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How do you do a Dihybrid cross?

What are ways you can do this?

1 Answer by Expert Tutors

Steve P. | Positive, Achievement-Oriented Tutor Specializing in PhysicsPositive, Achievement-Oriented Tutor Spe...
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A dihybrid cross maps out the possible combinations of two selected traits in offspring from two parent organisms.  This is done graphically on a Punnett Square.

For example, if two pea plants are crossed - and we choose to look at the traits of height and flower color - we need to start with the respective genotypes of the parents.  The genotypes tell you which two alleles each parent has for each trait.  Suppose that parent 1 is heterozygous for height.  This means it has one dominant, or tall, allele (T) and one recessive, or short, allele (t).  Also, parent 1 is homozygous recessive for flower color, so both alleles are recessive, or violet (ww).

Continuing with the example, let's suppose that parent 2 is heterozygous for both traits.  This results in the following genotypes in this example.

Parent 1:  Ttww
Parent 2:  TtWw

Since we don't know which height allele will go with which flower color allele in each parent's sex cell, here are the possible sex cells each parent can produce:

Parent 1:  Tw, Tw, tw, tw
Parent 2:  TW, tW, Tw, tw

In the Punnett Square, each combination from Parent 1 will have it's own column, and each combination from Parent 2 will have it's own row.  Each box in the matrix is just a combination of the row and column it is in, and it represents the genotype the offspring will have if those particular sex cells (1 from each parent) combine.

Tw                Tw                tw                tw

TW   TTWw          TTWw           TtWw           TtWw

tW    TtWw           TtWw            ttWw            ttWw

Tw   TTWw          TTww            Ttww           Ttww

tw    Ttww            Ttww             ttww            ttww

This tells us there is a 2 in 16 chance that the offspring from these two parent pea plants will have offspring that are short in height and violet in flower color.