Chapter 3

Mendel crossed peas having round green seeds with peas having wrinkled yellow seeds. All ${\mathrm{F}}_1$ plants had seeds that were round and yellow. Predict the results of testcrossing these $F_1$ plants.

Thalassemia is an inherited anemic disorder in humans. Affected individuals exhibit either a minor anemia or a major anemia. Assuming that only a single gene pair and two alleles are involved in the inheritance of these conditions, is thalassemia a dominant or recessive disorder?

The following are ${\mathrm{F}}_2$ results of two of Mendel's monohybrid crosses. (a) full pods 882 constricted pods 299 (b) violet flowers 705 white flowers For each cross, state a null hypothesis to be tested using ${\chi }^2$ analysis. Calculate the $x^2$ value and determine the $p$ value for both. Interpret the $p$ values. Can the deviation in each case be attributed to chance or not? Which of the two crosses shows a greater amount of deviation?

In assessing data that fell into two phenotypic classes, a geneticist observed values of $250:150.$ She decided to perform a $x^2$ analysis by using the following two different null hypotheses: (a) the data fit a 3: 1 ratio, and (b) the data fit a 1: 1 ratio. Calculate the $x^2$ values for each hypothesis. What can be concluded about each hypothesis?

The basis for rejecting any null hypothesis is arbitrary. The researcher can set more or less stringent standards by deciding to raise or lower the $p$ value used to reject or not reject the hypothesis. In the case of the chi- square analysis of genetic crosses, would the use of a standard of $p=0.10$ be more or less stringent about not rejecting the null hypothesis? Explain.

For decades scientists have been perplexed by different circumstances surrounding families with rare, early-onset auditory neuropathy (deafness). In some families, parents and grandparents of the proband have normal hearing, while in other families, a number of affected (deaf) family members are scattered throughout the pedigree, appearing in every generation. Assuming a genetic cause for each case, offer a reasonable explanation for the genetic origin of such deafness in the two types of families.

Tay-Sachs disease (TSD) is an inborn error of metabolism that results in death, often by the age of 2. You are a genetic counselor interviewing a phenotypically normal couple who tell you the male had a female first cousin (on his father's side) who died from TSD and the female had a maternal uncle with TSD. There are no other known cases in either of the families, and none of the matings have been between related individuals. Assume that this trait is very rare. (a) Draw a pedigree of the families of this couple, showing the relevant individuals. (b) Calculate the probability that both the male and female are carriers for TSD. (c) What is the probability that neither of them is a carrier? (d) What is the probability that one of them is a carrier and the other is not? [Hint: The $p$ values in (b), (c), and (d) should equal $1.]$

To assess Mendel's law of segregation using tomatoes, a truebreeding tall variety (SS) is crossed with a true-breeding short variety (ss). The heterozygous ${\mathrm{F}}_1$ tall plants (Ss) were crossed to produce two sets of ${\mathrm{F}}_2$ data, as follows. $\begin{array}{cc}\text{ Set I }& \text{ Set II }\\ 30\text{ tall }& 300\text{ tall }\\ 5\text{ short }& 50\text{ short }\end{array}$ (a) Using the $x^2$ test, analyze the results for both datasets. Calculate ${\chi }^2$ values and estimate the $p$ values in both cases. (b) From the above analysis, what can you conclude about the importance of generating large datasets in experimental conditions?