Dihybrid Crosses

Today in class we learned about doing punnett squares with 2 genes involved instead of 1. This type of problem in called a dihybrid cross.

To connect it to my previous post of Mendelian genetics, I will use an example with peas to show the dihybrid cross possibilities and punnett square set up.

There are two traits being crossed here: color and texture. We have both parents as heterozygous dominant for color and texture. This means we are crossing two round, yellow peas. (RrYy x RrYy)

When they are crossed together, they can produce every type of possibility for these traits. There are yellow and green smooth peas, as well as wrinkled.

This is coming from creating the possible gametes for each pea.






This is a clearer example that shows how the various gametes are formed for the different parents. In this example we are crossing two cats: a brown male and a white female. Since the females genotype is SSbb the only possible gamete to be crossed is Sb. As for the male, his genotype is ssBB. Once again, the only possible gamete formation for him is sB. When you cross these for the F1 generation the table is set up as:

      Sb   Sb   Sb   Sb
sB
sB
sB
sB

and we will only produce SsBb cats as shown in the F1 generation picture.

Then the F1 generation is crossed with itself and we see the other possible variations that come out from the gametes. Since the genotype is the same for both cats the possible gametes for SsBb are:

SB, Sb, sB, and sb

They are crossed and we have all the possibilities as shown in the picture.


Now that it is clearer on how to set up the tables, it is important to know how to solve for the different ratios.

Method 1: The Easier Way

You basically count and interpret the punnett square and determine the phenotypes and genotypes from the cross.

Method 2: Math Way

You find the fraction of the various possibilities and then multiply them by each other to get the exact probability of this happening.