Question

Assertion: A geneticist crosses two plants. He got 50% tall and 50% dwarf plant in the progeny.
Reason: One plant was heterozygous tall while the other was dwarf.
A. Both assertion and the reason are correct and the reason is the correct explanation for the assertion.
B. Both assertion and reason are correct but the reason is not the correct explanation for the assertion.
C. The assertion is correct but the reason is incorrect
D. Both assertion and reason are incorrect.

Answer 1

Gregor Johann Mendel is Known as the father of modern genetics. According to his law of heredity, when an allele passes from parent to offspring it separates independently during the formation of gamete and shows its phenotypic expression in the offspring without mixing with other alleles. He crosses a pea plant of two colours, purple and white. The cross shows the purple colour of the offspring. From the result, he concluded that the white colour and the purple colour did not mix rather shows its phenotypic expression independently. As the purple allele was dominant over the white colour allele, offspring showed purple colour plants.

Complete answer: Explanation:
In the given cross, geneticists found 50% tall and 50% dwarf plants. The dwarf phenomenon is the recessive character which can only be expressed in homozygous condition. Here the cross shows there is 50% population of the plant are dwarf. Thus it can be concluded that one of the parent plants of this plant must be heterozygous.
Let, the genotype of the heterozygous tall plant it Tt. Here T stands for tall and t expresses the dwarf phenomenon. As T is dominant over t it appears phenotypically.
The genotype of dwarf plant tt
Possible gametes come from the heterozygous tall plant and the homozygous dwarf are T, t and t, t respectively.
The cross is shown in the checkerboard:

Gametes	T	t
t	TtHeterozygous tall	ttDwarf
t	TtHeterozygous tall	ttDwarf

The cross shows a 50% tall plant and 50% dwarf plant.
So, the correct answer is “Option A”, both the assertion and the reason are correct and the reason is the correct explanation of the assertion..

Note:
When an organism involves two same alleles for a particular genotype is called homozygous condition. Example tall pea plant having a genotypic expression of TT. When an organism comprises two different alleles for a particular character and only one allele can be expressed phenotypically is called heterozygous condition. Example tall pea plant involves the genotypic expression of Tt.