When two heterozygotes are mated, the ratios of the offspring produced should be in a 1:2:1 ratio if normal Mendelian segregation is occurring. If one of the alleles is dominant, then the phenotypes observed should be present in a 3:1 ratio with the dominant phenotype more common than the recessive one. We can use a χ² goodness-of-fit test to test whether the ratio of offspring is indeed 3:1. Imagine a cross is performed and the number of offspring observed are 67 with the dominant phenotype and 33 with the recessive phenotype. What is the χ² value we would obtain?

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Quizplus · Accepted Answer

To calculate the χ^2 value, we first determine the expected numbers based on the 3:1 ratio for 100 offspring (75 dominant:25 recessive). Then, χ^2 = Σ[(observed - expected)^2 / expected]. For dominant: (67-75)^2 / 75 = 0.853, and for recessive: (33-25)^2 / 25 = 2.56. Adding these gives 0.853 + 2.56 = 3.413.

When Two Heterozygotes Are Mated, the Ratios of the Offspring