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Now that we have the chi-square value, we must determine the probability of this chi-square value being due to chance. To obtain this probability, we first calculate the degrees of freedom, which for a goodness-of-fit chi-square test are n — 1, where n equals the number of expected phenotypic classes. In this case, there are four expected phenotypic classes; so the degrees of freedom equal 4 — 1 = 3. We must now look up the chi-square value in a chi-square table (see Table 3.4). We select the row corresponding to 3 degrees of freedom and look along this row to find our calculated chi-square value. The calculated chi-square value of 2.7 lies between 2.366 (a probability of .5) and 6.251 (a probability of .1). The probability (P) associated with the calculated chi-square value is therefore .5 < P < .1. This is the probability that the difference between what we observed and what we expect is due to chance, which in this case is relatively high, and so chance is likely responsible for the deviation. We can conclude that the progeny do appear in the 1 : 1 : 1 : 1 ratio predicted by our genetic explanation.

COMPREHENSION QUESTIONS]_

* 1. Why was Mendel's approach to the study of heredity so successful?

2. What is the relation between the terms allele, locus, gene, and genotype?

* 3. What is the principle of segregation? Why is it important?

4. What is the concept of dominance? How does dominance differ from incomplete dominance?

5. Give the phenotypic ratios that may appear among the progeny of simple crosses and the genotypes of the parents that may give rise to each ratio.

6. Give the genotypic ratios that may appear among the progeny of simple crosses and the genotypes of the parents that may give rise to each ratio.

* 7. What is the chromosome theory of inheritance? Why was it important?

8. What is the principle of independent assortment? How is it related to the principle of segregation?

9. How is the principle of independent assortment related to meiosis?

10. How is the goodness-of-fit chi-square test used to analyze genetic crosses? What does the probability associated with a chi-square value indicate about the results of a cross?

11. What is incomplete penetrance and what causes it?

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