INTRODUCTION – Crosses involving two traits can be followed with a Punnett square just like a Monohybrid. It just requires a larger square. These types of crosses are called Dihybrids. The major difference is that when following one trait, the parent can produce a maximum of two gametes. With two triats, a dihybrid, the parent could produce as many as four gametes. For example, look at a plant that is heterozygous for tallness and heterozygous for pod color. Its genotype would be TtGg. It can produce four different gametes. Any one of these gametes could be passed on: TG, Tg, tG, and tg. Here is an example of a Dihybrid cross:

PROBLEMS – Use the table below to solve the following problems. Determine the gametes that will be produced. Then using a Punnet square, predict the genotypic ratio and phenotypic ratio for the following crosses. Always write down the genotype of the parents if it is not supplied for you.

1. Cross these two plants: TTBB X ttbb

What gametes can the first plant produce? _________ _________ _________ _________

What gametes can the second plant produce? _________ _________ _________ _________

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

What is the genotypic ratio _________________ What is the phenotypic ratio__________________

2. Cross two offspring from the cross in problem number 1.

What gametes can the first plant produce? _________ _________ _________ _________

What gametes can the second plant produce? _________ _________ _________ _________

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

What is the phenotypic ratio__________________

3. Cross two plants that are heterozygous for both round seed coats and green pod color.

What is the phenotypic ratio__________________