Download Codominance and Incomplete Dominance PPT

Punnett Squares:
Codominance and
Incomplete Dominance
Expressed means…
• The gene is “on” or “activated” and the protein
will be produced.
• We observe expressed genes as phenotypes.
Hair Color Expressed
Subject A
Subject B
Subject C
BB
Bb
bb
Genotype for Eye
Color
Phenotype for
Eye Color
• What eye color gene is
expressed?
• What eye color is
unexpressed?
• What is Ryan Reynolds’
phenotype for eye color?
Genotype for Eye
Color
Phenotype for
Eye Color
Subject A
Subject B
Subject C
BB
Bb
bb
• The examples we have seen so far are simple:
you either have brown eyes or blue eyes.
• It is often more complicated than just one gene
being dominant over another. Sometimes,
genes are considered codominant. This means
both alleles are expressed equally; the
phenotype will be a combination of both alleles.
Red blood cell
Blood types are a great example of co-dominance!
Your blood type results from a protein that is found in the
membrane of your red blood cells.
• If you have gene IA, then you make protein A.
• If you have gene IB, then you make protein B.
• If you have gene i, then you do not make a protein.
• If you have both gene IA and gene IB, then you make BOTH
proteins.
These alleles are co-dominant because they are
both always expressed!
Blood types are Codominant
A
1. If a person has genotype IAIA, their blood type (phenotype) will be ______
A
2. If a person has genotype IAi, their blood type (phenotype) will be ______
B
3. If a person has genotype IBIB, their blood type (phenotype) will be ______
B
4. If a person has genotype IBi, their blood type (phenotype) will be ______
O
5. If a person has genotype ii, their blood type (phenotype) will be ______
AB
6. If a person has genotype IAIB, their blood type (phenotype) will be ______
Codominance Examples
IA
IA
i
i
IA IAIA IAi
IB IAIB IBi
IA IAIA IAi
i
100%
% A = _____
0%
% O = _____
IAi
ii
IA
IA
IA
i
IA IAIA IAIA
IB IAIB IBi
IB IAIB IAIB
IB IAIB IBi
50%
% A = _____
25%
% A = _____
25%
0%
% B = _____
% B = _____
25%
% O = _____
% AB = _____
50%
% AB = _____
25%
% A = _____
0%
% B = _____
50%
0%
% O = _____
% AB = _____
50%
More Codominance Examples
IB
i
IB IBIB IBi
i
IBi
ii
75%
% B = _____
25%
% O = _____
i
i
IA
IA
IA
i
IA IAi
IAi
i
IAi
IAi
IA IAIA IAi
IB IBi
IBi
i
IAi
IAi
IB IAIB IBi
100%
% A = _____
50%
% A = _____
0%
% B = _____
% O = _____
50%
0%
% O = _____
0%
% AB = _____
% A = _____
50%
% B = _____
25%
0%
% O = _____
% AB = _____
25%
Incomplete Dominance
As we have seen, not all gene combinations are as simple
as having one dominant gene and one recessive gene.
Based on the diagram, how would you define incomplete
dominance?
→
X
RR
R
rr
Rr
R
r Rr
Rr
R
r
r
Rr
Incomplete dominance
•
Occurs when a
combination of genes
produces an
intermediate of the
two traits.
• One trait is not
completely dominant,
so the heterozygous
phenotype is a
mixture.
R
r
R
RR
Rr
r
Rr
rr
Animals, like these horses, often exhibit
incomplete dominance in the color of their
hair or skin. This produces a range of shades
from light to dark.
Photo by David Blaikie
Dark
Brown
Palomino
Cream
D
d
D DD
Dd
d Dd
dd
Dark
Brown
Palomino
Cream
d
d
D Dd
Dd
d dd
dd
R
Examining
Incomplete Dominance
R
1.A species of flowers exhibits
incomplete dominance between
the allele for red flowers and the
allele for white flowers.
r Rr
Rr
If a plant homozygous for red
flowers is crossed with a plant
that is homozygous for white
flowers, what percentage of
offspring will have pink flowers?
r Rr
Rr
100% pink flowers
KEY:
Examining
Incomplete Dominance
2. If two heterozygous, pink
plants are crossed, what
percentage of each color of
flower will appear in the
offspring?
25% red flowers
50% pink flowers
25% white flowers
R
r
R RR
Rr
r
rr
KEY:
Rr
Complete the following Punnett squares about incomplete
dominance and answer the questions go with each cross.
Subject A
Subject B
Subject C
RR
Rr
rr
Red
Pink
White
Genotype for
Flower Color
Phenotype for
Flower Color
R
r
r
Rr Rr
r
rr rr
50%
R
R
r
RR Rr
r
Rr
rr
3
r
R
r
Rr rr
R
R
R
RR RR
r
Rr rr
r
Rr Rr
50%
50%
Codominance vs. Incomplete Dominance
• Assume each chicken below is the results of a cross
between a parent with black feathers and a parent with
white feathers.
• Based on the definitions we have learned, which breed
shows codominance between feather color and which
shows incomplete dominance.
Chicken with Gray
Feathers
Incomplete Dominance
An intermediate trait
appears in the offspring.
Chicken with Black
& White Feathers
Codominance
Both traits appear in the
offspring.