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Dihybrid Crosses

We can also examine crosses with two different traits (a dihybrid cross) to see how the traits are inherited by the progeny. Consider a cross between two pure breeding strains, one with yellow seeds and purple flowers, and one with green seeds and white flowers (yellow is dominant to green and purple is dominant to white). We can represent the cross like this:

When the F1 generation is self-crossed, we can set up a Punnett square of all possible combinations of gametes. Look carefully at the F1 genotype. Each gamete must contain one allele for the gene for seed color and one allele for the gene for flower color. The analysis would look like this:

 

 

YP

Yp

yP

yp

YP

YYPP

YYPp

YyPP

YyPp

Yp

YYPp

YYpp

YyPp

Yypp

yP

YyPP

YyPp

yyPP

yyPp

yp

YyPy

Yypp

yyPp

yypp

 

By counting all the squares and combining the phenotypes, you should see that 9/16 of the progeny will have yellow seeds and purple flowers, 3/16 will have yellow seeds and white flowers, 3/16 will have green seeds and purple flowers, and 1/16 will have green seeds and purple flowers. Thus, the phenotypic ratio for a dihybrid cross is 9:3:3:1. 





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