what is a test cross used for and how do you know what genotypes to cross?

A test cross is used to discover the genotype of an individual by crossing two parents to see the phenoypic ratio of the offspring produced. A homozygous recessive individual (one with two recessive or small letter alleles) should be crossed with the unknown genotype individual. This is because the homozygous recessive individual will not influence the phenotype of the offspring because it only passes on the recessive allele. Remember that recessive means that the trait will be masked by a dominant allele (capital letter) and will only be shown phenotypically if the individual has two recessive alleles which is known as being homozygous recessive. So this cross will produce offspring who's phenotype are determined by the alleles of the other parent. You can therefore determine the genotype of this other parent by looking at the phenotype of the ofspring.