Question

Test cross in plants or Drosophila Involves crossing
A. Between two genotypes with a recessive trait
B. Between two ${{F}_{1}}$ hybrids
C. The ${{F}_{1}}$ hybrid with a double recessive genotype
D. Between two genotypes with the dominant trait

Answer 1

The organism with dominant traits is obliged to be crossed with an organism having recessive traits in a test cross. In addition, a test cross for a violet ${{F}_{1}}$ generation the test cross will be with white recessive.

Complete step by step answer: A test cross is a method that is used to check the genotype of an organism. The genotype of an organism is its genetic constitution for each trait that it expresses as its phenotype or physical characteristics. If we come to know the genetic constitution of an organism we can easily predict its phenotypic characteristics.
Now, in a test cross, an organism whose genotype is unknown is crossed over with a recessive organism or parent. Let us understand the test cross with the help of an example.

Consider a monohybrid cross involving a violet color flower with the dominant genotype of WW and a white flower with recessive genotype as ww. Here, W represents the violet trait and w represents the white trait.
The progeny of this cross or ${{F}_{1}}$ generation of this cross will be all violet flowers as the dominant trait expresses itself whenever it will pair up with a recessive gamete. Thus, the genotype of ${{F}_{1}}$ generation will be Ww. This is called the ${{F}_{1}}$ hybrid as a hybrid means a mixture of two traits.
Now, this ${{F}_{1}}$ generation with Ww genotype will be crossed with the recessive parent with genotype ww. This is called a test cross. The result of this cross will be half heterozygous dominant violet flower population with genotype Ww and half of the population with genotype ww that is double recessive with the white trait. Hence, from the above explanation, we can conclude that a test cross in plants or Drosophila will involve crossing the ${{F}_{1}}$ hybrid with a double recessive genotype.
Therefore, the right option is option C.

Note: In a test cross an offspring shows a recessive trait such as ww, the unknown genotype will be heterozygous like Ww. But if all the progeny shows dominant traits, then the unknown genotype will be homozygous dominant like WW. Also, to get a reliable result a large number of progeny is needed.