3.In Drosophila, the map positions of genes are given in map units numbering from one end of a
chromosome to the other. The X chromosome of Drosophila is 66 mu long. The X-linked gene
for body color—with two alleles, y
+
for gray body and y for yellow body—resides at one end of
the chromosome at map position 0.0. A nearby locus for eye color, with alleles w
+
for red eye
and w for white eye, is located at map position 1.5. A third X-linked gene, controlling bristle
form, with f
+
for normal bristles and f for forked bristles, is located at map position 56.7. Each
gene resides on the X chromosome, and at each locus the wild-type allele is dominant over the
mutant allele.
a. In a cross involving these three X-linked genes, do you expect any gene pair(s)
to show genetic linkage? Explain your reasoning.
b. Do you expect any of these gene pair(s) to assort independently? Explain your
reasoning.
c. A wild-type female fruit fly with the genotype y
+w
+
f/ywf
+
is crossed to a male fruit
fly that has yellow body, white eye, and forked bristles. Predict the frequency of
each progeny phenotype class produced by this mating.
d. Explain how each of the predicted progeny classes is produced.
4.Gene G recombines with gene T at a frequency of 7%, and gene G recombines with gene R
at a frequency of 4%.
a. Draw two possible genetic maps for these three genes, and identify the
recombination frequencies predicted for each map.
b. Assuming any desired genotype is available, propose a genetic cross whose
result could be used to determine which of the proposed genetic maps is correct.
5.On the Drosophila X chromosome, the dominant allele y
+ produces gray body color and the
recessive allele y produces yellow body color. This gene is linked to one controlling full eye
shape by a dominant allele lz
+ and lozenge eye shape with a recessive allele lz. These genes
recombine with a frequency of approximately 28%. The Lz gene is linked to gene F controlling
bristle form, where the dominant produces long bristles and the recessive produces forked
bristles. The Lz and F genes recombine with a frequency of approximately 32%.
a. Using any genotypes you choose, design two separate crosses, one to test
recombination between genes Y and Lz and a second between genes Lz and F.
Assume 1000 progeny are produced by each cross, and give the number of
progeny expected to be in each outcome category, male and female. (Remember
that Drosophila male X chromosomes do not undergo recombination.)
b. Will either of these crosses (or any other possible cross) reveal linkage between
gene Y and gene F? Why or why not?
c. Why is “independent assortment” the genetic term that best describes the
observations of a genetic cross between genes Y and F?
6,Nail-patella syndrome is an autosomal disorder affecting the shape of nails on fingers
and toes as well as the structure of kneecaps. The pedigree below shows the
transmission of nail-patella syndrome in a family along with ABO blood type.
a. Is nail-patella syndrome a dominant or a recessive condition? Explain your
reasoning.
b. Does this family give evidence of genetic linkage between nail-patella syndrome
and ABO blood group? Why or why not?
c. Using N and n to represent alleles at the nail-patella locus and I
A
, I
B
, and i to
represent ABO alleles, write the genotypes of I-1 and I-2 as well as their five
children in generation II.
d. Explain why III-6 has nail-patella syndrome and III-8 does not. Give complete
genotypes for these two individuals.
e. Explain why III-11 has nail-patella syndrome and III-12 does not. Give genotypes
for these two individuals.
1.Fifteen bacterial colonies growing on a complete medium are replica-plated to a minimal medium. Twelve of
the colonies grow on minimal medium.
a. Using terminology from the chapter, characterize the 12 colonies that grow on minimal medium and
the 3 colonies that do not.
b. The three colonies that do not grow on minimal medium are replica-plated to minimal medium plus
the amino acid serine (min + Ser), and all three colonies grow. Characterize these three colonies.
c. The serine biosynthetic pathway is a three-step pathway in which each step is catalyzed by the
enzyme product of a different gene, identified as enzymes A, B, and C in the diagram below.
3-Phosphoglycerate 3-Phospho-hydroxypyruvate 3-Phosphoserine Serine
Mutant 1 grows only on min + Ser. In addition to growth on min + Ser, mutant 2 also grows on min + 3-PHP
and min + 3-PS. Mutant 3 grows on min + 3-PS and min + Ser. Identify the step of the serine biosynthesis
pathway at which each mutant is defective.
1.Fifteen bacterial colonies growing on a complete medium are replica-plated to a minimal medium. Twelve of
the colonies grow on minimal medium.
a. Using terminology from the chapter, characterize the 12 colonies that grow on minimal medium and
the 3 colonies that do not.
b. The three colonies that do not grow on minimal medium are replica-plated to minimal medium plus
the amino acid serine (min + Ser), and all three colonies grow. Characterize these three colonies.
c. The serine biosynthetic pathway is a three-step pathway in which each step is catalyzed by the
enzyme product of a different gene, identified as enzymes A, B, and C in the diagram below.
3-Phosphoglycerate 3-Phospho-hydroxypyruvate 3-Phosphoserine Serine
Mutant 1 grows only on min + Ser. In addition to growth on min + Ser, mutant 2 also grows on min + 3-PHP
and min + 3-PS. Mutant 3 grows on min + 3-PS and min + Ser. Identify the step of the serine biosynthesis
pathway at which each mutant is defective.