Question

What is a test cross? Show phenotypic ratio 1:1 in a monohybrid test cross through a Punnett square.

Answer 1

Hint: The cross has a phenotypic ratio of 1:1:1:1 where the genes having two alleles and these two alleles segregate into different gametes. By this method, Mendel was able to determine the offspring produced will be linked or recombinant.

Complete answer:
When a cross is done between the hybrid progeny produced and the recessive homozygous parent then the cross is known as the test cross. It is also helpful in finding the nature of the parental individual with dominant characteristics whether it was homozygous or heterozygous. The test cross is done between hybrid offspring produced by parents and any one of the homozygous parents.
-This test helps in determining the nature of the dominant trait whether it is homozygous or heterozygous.
-In this test, if the parent(unknown) is taken to be homozygous dominant then the offspring that are produced will also be the dominant phenotype, while on the other hand if the parent (unknown) is heterozygous in nature than half of the total offspring will be dominant and the other half of the offspring produced will be recessive in nature.
-This cross also determines the linkage between the two genes after their cross with the known heterozygote.
-If the potential of the phenotype is in an equal ratio then genes are likely to be unlinked.
-The genes are said to be linked if the two phenotypes are present in a higher amount or present in a lower amount.
-Classic Mendelian ratio i.e. 1:1:1:1 phenotypic ratio is the result of a test cross. The two alleles of the gene here are independently assorted into the gametes.
In this example, we have taken two alleles ‘Y’ and ‘y’. ‘Y’ for dark color and ‘y’ for a light color.

Note:
The phenomenon of test cross was first introduced by the Scientist Gregor Mendel. The number of offspring utilized in performing the test cross experiment is very large as the data that is noted must be very accurate with a meaningful conclusion. Due to the advancement of genetic screening and genome mapping tests cross is not commonly used nowadays.