Question

Why is a testcross necessary to determine genotype when an organism shows a dominant trait?

Answer 1

C an organism only needs one dominant allele, so the genotype could be 2 dominant or 1 dominant 1 recessive and you could only find out using a testcross

Answer 2

Answer:

option A, an organism only needs one dominant allele to show the dominant trait.

Explanation:

A test cross between two individuals helps to determine the genotype of off springs and thus decide their phenotypic characteristics.  

Since one single dominant allele is enough to express the dominant trait, it becomes essential to know the exact genotype of organism showing dominant trait

For example –  

Let red colour be a dominant trait of flower represented by “R”

And white colour be a recessive trait of flower represented by “w”

If a homozygous red and white flower are crossed the following offspring would form –

R R

w Rw Rw

w Rw Rw

Now , since red is dominant over white all the offs pings flower would be of red color. Now in case if exact genotype is not determined , then one would assume that all red offspring have phenotype “RR” and this will create problem in predicting output of future crosses carried out between offspring.  

Thus, knowing the genotype is extremely important to determine both the genotype and phenotype of future generations.