Name: ____________________________________ Date: ________ Period: ____
Genetics: Punnett Squares and Incomplete vs Codominance
Most genetic traits have a stronger, dominant allele and a weaker, recessive allele. In an individual
with a heterozygous genotype, the dominant allele shows up in the offspring and the recessive allele
gets covered up and doesn’t show; we call this complete dominance.
However, some alleles don’t completely dominate others. In fact, some heterozygous genotypes allow
both alleles to partially show by blending together how they are expressed; this is called incomplete
dominance. Other heterozygous genotypes allow both alleles to be completely expressed at the same
time; this is called codominance.
Complete dominance =
Incomplete dominance =
Codominance =
Hybrid = Purebred Line =
Practice Problems
1. Snapdragons are incompletely dominant for color; they have phenotypes red, pink, or white. The
red flowers are homozygous dominant, the white flowers are homozygous recessive, and the
pink flowers are heterozygous. Give the genotypes for each of the phenotypes, using the letters
“R” and “W” for alleles:
a. Red snapdragon b. Pink snapdragon c. White snapdragon
genotype: ______ genotype: ______ genotype: ______
2. Show genetic crosses between the following snapdragon parents, using the punnett squares
provided, and record the genotypic and phenotypic ratios below:
a. pink x pink b. red x white c. pink x white
Genotypic Genotypic Genotypic
Ratio: ____:____:____ Ratio: ____:____:____ Ratio: ____:____:____
Phenotypic Phenotypic Phenotypic
Ratio: ____:____:____ Ratio: ____:____:____ Ratio: ____:____:____
3. In horses, some of the genes for hair color are incompletely dominant. Genotypes are as follows:
brown horses are BB, white horses are WW and a BW genotype creates a yellow-tannish colored
horse with a white mane and tail, which is called “palomino”. Show the genetic crosses between
the following horses and record the genotypic and phenotypic ratios:
a. brown x white b. brown x palomino c. palomino x palomino
Genotypic Genotypic Genotypic
Ratio: ____:____:____ Ratio: ____:____:____ Ratio: ____:____:____
Phenotypic Phenotypic Phenotypic
Ratio: ____:____:____ Ratio: ____:____:____ Ratio: ____:____:____
4. Can palominos be considered a purebred line of horses? Why or why not?
5. Which two colors of horse would you want to breed if you wanted to produce the maximum
numbers of palominos in the shortest amount of time?
6. In mice, there is a lethal mutation that causes embryos with the homozygous dominant genotype
YY to die before birth. Baby mice with the genotype YG appear yellow, and GG mice are gray.
a. If 2 gray parent mice mate, what % of their offspring
would be expected to die before birth?
b. If 1 gray mouse and 1 yellow mouse mate, what % of their
offspring would be expected to die before birth?
c. If 2 yellow parent mice mate, what percent of their offspring
would be expected to die before birth?
Codominance Worksheet (Blood types)
Human blood types are determined by genes that follow the CODOMINANCE pattern of inheritance.
There are two dominant alleles (I
A
and I
B
) and one recessive allele (i).
Blood Type
(Phenotype)
Genotype Can donate blood to: Can receive blood from:
O
ii
A,B,AB and O
(universal donor)
O
AB
I
A
I
B
O, AB
A,B,AB and O
(universal receiver)
A
I
A
I
A
or I
A
i
AB, A
O,A
B
I
B
I
B
or I
A
i
AB,B
O,B
1. Write the genotype for each person based on the description:
a. Homozygous for the “B” allele ______
b. Heterozygous for the “A” allele ______
c. Type O ______
d. Type “A” and had a type “O” parent ______
e. Type “AB” ______
f. Blood can be donated to anybody ______
g. Can only get blood from a type “O” donor ______
2. Pretend that Brad Pitt is homozygous for the type B allele, and Angelina Jolie is type “O.” What
are all the possible blood types of their baby?
3. Draw a Punnett square showing all the possible blood types for the offspring produced by a type
“O” mother and an a Type “AB” father
4. Mrs. Clink is type “A” and Mr. Clink is type “O.” They have three children named Matthew,
Mark, and Luke. Mark is type “O,” Matthew is type “A,” and Luke is type “AB.” Based on this
information:
a. Mr. Clink must have the genotype ______
b. Mrs. Clink must have the genotype ______ because ___________ has blood type ______
c. Luke cannot be the child of these parents because neither parent has the allele _____.
5. Two parents think their baby was switched at the hospital. Its 1968, so DNA fingerprinting
technology does not exist yet. The mother has blood type “O,” the father has blood type “AB,”
and the baby has blood type “B.”
a. Mother’s genotype: _______
b. Father’s genotype: _______
c. Baby’s genotype: ______ or ________
d. Punnett square showing all possible genotypes for children produced by this couple
e. Was the baby switched?
6. Two other parents think their baby was switched at the hospital. The mother has blood type “A,”
the father has blood type “B,” and the baby has blood type “AB.”
a. Mother’s genotype: _______ or ________
b. Father’s genotype: _______ or ________
c. Baby’s genotype: ______
d. Punnett square that shows the baby’s genotype as a possibility:
e. Was the baby switched?
7. Based on the information in this table, which man could not be the father of the baby? Justify
your answer with a Punnett square.
Name Blood Type
Mother Type A
Baby Type B
Sammy the player Type O
George the sleeze Type AB
The waiter Type A
The cable guy Type B
8. Based on the information in this table, which man could not be the father of the baby? Justify
your answer with a Punnett square.
Name Blood Type
Mother Type O
Baby Type AB
Bartender Type O
Guy at the club Type AB
Cabdriver Type A
Flight attendant Type B
9. Explain why blood type data cannot prove who the father of a baby is, and can only prove who the
father is not.
