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Monohybrid Crosses

When only one trait in a cross is examined, the mating is called a monohybrid cross. We can represent the cross in a simple manner and make predictions about the outcome. In crosses, the dominant allele is usually represented by a capital letter and the recessive allele is represented by the lower case. In addition, the dominant allele is used to represent the gene itself. For example, since yellow is dominant to green seeds, the letter Y is chosen to represent the gene for seed color.  The dominant allele is Y while the recessive allele is represented as y. Recall that Mendel used only pure breeding strains in his experiments. Therefore, all his parental plants were homozygous.
 
If we consider the example from above, where Mendel crossed plants having yellow seeds with those having green seeds, we can represent the cross as follows:
 

Each parent will contribute one allele (through the gametes) to the offspring. From the homozygous dominant parent, the only allele that can be passed on to the offspring will be Y, while in the homozygous recessive parent, only a y allele can be transmitted. Therefore, all the offspring will be Yy. Remember that Mendel self crossed the F1 progeny to obtain the F2.  The results of this cross are shown below:

There are three genotypes in the F2 generation (YY, Yy, and yy) but only two phenotypes (yellow and green seeds). Moreover, there is a 3:1 ratio of yellow to green seeds.

 

How can you determine the genotypes of the progeny from a cross? Typically, a matrix can be arranged to predict the outcomes of a cross. This matrix is called a Punnett square. On one side of the square, all the possible gametes from one parent are listed, while on the other side, all the gametes from the other parent are listed. The gametes are then combined in the boxes. Consider our example of the parental cross (P) above:

 

 

Y

(P gamete)

Y

(P gamete)

y

(P gamete)

Yy

(F1 progeny)

Yy

(F1 progeny)

y

(P gamete)

Yy

(F1 progeny)

Yy

(F1 progeny)

  

All the progeny (the F1 generation) are genotypically heterozygotes, and each has a yellow phenotype. When the F1 individuals are crossed, the Punnett square looks like this:

 

 

Y

(F1 gamete)

y

(F1 gamete)

Y

(F1 gamete)

YY

(F2 progeny)

Yy

(F2 progeny)

y

(F1 gamete)

Yy

(F2 progeny)

yy

(F2 progeny)

 

The F2 generation shows three genotypes: one quarter homozygous dominant (YY), one quarter homozygous recessive (yy) and one half heterozygous (Yy). The corresponding phenotypes are three quarters yellow and one quarter green, or a 3:1 ratio.





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