Post on 01-Jan-2016
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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