## Dihybrid Cross

### Problem 3: A genetic cross yielding a 9:3:3:1 ratio of offspring.

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?

Tutorial

#### Predicting the genotype of offspring

 Determine all possible combinations of alleles in the gametes for each parent. 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.

#### 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

 A phenotypic ratio of 9:3:3:1 is predicted for the offspring of a SsYy x SsYy dihybrid cross.
 9 spherical, yellow 3 spherical, green
 3 dented, yellow 1 dented, green

The Biology Project
University of Arizona
Wednesday, August 14, 1996
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