2 AaBBx AABb a Possible gamete genotypes for AaBB b Possible gamete genotypes for AABB c Punnett square d Offspring genotype ratio You can go further If you know the total number of offspring that result from the mating you can determine how many of them should have a specific genotype assuming that your understanding of the genetic system is correct For example if a pair of fruit flies following this genetic system had 40 offspring we would expect that 40 0 25 10 of the offspring would have genotype AA We can use this to test hypotheses about the relationships between different genes alleles and even to determine possible parent genotypes In fact this is the basic logic of paternity tests used in courts of law tabloid talk shows STATISTICALLY COMPARING GENOTYPES The chi square test The chi square x pronounced kai square test is very useful in Mendelian genetics Categorical data are usually counts of occurrences in different mutually exclusive categories Each genotype is a category as are phenotypes Thus these are categorical data and a chi square test is appropriate This chi square test uses the observed numbers and the numbers expected if the results follow the hypothesis predicted by the Punnett Square which follows Mendelian principle of independent assortment to calculate the probability that the observed results could have occurred by chance The chi square test is statistically testing these hypotheses when analyzed with Mendelian genetics ratio data Ho The observed ratio is not different from the expected ratio as predicted by Mendelian genetics Ha The observed ratio is different from the expected ratio as predicted by Mendelian genetics