Question

If you examine 100 F₁ plants from the testcross and find that 44 have purple flowers with tall stems and 56 have purple flowers with short stems, you should conclude that the genotype of your original pea plant is ____________ .

Answer 1

Answer:

PPTt

Explanation:

Let the gene for flower colour be denoted by P. P is the dominant allele and produces purple colour, p is the recessive allele and produces the recessive colour. Let the gene for stem height be denoted by T. T is the dominant allele and produces tall stem, t is the recessive allele and produces short stem.

A test cross was done which means that one of the parents was homozygous recessive for both the traits i.e. pptt

In the progeny all the flowers had purple colour which is only possible if the test parent was homozygous dominant for the trait i.e. it was PP

PP x pp :

    P    P

p  Pp  Pp

p  Pp  Pp

As evident, all the flowers will have Pp genotype and hence purple colour.

In the progeny, around half of the flowers had tall stems and half had short stems. This is only possible if the test parent was heterozygous for the trait i.e. it was Tt

Tt  X  tt :

   T    t

t  Tt   tt

t  Tt   tt

As evident, half plants have Tt genotype so they will have tall stems. Half of them have tt genotype hence will have short stems.

Hence, combining the genotype for two traits we obtain the genotype PPTt of the original test pea plant.