Which of the following characteristics does not occur in pairs?

In $Pisum$ $sativum$,two of the seven traits studied by Mendel were linked,as they are located on chromosome number $4$. These traits were:

$A$: Mendel proposed postulates like "principles of segregation and principles of independent assortment" after studying seven pairs of contrasting traits in garden pea.
$R$: He was lucky in selecting seven characters in pea that were located on seven different chromosomes.

What is the primary purpose of a test cross?

Test cross is

Assertion: The genetic complement of an organism is called genotype.
Reason: Genotype is the type of hereditary properties of an organism.