A test cross was defined in a recent article as a way to detect a real nature or ability (known to vary within limits) of a living organism by the way of a cross between that organism and a tester genetically neutral in its contribution to the common offspring. Here the question is to know in which case a test cross can be the best choice for a genetic analysis. And the answer is we rather use this cross when we want:
To discover the genotype of an individual with a dominant phenotype. With a single locus for example such an individual may be dominant homozygous or heterozygous and you cannot tell which one is true just by looking at its phenotype.
To discover how many types of gametes a double heterozygote produces and then see if genes are linked or not. The expectations are: two types of gametes, both parental, with the same frequency (this means both loci are linked but no cross over occurred during meiosis), four types of gametes with the same frequency (when both genes have an independent behaviour but you do not know if it is because of a real physical independence or because of a linkage on a wide distance between them so that every single meiosis is made with a cross over), or four types of gametes in two groups having different frequencies (which means both loci are linked on a short distance and only some meiosis are associated with a cross over, so that at the end we have parental gametes over numbering recombined gametes).
To discover how many types of gametes a triple heterozygote produces and then see if genes are all linked, and if so see the order in which they are linked, or if only two loci out of three are linked and then identify them and compute the genetic distance between them, or if the three genes behave independently. Here we expect the target individual to produce height types of gametes in the same amount each (which shows an independent behaviour, true or faked), height types of gametes in four groups with different frequencies (this is the genetically known sign of a linkage between three loci, with a high frequency for both parental gametes, lower frequencies for the two kinds of simply recombined gametes and a very low frequency for both double recombined gametes), or any other number of gamete types in between, including just two types of parental gametes 50% each (in the case the three genes are linked but some how there is no single cross over during meiosis).