When mating two heterozygotes for alleles in which one is dominant to the other, if the usual Mendelian segregation process is occurring, the ratio of the offspring phenotypes produced should be 3:1. The dominant phenotype would be more common than the recessive one. Imagine a situation in which two heterozygotes are mated and among their 200 offspring, 165 show the dominant phenotype while 35 show the recessive phenotype. What is the P-value of a two-sided binomial test using the normal distribution to approximate the binomial?

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Quizplus · Accepted Answer

The expected ratio of dominant to recessive phenotypes is 3:1, meaning out of 200 offspring, 150 should show the dominant phenotype and 50 the recessive. The observed numbers are 165 dominant and 35 recessive. Using a two-sided binomial test with these observed and expected frequencies, the P-value closest to the result of such a test is 0.018, indicating the probability of observing such a deviation from the expected 3:1 ratio by chance is low.

When Mating Two Heterozygotes for Alleles in Which One Is