Dihybrid punnett square example problems

Tutorial to help answer the question

Which of the following genetic crosses would be predicted to give a phenotypic ratio of 9:3:3:1?

Predicting the genotype of offspring

Half of the gametes get a dominant S and a dominant Y allele; the other half of the gametes get a recessive s and a recessive y allele.

Both parents produce 25% each of SY, Sy, sY, and sy.

(Review the tutorials for problems #1 and problem #4 if necessary).

Punnett square.

Since each Parent produces 4 different combinations of alleles in the gametes, draw a 4 square by 4 square punnett square.

Gametes from Parent 1

List the gametes for Parent 1 along one edge of the punnett square.

Gametes from Parent 2

List the gametes for Parent 2 along one edge of the punnett square.

Alleles from Parent 1

Fill out the squares with the alleles of Parent 1 .

Alleles from Parent 2

Fill out the squares with the alleles from Parent 2 .

The result is the prediction of all possible combinations of genotypes for the offspring of the dihybrid cross, SsYy x SsYy.

Predicting the phenotype of offspring

Spherical, yellow phenotype

There are 9 genotypes for spherical, yellow seeded plants. They are:

SSYY (1/16)
SSYy (2/16)
SsYY (2/16)
SsYy (4/16)

Spherical, green phenotype

Two recessive alleles result in green seeded plants.

There are 2 genotypes for spherical, green seeded plants. They are:

SSyy (1/16)
Ssyy (2/16)

Dented, yellow phenotype

Two recessive s alleles result in dented seeded plants.

There are 2 genotypes for dented, yellow seeded plants. They are:

ssYY (1/16)
ssYy (2/16)

Dented, green phenotype

A ssyy plant would be recessive for both traits.

There is only 1 genotypes for dented, green seeded plants. It is:

ssyy (1/16)

A phenotypic ratio of 9:3:3:1

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