Genetics 371B, Autumn 1999

Problem set 2 -- based on lectures 5-8

Due Fri Oct. 15 at the start of class

1.

Wings in Drosophila can be bent (bw) or normal (bw+); eyes can be star-shaped (s) or normal (s+); and bristles can be forked (f) or normal (f+). Star-eyed, forked-bristle males were crossed with star-eyed, bent-wing females. (If a phenotype is not mentioned, assume that it is normal e.g., the males had normal wings and the females had normal bristles.) The F1 progeny consisted of 118 star-eyed flies and 42 completely normal flies. Males and females were present in equal proportions in each phenotypic class. Normal F1 males were crossed with normal F1 females, yielding progeny of the following phenotypes with respect to wing and bristles:

Phenotype and sex Number of progeny
normal females 1212
bent-wing females 398
normal males 602
forked-bristle males 590
bent-wing males 203
bent-wing, forked bristle males 195

(Each individual cross gave the same phenotypes of progeny and in the same proportions.)

(a) Do you expect to see star-eyed flies in these F2 progeny? Why, or why not?
(b) What is the mode of inheritance for bent wing and forked bristle? Briefly explain how you arrived at your answer, giving the genotypes for all three generations.
(c) Perform a chi-square test to see if the F2 numbers match your proposed modes of inheritance for the two traits. Show your work, including the chi-square calculations, the number of degrees of freedom, the P value you get, and your interpretation of what that P value means.

2. What is the probability that a girl has inherited a complete set of chromosomes from her maternal grandmother? What is the probability that a boy has inherited a complete set of chromosomes from his paternal grandfather? [Suggestion: If this question confuses you, take just one pair of homologs for either the boy or the girl, and trace it back up the generations to see where those two homologs might have come from. Then think about a full chromosome set.]

3. The following pedigrees (from separate families) show inheritance of two independently assorting traits. Pedigree 1 shows inheritance of male pattern baldness, which is a sex-influenced trait: in males, the allele that promotes baldness (Hb) is dominant to the allele that allows a full head of hair (Hh), while in females Hh is dominant to Hb. Pedigree 2 shows inheritance of Achoo syndrome (filled symbols), the tendency to sneeze in response to bright light (e.g., looking at the sun).
 
 
(a) What are the genotypes of the individuals in Pedigree 1?
(b)
For Pedigree 2, which of the following modes of inheritance can you eliminate, and why?
(i)
autosomal recessive
(ii)
autosomal dominant
(iii)
X-linked recessive
(iv)
X-linked dominant
(v)
Y-linked
(vi)
sex-limited
(vii)
sex influenced, dominant in males
(viii)
sex-influenced, dominant in females
 
(c)
If individual III-4 from Pedigree 1 marries IV-1 from Pedigree 2, what is the probability that their first child will be:
(i)
a bald, sneezing male?
(ii)
a bald, sneezing female?
(iii)
a hairy, sneezing male?

4. After talking to a patient who has come in for counseling, a genetic counselor draws up a pedigree for the patient's family. Which of the modes of inheritance listed in question 3(b) can explain this pedigree? (For each mode that you eliminate, briefly explain why you did so.) Suggest three independent explanations for this pedigree.

5.

Fleet-footedness in the Puyallup Pig is controlled genetically (pigs can be fast or slow), as is Oreo-craving (pigs can love or hate Oreo cookies). The two traits segregate independently. Two crosses are performed. In Cross 1, true-breeding slow Oreo-lovers are mated with true-breeding slow Oreo-haters. In Cross 2, true-breeding fast Oreo-lovers are mated with true-breeding slow Oreo-haters. The progeny from Cross 1 are mated with the progeny from Cross 2. The resulting progeny are as follows:

Phenotype Number of progeny
fast Oreo-hater 61
slow Oreo-hater 58
slow Oreo-lover 22
fast Oreo-lover 19

Explain these crosses (genotypes and phenotypes, dominant vs. recessive), using F and f to describe dominant and recessive with respect to speed, and L and l for dominant and recessive with respect to Oreo-loving.

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