How to Solve a Dihybrid Cross

Step 1: Identify Parental Genotypes

Determine the genotype of the parental organisms for the two traits you're studying. For example, if you're examining seed color (Y for yellow, y for green) and seed shape (R for round, r for wrinkled) in pea plants, you might start with parents of genotypes YYRR (pure-breeding for yellow round seeds) and yyrr (pure-breeding for green wrinkled seeds).

Create a "key" for the possible phenotypes

YY = Yellow
Yy = Yellow
yy = green

RR = round
Rr = round
rr = wrinkled

Step 2: Determine Possible Gametes

Identify the possible gametes each parent can produce through the process of Mendelian segregation. Each parent will produce haploid gametes containing one allele for each trait.

For the parents YyRr and YyRr:

Use the "FOIL" method to determine the gametes - first, outer, inner, last

dihybrid foil

Step 3: Set Up a Punnett Square

Construct a Punnett square to visualize the potential genotypes of offspring resulting from the cross between the parental organisms. Label the rows and columns with the possible gametes from each parent.

Step 4: Analyze Offspring Genotypes

Fill in the Punnett square by combining the gametes from each parent. This will give you the genotype combinations of the offspring. There are 16 possible combinations of genotypes.

punnett square

Step 5: Determine Phenotypes

Once you have the genotypes of the offspring, determine the phenotypes associated with each genotype based on the expression of the traits being studied. Refer to your "key" to help your determine the phenotypes. It may also be helpful to colorcode the squares to help you count.

Step 6: Calculate Phenotypic and Genotypic Ratios

Count the number of each phenotype and genotype in the offspring to determine the phenotypic and genotypic ratios.

Yellow, Round = 9/16
Yellow, Wrinkled= 3/16
Green, Round = 3/16
Green, Wrinkled = 1/6

9:3:3:1

Practice Sets on Genetic Crosses

Advanced Bunny Genetic Crosses with two traits - also includes fruit fly genetics

Bunny Genetics 2 - basic crosses, rabbits have gray or white hair, black or red eyes

Explore the Genetics of Corn Snakes - dihybrid crosses with corn snakes, color is polygenic

Multiple Allele Traits in Chickens - shows how combs are inherited (rrpp x RRpp)