How can a testcross help you find the unknown genotype of the plant?
1 Answer
We can determine the genotype of the plant by observing the phenotypes of the offsprings resulting from test cross.
Explanation:
In a test cross the plant is crossed with a recessive plant for that trait of which the genotype is to be determined. Let us explain by taking example of pea plant.
The genotype of a tall pea plant is not known, whether it is homozygous (TT) or heterozygous (Tt).
This tall pea plant is crossed to dwarf pea plant (tt). Dwarfness is recessive to dominant allele T for tallness. according to Mendel's law of dominance, recessive allele t can not express in phenotype in presence of dominant allele T.
Thus when a tall pea plant with unknown genotype is crossed with dwarf pea plant (test cross), the ratio of the phenotypes will be the same as the ratio of the gametes formed by tall plant as recessive alllele contributed by dwarf plant will not have any influence on the phenotpe in presence of dominant allele.
Thus if the plant is homozygous tall (TT) it will be produce only one type of gametes carrying T allele.
The heterozygous tall (Tt) plant will produce 2 types of gmaetes carrying T allele and t allele in ratio of 1:1.
Thus if all the offsprings in test cross are tall the given tall plant is homozygous tall and if tall and dwarf offsprings are produced in the ratio of 1:1, the given tall plant is heterozygous tall.
