Codominance , Incomplete Dominance, Polygenic Traits, & Multiple Alleles

April 22, 2023

I. What is codominance?A. Some traits do not have a clear dominant &

recessive allele. B. In this case, BOTH alleles are expressed.

1. Example: Red flowers and White flowers would produce Red & White spotted flowers

C. This is called codominance because BOTH alleles act like they’re dominant.

Example #1

1. Cattle have red, roan, & white coat colors. Roan is a “mixture” of the genes for red & white.

a. There are three possible expressions of this trait… what would the genotypes be?

Red Cattle: White Cow:

Roan bull:

RR WW

RW

b. A roan bull is mated with a white cow. What are the genotypes of the parents?

P1=___________ P2=__________

c. What is the phenotypic ratio for the F1 generation? R W

RW WW

W

W

RW

RW

WW

WW

Example #2

2. A red bull mated with a white cow produces a roan calf.

a. What are the genotypes of the bull, cow and calf?

Bull_____ Cow_____ Calf_____RR WW RW

b. What would be the probability of having a RED calf if the two were mated again?

0%

Incomplete Dominance Notes

April 22, 2023

I. What is incomplete dominance?A. With incomplete

dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.

1. Its like mixing paints, red + white will make pink. 2. Red doesn't totally block (dominate) the white, instead

there is incomplete dominance, and we end up with something in-between.

II. Punnett Squares & Incomplete Dominance

A. We can still use the Punnett Square to solve problems involving incomplete dominance.

1. The only difference is that instead of using a capital letter for the dominant trait & a lowercase letter for the recessive trait, the letters we use are both going to be the same capital letter but the second letter will have a tick mark on it (because neither trait dominates the other).

B. What are the genotypes for each of the flowers types we used in the earlier example?

C. So the cross from the example we used up above would look like this:

Red flower = White flower =

Pink flower =

RR R’R’

RR’

R R

R’

R’

RR’

RR’

RR’

RR’

Genotype100% RR’

Phenotype100% Pink

flowers

What is polygenic inheritance?• In polygenic inheritance there is more than

one gene responsible for a trait. This usually results in continuous variation.

• In humans, for example, traits such as height, shape, weight, color, and metabolic rate are governed by the cumulative effects of many genes

How do we recognize polygenic traits?

• Polygenic traits are recognizable by their expression as a continuous variation of small differences. The results form a bell shaped curve, with a mean value & extremes in either direction.

Let’s Do Some Practice!• Complete the practice problem worksheet

with the people sitting around you.• Be prepared to share your answers!!

Mutiple Alleles

II. What are multiple alleles?

A. Sometimes there are more than two alleles possible for a trait.

1. However, each individual organism will still only have two alleles (one from Mom and one from Dad) even if there are more

2. Examples: blood types, hair color, and eye colorB. This can lead to more possible genotypic

and phenotypic ratios

C. Blood Types1. Human blood type is a good example of BOTH

multiple alleles and codominance. a. There are 3 possible alleles & two of them act

dominant (A and B are dominant while o is a recessive allele)

2. Blood Type Allelesa. Type A: causes cells to make protein A for the

surface of the RBC’sb. Type B: causes cells to make protein B for the

surface of the RBC’sc. Type O: causes cells to make no proteins for the

surface of the RBC’s

3. Both A and B alleles are DOMINANT over the O allele because if any A or B allele is present, they will be expressed and the A or B proteins will be made

• Blood typing and Rh factor

4. Label the possible genotypes of the following three phenotypes:

(RBC) A

A A

A

A

(RBC) B

B B

B

B

(RBC)

Phenotype:

Genotype:

A

AA, AO

B

BB, BO

O

OO

Example #1

1. Suppose a child is of blood group B and its mother is of blood group O.

a. To what genotypic groups may the father have belonged?

b. The child?c. The mother?

AB, BB, BOBO

OO

Example #22. A child has blood group AB. What 4

PHENOTYPIC blood type combinations could its parents have?

1. Blood type______ x Blood type_____2. Blood type______ x Blood type______3. Blood type______ x Blood type______4. Blood type______ x Blood type______

A BA AB

AB B

AB AB

Example #33. A man has blood type A and his wife has blood

type B. A child is born with blood type O. The man accuses his wife of infidelity saying that this child could not possibly he his. Do you agree or disagree? Explain why.

Disagree both could be AO and BO, so therefore both O alleles could be given to child