Chapter 16, Problem 8STP

Summary Introduction

To perform:

The test cross to find out the genotype of black guinea pig and show both the Punnett square result.

Introduction:

Monohybrid cross deals with one contrasting character at a time which is controlled by a single gene/ a pair of alleles. The allele can be dominant or recessive. Dominant allele expresses itself in the offspring in both homozygous as well as in heterozygous conditions, whereas a recessive allele can only express itself in homozygous conditions. Homozygous means when both the alleles present are of a single nature, i.e., either both are dominant or recessive, while in heterozygous both dominant and recessive alleles are present. Here, in the given case, the dominant phenotype is Black fur while the recessive phenotype is white fur controlled by allele B and b respectively.

Explanation of Solution

A test cross is widely used to find out the zygosity of the dominant phenotype, i.e., either it would be due to homozygous condition or due to heterozygous condition. Test cross involves the crossing of the questioned individual with the recessive individual as we know recessive condition will always be due to the homozygosity.

Here, Black fur individuals can have (BB) genotype or (Bb) whereas, White fur would have genotype (bb) only.

So, two test crosses are possible in this case:

1. BB with bb
2. Bb with bb

1)BB with bb

Gametes	b	b
B	Bb	Bb
B	Bb	Bb

If all the offspring are of the same phenotype, i.e., Dominant phenotype, then the individual in question must be in Homozygous dominant.

2) Bb with bb

Gametes	b	b
B	Bb	Bb
B	bb	bb

If half the offspring are of dominant phenotype and half are of recessive phenotype, then the individual in question must be in Heterozygous dominant.

Conclusion

Test cross can help us in finding out the zygosity of the dominant individual. If all the offspring of that individual have a dominant phenotype, then the dominant individual must be of homozygous dominant condition.Whereas if 50% of offspring are of dominant and 50% are of recessive phenotype, then the individual must have a heterozygous dominant condition. Therefore, the test cross isa very helpful method to predict zygosity. This method was first introduced by Gregor Mendel who is also known as Father of Genetics.