#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
Date ________
Period _______
Intro to Dihybrid Crosses Info
When we study two unlinked traits on different chromosomes at one time we call this a dihybrid cross. Essentially, you will follow the same
steps as in a monohybrid cross, but each parent will produce 4 possible gamete combinations and there will be 16 possibilities instead of 4 for the
offspring's genotype, so you will have a larger Punnett Square.
Ex. A female guinea pig is heterozygous for both fur color and coat texture. She is crossed with a male that has light fur color and is
heterozygous for coat texture. What possible offspring can they produce?


Dark fur color is dominant (D) and light fur (d) is recessive.
Rough coat texture (R) is dominant, while smooth coat (r) is recessive.
Step 1: Determine genotypes of parents.
 Female = DdRr
 Male = ddRr
Step 2: Determine the possible combinations of genes in each gamete. The "FOIL" method or the "branching" method ("combination tree") will
be very useful here.
 Here is the FOIL method you may have learned from math class too. You write out the alleles for gene A on the
left and alleles for gene B on the right. Then "FOIL" to determine the gametes.
o F: Combine the FIRST allele of each gene (left eyebrow) to make one possible gamete.
O: Combine the OUTER alleles (big smile) to make one possible gamete.
I: Combine the INNER alleles (nose curve) to make one possible gamete.
L: Combine the LAST allele of each gene (right eyebrow) to make one possible gamete.

Another method is the BRANCHING method, which is especially useful when considering even more genes.
In this case, the alleles from gene A have been separated vertically (see left column). From allele "A", it is
possible to have either (branching arrows) "B" or "b" allele passing (big horizontal arrow) to the gamete
combination (letters in oval). The possible gamete combinations are exactly the same ones you got with
the FOIL method.
o
o
The females' gametes would be = DR, Dr, dR, dr
The females' gametes would be = dR, dr, dR, dr (***Note that both combinations are
repeated twice due to the fact that the male has 2 copies of the same allele for dark fur...
Step 3: The Punnett square will be larger now because there are more possible sperm and egg combinations. Filling-in the Punnett square follows
the same rules as in a monohybrid cross, except there are 16 boxes instead of four. It should look like the one started below. Finish filling in the
blank squares in the Punnett square below. (*Note you can simplify your Punnett Square by representing each combination only once.)
Step 4: After filling-in the Punnett square you should obtain the following genotypic ratio: *remember the numbers should add up to the number
of squares filled in = 0 DDRR: 0DDRr: 0DDrr: 1 DdRR: 2 DdRr: 1 Ddrr: 1 ddRR: 2 ddRr: 1 ddrr
Step 5: There will be only 4 different phenotypes because the 4 DdRr and the 2 DdRR will have dark fur with rough coat, and the 4 with ddRr
and the 2 ddRR will have light fur with rough coat, while the 2 Ddrr will have dark fur with smooth coat and the 2 ddrr will have
light fur with smooth coat. So, the phenotypic ratio would be = 3 dark, rough: 1 dark, smooth: 3 light, rough: 1 light, smooth.
#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
Date ________
Period _______
Intro to Dihybrid Crosses Problems
1. In zonks, puffy lips (P) are dominant to thin lips (p) and red lips (R) are dominant to purple lips (r). A female
zonk with the genotype PpRr is crossed with a male zonk with the genotype PPRr. Use the Punnett square
below to answer questions related to this cross.
a. What is the expected genotypic ratio for the offspring?
b. What is the expected phenotypic ratio for the offspring?
c. Are there any traits that do not show up in any of the offspring? If so, explain why.
#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
Date ________
Period _______
Pea Plant Problems
2. In pea plants, the smooth seed allele (S) is dominant over the wrinkled seed allele (s), and the tall stem allele
(T) is dominant over the short stem allele (t). The genes for these traits are on different chromosomes.
a. In the table below, list all the possible genotypes for all of the possible phenotypes (given).
b. In the table below, list all the unique gametes that can be produced. (Remember each gamete will have 1
allele for each trait or two total alleles.)
#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
Date ________
Period _______
3. In pea plants, the smooth seed allele (S) is dominant over the wrinkled seed allele (s), and the tall stem allele
(T) is dominant over the short stem allele (t). The genes for these traits are on different chromosomes.

Cross of a homozygous smooth, homozygous tall plant (# 1) with a wrinkled, short plant (# 2).
a. The genotype of plant 1 = __________________ and the genotype for plant 2 = __________________
b. The possible combinations of gametes from plant 1 = _________, _________, _________, _________
c. The possible combinations of gametes from plant 2 = _________, _________, _________, _________
d. Construct a Punnett square for this cross.
e. What is the expected genotypic ratio for the offspring?
f. What is the expected phenotypic ratio for the offspring?
4. Using the info above:

Cross two F1 plants.
a. The genotype of plant 1 = __________________ and the genotype for plant 2 = __________________
b. The possible combinations of gametes from plant 1 = _________, _________, _________, _________
c. The possible combinations of gametes from plant 2 = _________, _________, _________, _________
d. Construct a Punnett square for this cross.
e. What is the expected genotypic ratio for the offspring?
f. What is the expected phenotypic ratio for the offspring?
#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
Date ________
Period _______
5. In pea plants, the smooth seed allele (S) is dominant over the wrinkled seed allele (s), and the yellow seed
allele (Y) is dominant over the green seed allele (y). The genes for seed texture and those for seed color are on
different chromosomes. Plant 1 which is heterozygous for seed texture and seed color is crossed with plant
2 which has wrinkled seeds and is heterozygous for seed color.
a. The genotype of plant 1 = __________________ and the genotype for plant 2 = __________________
b. The possible combinations of gametes from plant 1 = _________, _________, _________, _________
c. The possible combinations of gametes from plant 2 = _________, _________, _________, _________
d. Construct a Punnett square for this cross.
e. What is the expected genotypic ratio for the offspring?
f. What is the expected phenotypic ratio for the offspring?
6. In pea plants, the tall stem allele (T) is dominant over the short stem allele (t), and the yellow seed allele (Y) is
dominant over the green seed allele (y). A plant of genotype was TTYy was crossed with one of genotype
ttYy. The cross yields 800 seeds.
a. What is the expected genotypic ratio for the offspring?
b. How many seeds of the 800 were produced for each genotype?
c. What is the expected phenotypic ratio for the offspring?
d. How many seeds of the 800 were produced for each phenotype?
#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
Date ________
Period _______
Genetic Disorders
7. In humans there is a disease called Phenylketonuria (PKU) which is caused by a recessive allele. People with
this allele have a defective enzyme and cannot break down the amino acid phenylalanine. This disease can
result in mental retardation or death. Let “E” represent the normal enzyme and "e" represent the allele for PKU.
In humans there is also a condition called galactose intolerance or galactosemia, which is also caused by a
recessive allele. Let “G” represent the normal allele for galactose digestion and "g" represent the allele for
galactose intolerance (galactosemia). Two adults are heterozygous for both traits.
a. The genotype of parent 1 = _________________ and the genotype for parent 2 = __________________
b. The possible combinations of gametes from parent 1 = _________, _________, _________, _________
c. The possible combinations of gametes from parent 2 = _________, _________, _________, _________
d. Construct a Punnett square for this cross.
e. What are the chances of having a child that is completely normal?
f. What are the chances of having a child that has just PKU?
g. What are the chances of having a child that has just galactosemia?
h. What are the chances of having a child that has both diseases?
#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
Date ________
Period _______
Watermelons
8. In watermelons, solid green (G) is dominant over striped pattern (g), and short (S) is dominant over long (s).

Cross a homozygous green, heterozygous short watermelon with a heterozygous green, long watermelon.
a. The genotype of watermelon 1 = _______________ and the genotype for watermelon 2 = ______________
b. The possible combinations of gametes from watermelon 1 = _________, _________, ________, ________
c. The possible combinations of gametes from watermelon 2 = _________, _________, ________, ________
d. Construct a Punnett square for this cross.
e. What is the expected genotypic ratio for the offspring?
f. What is the expected phenotypic ratio for the offspring?
9. Using the info above:

Cross a striped, homozygous short watermelon with a green, short watermelon (heterozygous for both).
a. The genotype of watermelon 1 = _______________ and the genotype for watermelon 2 = ______________
b. The possible combinations of gametes from watermelon 1 = _________, _________, ________, ________
c. The possible combinations of gametes from watermelon 2 = _________, _________, ________, ________
d. Construct a Punnett square for this cross.
e. What is the expected genotypic ratio for the offspring?
f. What is the expected phenotypic ratio for the offspring?
#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
Date ________
Period _______
Guinea Pig Problems

In guinea pigs, black coat color is dominant to white, and short hair length is dominant over long hair.
10. Cross a homozygous black, heterozygous short-haired guinea pig with white, long-haired guinea pig.
a. Indicate the phenotype(s) of the F1 generation.
b. If two hybrids from the F1 generation are mated,
determine the phenotypic ratio of the F2 generation.
11. Cross a guinea pig that is homozygous for both white and short hair with a guinea pig that is homozygous for
both black and long hair.
a. Indicate the phenotype(s) of the F1 generation.
b. If two hybrids from the F1 generation are mated,
determine the phenotypic ratio of the F2 generation.
#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
Date ________
Period _______
Fruit Fly (Drosophila) Problems

In Drosophila, the allele for short legs (d) is recessive to the allele for normal legs (D), and the allele for
hairy body (h) is recessive to the allele for normal body (H).
12. Make a Punnett square for the following cross: DDHh x Ddhh
a. What proportion of the offspring from the cross in would
be expected to show the recessive phenotype for both traits?
13. Make a Punnett square for the following cross: DdHh x ddhh
a. What proportion of the offspring from the cross in would
be expected to show the normal phenotype for both traits?
#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
Date ________
Period _______
Shrek Problems
 Lord Forquad’s big jaw and straight hair traits are on different chromosomes. Big jaw is dominant to
small jaw, and wavy hair is dominant to straight hair.
14. Lord Forquad is hybrid for his jaw. If Lord Forquad marries Fiona who has a small jaw and is hybrid for her
wavy hair:
a. What will be the predicted phenotypes of their future children?

Protruding ears are dominant over short ears and furry eyebrows are dominant over thin eyebrows.
Both traits are on separate chromosomes.
15. Shrek is heterozygous for both protruding ears and furry eyebrows. What is his genotype?
16. Shrek marries an ogre that is also hybrid for ear and eyebrow traits.
a. What will be the genotypic ratio for their future offspring?
b. What will be the phenotypic ratio for their future offspring?
c. Is this a true dihybrid cross?
#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses

Date ________
Period _______
Donkey’s ability to talk and his big teeth are on separate chromosomes. Big teeth are dominant over
small teeth and the ability to talk is dominant over the inability to talk.
17. Donkey is homozygous dominant for his big teeth and hybrid for his ability to talk. He marries a donkey that
is hybrid for big teeth and not able to talk.
a. What will be the genotypic ratio for their future offspring?
b. What will be the phenotypic ratio for their future offspring?
c. Is this a true dihybrid cross?
Hair and Eye Color Problems
 Black hair (B) is dominant over blonde hair (b) and brown eyes (E) are dominant over blue eyes (e).
Both traits are found on separate chromosomes.
18. The father is heterozygous for black hair and brown eyes. The mother is blonde-haired and blue-eyed.
a. How many offspring could be expected to have blonde hair and blue eyes?
19. Both parents are heterozygous for black hair and for brown eyes.
a. What is the phenotypic ratio of the offspring?
#_____ Name _________________________________
300 Bio
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
Date ________
Period _______
Snow People Problems

In snow people, eyes made of coal are dominant over eyes made of gumdrops and carrot noses are
dominant over ice cream cone noses.
20. Frosty is heterozygous for both traits. Crystal is also heterozygous for both traits.
a. What is Frosty’s genotype? __________________________
b. What is Crystal’s genotype? __________________________
c. What is the expected genotypic ration for their offspring?
d. What is the expected phenotypic ration for their offspring?