Question

What is a test cross?

Answer 1

When $F_1$ generation is crossed with a parent with recessive traits, it is called a test cross.

It helps determine the genetic composition (heterozygous/homozygous) of the progeny.

In the given cross the $F_1$ is heterozygous and so the ratio is 1:1

If it were homozygous all the progeny will be of the same phenotype.