3.4 - D - Test Crossing
Monohybrid crosses and Punnett squares can be used to determine the genotype of an unknown organism. When doing a test cross, the unknown organism should always be crossed with a homozygous recessive individual. See the following example.
The purple flower shown at the top can be one of two genotypes (PP or Pp). In order to figure this out, we can cross the purple flower, with a white one (pp). Then, based on the results we can deduce what the genotype of the original flower was. If it was PP, then all offspring will be purple. If it is Pp, then 50% will purple and 50% will be white.
Predicted vs. Actual Outcomes
Actual outcomes of genetic crosses do not always correspond with predicted figures. This is because no matter what, there is always an element of chance concerning whether or not something will occur. With enough samples, though, the observed number typically reflects the prediction. Later, we will discuss how to determine statistic significance.