1. Punnett Squares:
Codominance and
Incomplete Dominance

2. Quick Review
Which organelle carries the information needed for making proteins in a eukaryotic cell? ________
nucleus
nucleus
The nucleus has one job, and that is to store the ______.
DNA 2

3. Quick Review
Image by Richard Wheeler
DNA is packaged into a very long molecule called a _____________. When you talk about chromosomes you need to realize that you are talking about ______.
chromosome
DNA
Human Chromosomes 3

4. sequence nitrogen bases
The instructions or __________ for making proteins are found in the ______. The information is carried in the ______________________. The sequences of DNA that helps to create and regulate proteins are known as a ______. The different versions of a gene are known as _______.
blueprint
DNA
sequence nitrogen bases
genes
alleles
Madeleine Price Ball

5. Phenotype for Eye Color
The combinations of alleles in an organism make up the organism’s _____________. If a gene affects the physical appearance of an organism, we say that it has been ____________. If a gene does not affect the physical appearance of an organism, we say that it is ______________.
genotype
expressed
unexpressed
Subject A
Subject B
Subject C
Genotype for Eye Color BB Bb bb
Phenotype for Eye Color

6. Phenotype for Eye Color
The physical appearance of an organism that results from the expression of genes is known as the organism’s ____________.
phenotype
Subject A
Subject B
Subject C
Genotype for Eye Color BB Bb bb
Phenotype for Eye Color
Brown Eyes
Blue Eyes
Brown eyed photo by Erin Silversmith
Blue eyed photo by J Drewes

7. Define the following words using a vocabulary reference sheet if available.
Define homozygous: ____________________________________________________________________________________
Define heterozygous: ____________________________________________________________________________________
Homozygous gametes form when the same genes join together. (GG, gg, TT, ss, etc.)
Heterozygous gametes form different genes join together. (Gg, Ss, Tt, Bb, etc.)

8. FF
Solid gray fur Tt
Long tails bb
Blue eyes Ff
Solid gray fur TT
Long tails ff
Spotted fur BB
Brown eyes Bb
Brown eyes

9. Codominant genes are BOTH expressed in the phenotype!
What you have been taught so far is a very simplified version of how genes interact. It is often more complicated than just one gene being __________ over another. Sometimes, genes are considered ______________. This means that they are both __________ (made into proteins). Blood types are a perfect example of this.
dominant
codominant
expressed
Codominant genes are BOTH expressed in the phenotype!

10. If a person has genotype IAIA, their blood type (phenotype) will be ______
If a person has genotype IAi, their blood type (phenotype) will be ______
If a person has genotype IBIB, their blood type (phenotype) will be ______
If a person has genotype IBi, their blood type (phenotype) will be ______
If a person has genotype ii, their blood type (phenotype) will be ______
If a person has genotype IAIB, their blood type (phenotype) will be ______ A A B B O AB

11. red A B do NOT both expressed
Red blood cell
Your blood type results from a protein that is found in the membrane of your _____ blood cells. If you have gene IA, then you make protein ___. If you have gene IB, then you make protein ___. If you have gene i, then you _________ make a protein. If you have both gene IA and gene IB, then you make ______ proteins. They don’t prevent each other from being ___________ (made into proteins), so they are codominant over gene i.
red A B
do NOT
both
expressed

12. % A = _____ % O = _____ 100% % A = _____ % B = _____ % O = _____
IA i
IA i
IA IA
IA i IA IB i IA IB IB
IAIA IAi
IAIB IBi
IAIA IAIA
IAIB IBi
IAIA IAi
IAi ii
IAIB IAIB
IAIB IBi
% A = _____
% O = _____
100%
% A = _____
% B = _____
% O = _____
% AB = _____
25%
% A = _____
% B = _____
% AB = _____
50%
% A = _____
% B = _____
% O = _____
% AB = _____ 0% 0%
25% 0%
50%
25%
50% 0%
25%
50%

13. % B = _____ % O = _____ 75% % A = _____ % B = _____ % O = _____
IB i
i i
IA IA
IA i IB i IA IB
IBIB IBi
IAi IAi i IA IB
IAi IAi
IAIA IAi
IBi ii
IBi IBi
IAi IAi
IAIB IBi
% B = _____
% O = _____
75%
% A = _____
% B = _____
% O = _____
% AB = _____
50%
% A = _____
% O = _____
100%
% A = _____
% B = _____
% O = _____
% AB = _____
50%
25% 0%
50%
25% 0% 0% 0%
25%

14. As we have seen, not all gene combinations are as simple as having one dominant gene and one recessive gene. Below is an example of another exception known as ___________ dominance. Based on the diagram, how would you define incomplete dominance?
incomplete
R R r
R r → X RR rr Rr

15. Incomplete dominance occurs when a combination of genes produces an _____________ trait. One trait is not completely dominant, and a diluted phenotype appears that is in between the homozygous dominant and recessive phenotypes.
R r R r
R R
R r
intermediate
R r
r r

16. Examining Incomplete Dominance
1. A species of flowers exhibits incomplete dominance between the allele for red flowers and the allele for white flowers. 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 R r
R r
R r
R r
R r
100% pink flowers

17. Examining Incomplete Dominance You get 3 different phenotypes!
2. A species of flowers exhibits incomplete dominance between the allele for red flowers and the allele for white flowers. If two heterozygous, pink plants are crossed, what percentage of each color of flower will appear in the offspring?
R r R r
R R
R r
R r
r r
25% red flowers
50% pink flowers
25% white flowers
You get 3 different phenotypes!

18. Genotype for Flower Color Phenotype for Flower Color
Complete the following Punnett squares about incomplete dominance and answer the questions go with each cross.
Subject A
Subject B
Subject C
Genotype for Flower Color RR Rr rr
Phenotype for Flower Color
Red
Pink
White
r r
R r
R r
R R R r
Rr Rr R r
RR Rr r
Rr rr R r
RR RR
rr rr
Rr rr
Rr rr
Rr Rr
50% 3
50%
50%

19. Codominance vs. Incomplete Dominance
Two different types of chickens are shown below. Assume each the 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 co-dominance between feather color and which shows incomplete dominance.
Chicken with Black & White Feathers
Chicken with Gray Feathers
Incomplete Dominance
An intermediate trait appears in the offspring.
Codominance
Both traits appear in the offspring.

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