Question

mendel tested a dominant phenotype plant to determine if it was a heterozygote by using a
o true - breeding cross
o testcross
dihybrid cross
o monohybrid cross

Explanation:

A testcross is a cross between an individual with an unknown genotype (in this case, a dominant - phenotype plant) and a homozygous recessive individual. If the dominant - phenotype plant is heterozygous, the offspring of the testcross will show a 1:1 ratio of dominant to recessive phenotypes. If it is homozygous dominant, all of the offspring will show the dominant phenotype.
A monohybrid cross is a cross between two individuals that differ in one trait. A dihybrid cross is a cross between two individuals that differ in two traits. A true - breeding cross is a cross between two homozygous individuals (either both dominant or both recessive).

Answer:

testcross