B Type O

The possible offspring blood types are:Type AB, Type A, Type B, and Type O

IB

Back to the dog example from yesterday…Back to the dog example from yesterday…

Dam (mom): KkYY Sire (dad): kkyy

There are TWO alleles for Coat ColorK = Blackk = Not BlackY = Redy = Tri-color

The Pups are:

KkYy = Black

kkYy = Red

To Sum Up Classic Dominance:

Incomplete Dominance:

Codominance:

Dominant Phenotype

XAn “in between” of the dominant and recessive

X Both alleles are displayed