1. How would you identify sex-linked mutations in the magpie moth (a species in which the female is the heterogametic sex)?

2. Can you name a nongenetic characteristic that is inherited in families in many societies as a sex-linked, strict paternal-effect trait?

    

3. Candice Cane has been researching peppermint stick breath disease (PSB), a rare and completely fictitious genetic disease in humans. Two people afflicted with the disease, Peter and Penelope, are thinking of having children and are worried that the children will be afflicted. They seek Candy's advice and she collects the following pedigrees from them.

1. What conclusions do you draw about the inheritance of PSB from each pedigree separately?

2. Based on your answer for (a), what is the probability of Penelope and Peter's child having the disease if it is (i) a boy or (ii) a girl? Explain your answer.

3. If Peter and Penelope have a daughter, what is the probability of her having an affected child (assuming that she does not marry into a family with a history of PSB)?  Will the sex of her child make a difference for your answer?

4. Females from each of the indicated true-breeding Drosophila strains (each having a different phenotype, e.g., Strain A has the A phenotype) are mated to wild-type males. The males and females from this cross are mated to each other. What can you say about the expression (i.e., dominant or recessive) and the linkage (sex-linked vs. autosomal) of the underlying mutations from the following results? Explain your answers by indicating the genotypes of the F1 animals and by giving a Punnett Square showing their expected progeny.

1. Strain A: 3/4 females are A and 3/4 males are A

2. Strain B: 1/2 females are B and 1/2 males are B

3. Strain C: All females are C and 1/2 males are C.

5. Females of a true-breeding Drosophila strain showing the A phenotype (i.e., they are mutant in gene a) are mated to males of another true-breeding strain showing the B phenotype (i.e., they are mutant in gene b). The males and females from this cross are mated to each other. What can you say about the expression and the linkage of the a and b mutations from the following results?  Explain your answers by indicating the genotypes of the F1 animals and by giving a Punnett Square showing their expected progeny.

1. 1/4 females are A        1/4 males are A

3/4 females are B        3/4 males are B

2. 1/4 females are A        1/4 males are A

1/2 females are B        1/2 males are B

3. All females are A        1/2 males are A

3/4 females are B       3/4 males are B

6. You have obtained a true-breeding mutant strain of Caenorhabditis elegans. By looking at the progeny from a mating of mutant hermaphrodites with wild-type males you should be able to determine (i) whether a mutation is dominant or recessive and (ii) if it is recessive, whether it is X-linked or autosomal.

1. Demonstrate how these conclusion can be drawn by indicating what progeny you expect from this mating. Remember that the hermaphrodites will also produce self-progeny.

2. Why will this mating not tell you whether a dominant mutation is autosomal or X-linked?

3. Mating the males from this cross to wild-type hermaphrodites, however, will allow you to determine whether a dominant mutation is autosomal or X-linked. Explain how.

7. In 1921, geneticist Lilian V. Morgan discovered an unusual chromosome in Drosophila that was formed from the joining of two X chromosomes. This chromosome, called an attached X, is symbolized X^X. The attached X always segregates as a unit (i.e., it doesn't break apart).

1. When X^X females are mated to wild-type males, fertile females and sterile males are produced. Why (a Punnett Square might help)?

2. What can you say about the fertility of this cross compared to wild-type (by fertility I mean the number of progeny that will be produced). Explain your answer.

3. How could you use an attached X female to determine that a mutation is sex linked? How would the segregation pattern be different from that of an autosomal, recessive mutation and an autosomal, dominant mutation?

4. How does the pattern for sex-linkage in (c) differ from that normally found in Drosophila?

5. A better experiment than that in (c) is one in which the females from the mating in (a) are used for the sex-linkage studies. What progeny would you expect and why is this a better experiment?

8. Assume that you have been given a strain of Drosophila that has an elevated level of nondisjunction of the X chromosome and no recombination (these properties make the analysis easier). By mating females that are heterozygous for two X-linked mutations, a and b, and homozygous for a third mutation c (i.e.,they are a+c/+bc) and males that are hemizygous for a and b (i.e., they are abY), you should be able to determine whether nondisjunction occurs at the first or second meiotic division. How could you do this?  (Hint: draw out what you expect when nondisjunction occurs and when it does not.) Why is the mutation c needed?

9. Answer TRUE or FALSE. If the statement is TRUE, explain or give a supporting example. If the statement is FALSE, either correct the statement or give a counterexample.

1. The same collection of chromosomes is found in each cell at the end of mitosis and meiosis I.

2. Nondisjunction can cause trisomy.

3. In species with two sexes, males are always the heterogametic sex.

4. If a+c/+bc Drosophila females are mated to abY males (where a and b are recessive mutations), nondisjunction in the female germ line will be seen by the production of females whose phenotypes are A or B.

5. Meiosis II is longer than meiosis I.

6. Nondisjunction of the chromosome III (which contain the sex-determination gene tra-1) results in males in C. elegans.

7. Trisomy always leads to lethality.

8. For maternal effect mutations the genotype of the mother causes a phenotype in the progeny.

9. In ZW/ZZ animals the heterogametic sex is male.

Answers