# name key date 15-16 300 bio period genetics problem …s genotype, so you will have a larger...
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#_____ Name ____Key_________________________ Date _15-16___
300 Bio Period _______
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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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 The females' gametes would be = DR, Dr, dR, dr o 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 ____Key_________________________ Date _15-16___
300 Bio Period _______
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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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.
#_____ Name ____Key_________________________ Date _15-16___
300 Bio Period _______
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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Pea Plant Problems
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Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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#_____ Name ____Key_________________________ Date _15-16___
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Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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Genetic Disorders
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Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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Watermelons
#_____ Name ____Key_________________________ Date _15-16___
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Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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Guinea Pig Problems
#_____ Name ____Key_________________________ Date _15-16___
300 Bio Period _______
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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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 Dd
13. Make a Punnett square for the following cross: DdHh x ddhh
#_____ Name ____Key_________________________ Date _15-16___
300 Bio Period _______
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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#_____ Name ____Key_________________________ Date _15-16___
300 Bio Period _______
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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#_____ Name ____Key_________________________ Date _15-16___
300 Bio Period _______
Genetics Problem Set #2 Dihybrid (Two Trait) Crosses
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Hair and Eye Color Problems