Download Genetics 275 Problem Assignment #3 March 2001

Genetics 275
Problem Assignment #3
March 2001
1. In a strain of garden peas, mutant allele a is recessive to its wild type allele a+ (designated simply
as + below); similarly mutant alleles b, c, and d are recessive to b+, c+, and d+ (also shortened to
+ in the data set below). When F s from a cross between two true-breeding lines were test
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crossed, they gave the following test cross progeny:
Phenotype
Number
++++
399
a+++
73
+b++
38
++c+
3
+++d
388
ab++
3
a+c+
36
a++d
72
+bc+
71
+b+d
39
++cd
3
abc+
382
ab+d
3
a+cd
35
+bcd
72
abcd
383
Σ=
2000
a) From the above data, deduce and list all of the genotypes of the possible sets of parents of the F .
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b) Calculate and draw the best genetic map possible for these data.
c) Determine the amount of genetic interference, if any.
2. At least four of the genes necessary for histidine biosynthesis are known in Neurospora. As
shown below, each of the four loci is located on a different chromosome. The map unit distances
are not drawn to scale on the chromosome figures below. O = centromeres and ! = the site of the
respective loci.
his-1
ade-3
his-2
_________!________!_O_________
______________O_____!________________
1m.u.
1 m.u.
his-3
_________O__________________!_____________
his-4
___O_________________________!_______
10 m.u.
40 m.u.
The his-1 gene is closely linked (1 m.u.) to both ade-3 and the centromere. The his-2, his-3, and his-4
genes are 1, 10, and 40 m.u., respectively, from their centromeres. In an experiment, you begin with
an ade-3 mutant strain from which you recover a strain that also requires histidine. Assume this his+
to his- change was due to a mutational event. You wish to determine which of the four his genes is
involved (ie. the new mutation). You cross the ade-3 his? strain with a wild type (+, +) strain and
analyze only twelve tetrads: three are PD, six are TT, and three are NPD. From these rather scanty
data, which of the four his genes is most probably the one mutated to his- in the original ade-3 strain?
Why?
3. In domestic cats at least one gene affecting fur color is sex-linked (on the X chromosome). One
allele of this gene determines an orange coat color while another allele determines a black coat
color. A Tortoise shell cat is described as a mosaic with orange and black patches.
a) A tortoise shell cat has a litter of eight kittens: one orange male, two black males, two orange
females and three tortoise shell females. What is the probable color of the father of this litter?
b) A black cat has a litter of seven kittens: three black males, one black female, and three tortoise
shell females. Comment on the probable paternity of this litter.
c) An orange cat has a litter of six kittens: three orange and three tortoise shells. What is the
probable sex of the orange kittens and why?
d) Why isn’t it possible to get a true-breeding line of tortoise shell cats?
4. In Drosophila, the X-linked genes cut (ct), lozenge eye (lz) and forked bristle (f) are the following
map distances apart: ct to lz is 7.7 m.u., lz to f is 29 m.u. and lz is the middle gene on the map.
Assuming that there is no genetic interference, what are the expected numbers of each of the eight
genotypic classes out of 1000 flies recovered from a cross between a triple heterozygous female (when
one of her parents was from a true-breeding triple mutant strain) and a test-cross male? You may take
your answer to the nearest whole fly when necessary. Repeat the above question but assume that the
interference is such that the observed D C/O are only ½ of the expected D C/O.