The degrees of freedom associated with the chi-square test (Chapter 3) are n — 1, where n equals the number of expected classes. Here, there are two expected phenotypes; so the degree of freedom is 2 — 1 = 1. Looking up our calculated chi-square value in Table 3.4, we find that the probability associated with this chi-square value is between .30 and .20. Because the probability is above .05 (our critical probability for rejecting the hypothesis that chance produces the difference between observed and expected values), we conclude that there is no significant difference between the 1: 1 ratio that we expect in the progeny of the testcross and the ratio that we observed.

We next compare the observed and expected ratios for the second locus, which determines the type of wing. At this locus, a heterozygote and homozygote also were crossed (cv+cv X cvcv) and are expected to produce 1/2 cv+cv straight-winged progeny and 1/2 cvcv curved-wing progeny. We actually observe 63 + 32 = 95 straight-winged progeny and 77 + 28 = 105 curved-wing progeny; so the calculated chi-square value is:

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