What is the cross between the $F_1$ progeny and the homozygous recessive parent called? How is it useful?

The crossing of an F${}_1$ progeny with a double (homozygous) recessive parental progeny is done in order to determine whether the progeny is homozygous or heterozygous for a character under consideration. This is called as test cross. It was first introduced by Gregor Mendel. Those that are heterozygous will have one dominant and one recessive allele individuals and those that are homozygous dominant will have two dominant alleles, and those that are homozygous recessive will have two recessive alleles. The cross is useful to determine the genotype of the unknown organism.