Genetics 371B, Autumn 2000 -- Week 2 Practice problems

Genetics 371B Practice problems--Autumn 2000 week 2

Questions 2, 5, and 11 will be covered in quiz section.

Three Drosophila questions to start off with...

(a)

Female fruit flies with normal wings were crossed to males with malformed "cut" wings. The F₁ progeny were:

1/4 normal females
1/4 cut-wing females
1/4 normal males
1/4 cut-wing males

State if each of the following modes of inheritance can explain these results, and if so, what the genotypes of the parents must be.

(i) Cut wing is autosomal recessive

(ii) Cut wing is autosomal dominant

(iii) Cut wing is X-linked recessive

(iv) Cut wing is X-linked dominant

(b)

The normal F₁ female progeny from the cross in (a) were crossed to the normal F₁ males. The resulting F₂ progeny were:

1/2 normal females

1/4 normal males

1/4 cut-wing males

What is the mode of inheritance of cut wing, and what were the genotypes of the parents, F₁, and F₂?

(c)

The cut-wing F₂ males were crossed to the normal F₂ females. What fraction of the crosses will give at least some cut-wing flies in the progeny? (--i.e., for each cross, ask yourself -- will there be cut-wing flies among the progeny, yes or no?) In those crosses where cut-wing progeny are seen, what will be the proportions of male and female, normal and cut-wing flies?

Bar-eyed female fruit flies were crossed to normal males. The F₁ progeny were:

1/4 normal females
1/4 bar-eyed females
1/4 normal males
1/4 bar-eyed males

The normal F₁ female progeny were crossed to the bar-eyed F₁ males. The F₂ progeny were: 1/2 bar-eyed female, 1/2 normal male. Explain the results. Your explanation should state the mode of inheritance and give the genotypes of all the flies.

3.	Female flies with club feet were crossed with normal males. The progeny were: 312 normal females 321 normal males 304 club-foot females Explain the results. Your explanation should state the mode of inheritance and give the genotypes of all the flies.

4.	Why can the disease in this pedigree not be X-linked recessive? (Assume that the disease is rare.)

For each of the following modes of inheritance, state whether you can find evidence in the pedigree against that mode of inheritance. Give reasons. Assume that the disease is rare and that there is complete expressivity and penetrance.

(i) autosomal recessive
(ii) autosomal dominant
(iii) X-linked recessive
(iv) X-linked dominant
(v) Y-linked
(vi) sex-influenced
(vii) sex-limited

How many distinct gamete classes (genotypes) do you expect from a diploid organism of genotype Aa Bb Dd EE ff? Assume independent assortment of the determinants.

The diploid number in Drosophila is 8 -- four chromosomes are paternally derived and four are maternal. What is the probabilty that a Drosophila female will pass on in an egg none of the paternally derived centromeres? (Assume that there is no aberrant event.)

The location of a gene on a particular plant chromosome is shown. Using A and a to designate dominant and recessive alleles of this gene, diagram the chromosomes for

(i) a homozygous dominant in the G₁ phase of a vegetative cell cycle

(ii) a homozygous recessive in the G₂ phase of a vegetative cell cycle

(iii) a heterozygote in the G₂ phase of a vegetative cell cycle

Your diagrams should be accurate with respect to the number of chromosomes and the alleles marked on the chromosomes.

Using the heterozygote from question 8 as the starting point, draw a simple meiosis diagram showing metaphase I, the end of meiosis I, metaphase II, and the end of meiosis II. Again, your representation should be accurate with respect to the number of chromosomes/chromatids and the alleles marked (and that's all this question is looking for). Don't worry about crossovers in your diagram.

10.	A couple, both heterozygous for albinism, have five children.
	(a)	What is the probability that the first child will be a son, and the next four children will be daughters?
	(b)	What is the probability that the children will have the following phenotypes in the order stated? 1st child = normal; 2nd child = normal; 3rd child = albino; 4th child = albino; 5th child = albino
	(c)	What is the probability that of the five children, two will be normal and three albino, in any order?
	(d)	What is the probability that all five will be normal?

11.	You are hired by a friendly neighborhood billionaire to go to a tropical island and work on a secret project. You get there and find that the project involves breeding dinosaurs--Tyrranosaurus rex in particular. Your predecessor has disappeared under mysterious circumstances, but from her lab notebook you find out that the T. rex specimens come in two varieties. One variety is tall and a lovely shade of green. Unfortunately (say her notes), it is clawless and squeaks when it tries to roar. The other variety is a puny gray kind, but it has impressive claws and a mighty roar all out of proportion with its small body. A cross between the two varieties produced a clutch of babies that all grew up to be tall, gray, clawed squeakers. Your predecessor had disappeared before she could obtain F₂ progeny.
	(a)	Explain the result of the cross between the two varieties, giving the genotypes of the parents and the offspring. (Assume independent assortment.)
	(b)	What phenotypic classes do you expect in the F₂ progeny, and in what proportions?
	(c)	Your predecessor was hoping to obtain a true-breeding line of tall, green, clawed roarers. You decide it might be wiser to go for true-breeders of the puny, green, clawless, squeaky kind. Do you expect to obtain either kind of true-breeder in the F₂? If so, what fraction of the F₂ do you expect for each kind?

12.

This is an exam question from last year. Not very many people got part (a). The answers are in the back of your lecture notes, but try not to peek!

(a)

Hemophilia is an X-linked trait in humans. Using long lines for X chromosomes and short lines for Y, show the possible sex chromosome configurations that might be seen at the end of Anaphase I of Meiosis in a Klinefelter male who is heterozygous for hemophilia (X^HX^hY). Your diagrams should be accurate with respect to the number of chromatids , and show the appropriate alleles of the hemophilia locus. (For this part of the question, ignore the temporary checkpoint delay. Also ignore the autosomes. And don't worry about his sterility or fertility!)

(b)

A tissue culture cell line was established from a normal (XY) male. Tissue culture cells that were just about to enter mitosis were incubated with radioactively labeled DNA precursors. Cell samples were withdrawn at regular intervals, and the amount of radioactive DNA in each sample was plotted:

(i)	In the plot, circle and label every S phase that you can detect.
(ii)	Approximately how many hours does one complete cell cycle last in the XY cells? (Plus or minus ~2 hours is good enough.)
(iii)	Do you think the presence of the extra X chromosome in XXY cells will cause a mitotic checkpoint delay? Explain in ONE sentence.