Download Sex Linked, Mapping and Pedigree Practice Problems

Genetics Assignment: Sex Linked Traits
Do the following questions and show all your work neatly!
1.
A recessive sex-linked gene (h) located on the X chromosome increases blood clotting time. This causes
hemophilia. A normal woman marries a hemophiliac man. They have 3 children. A normal girl and boy and a
hemophiliac girl. The boy marries a normal woman and they have a hemophiliac son.
a) Draw a pedigree of this family
b) Indicate the genotypes of the individuals in the family.
c) What is the probability that the first man and woman will have a son who is a hemophiliac? (Show your work)
d) What will be the results of mating between a normal (non-carrier) female and a hemophiliac male?
2.
In Drosophila, eye color is determined by two different genes located on different chromosomes. A recessive
gene (bb) found on the second chromosome produces brown eye color. The second chromosome is an
autosomal chromosome. A recessive sex linked gene (vv) causes vermillion eye color to be produced. The
presence of at least one dominant allele for each trait (B) and (V) results in wild color. The presence of both the
brown and vermillion alleles in the homozygous recessive form results in white eyes.
a) Indicate the phenotypes of the following:
i)
ii)
iii)
iv)
BBXVXV
BbXvXv
bbXVY
bbXvY
b) Indicate the genotypes and phenotypes produced from the following cross:
BbXvY x bbXVXV
3.
In a cross between a white eyed female fruit fly and wild (red-eyed) male, what percent of the female offspring
will have white eyes? (white eyes are X-linked recessive)
4. A human female “carrier” who is heterozygous for the recessive, sex linked trait causing red-green color
blindness, marries a normal male. What proportion of their male progeny will have red-green color blindness?
Mapping Practice
Answer the following questions in your notebook.
1. Working in the famous “fly room” at Columbia University, you cross a homozygous
recessive purple-eyed, vestigial-winged fruit fly (ppvv) with a heterozygous normaleyed, normal-winged fly (PpVv). Both genes are found on chromosome 2. You
discover the following percentages of flies in the F1 generation: 42 percent with
normal eyes and normal wings; 46 percent with purple eyes and vestigial wings; 6
percent with normal eyes and vestigial wings; and 6 percent with purple eyes and
normal wings.
a) What is the expected outcome for a cross involving two linked genes? Why do you
think your results differed from the expected outcome?
b) Which of the offspring are recombinant types?
c) Determine the recombination frequency for this cross.
d) Based on your findings, how many map units apart are the genes for eye colour
and wing type on chromosome 2?
2. In the same lab, your colleague is studying the genes for eye colour and body colour
found on chromosome 2. She crosses a homozygous recessive purple-eyed, blackbodied fruit fly (ppgg) with a heterozygous normal-eyed, normal-coloured fly
(PpGg). She counts 1000 offspring and finds 454 flies with normal eyes and normal
body colour, 466 flies with purple eyes and black body colour, 42 flies with normal
eyes and black body colour, and 38 flies with purple eyes and normal body colour.
a) What are the recombination frequency and map distance between the two genes?
b) From the data gathered by a third colleague, you know that the gene for wing type
and the gene for body colour are 4 map units apart. Combining your colleagues’
data with your own findings in question 1, draw a chromosome map showing the
linear arrangement of all three genes on chromosome 2.
3. In another experiment, you decide to cross a white-eyed female fruit fly (Xr Xr) with a
red-eyed male (XR Y).
a) Draw a Punnett square for this cross showing the F1 generation.
b) What is the phenotype ratio for this cross?
c) Which fly would you cross with the white-eyed male F1 offspring to get an F2
generation of all white-eyed flies? Use a Punnett square to support your answer.
4. In fruit flies, the following mutant genes have been identified: d causes short legs, vg
causes vestigial wings, and pr causes purple eye colour. All three genes reside on
chromosome 2.
Using linked gene notation, write the genotypes of fruit flies that are
a) homozygous recessive
b) heterozygous
c) homozygous dominant for these genes
5. A geneticist performs the following cross: vg d/+ +  vg d/vg d. Of 1000 offspring,
385 are wild type, 102 have vestigial wings and normal legs, 108 have normal wings
and short legs, and 405 have vestigial wings and short legs.
a) What is the map distance between the genes for wing type and leg length?
b) Previous data shows that the recombination frequency for the genes for wing type
and eye colour is 10 percent, while that for leg length and eye colour is 31 percent.
All three genes are found on chromosome 2. Draw a chromosome map showing the
relative distances between these linked genes.
Pedigree Practice
1. Marfan syndrome is an inherited condition that affects the connective tissue, resulting
in unusually long bones and spinal curvature, as well as vision, cardiac, and
respiratory problems. The syndrome tends to become increasingly severe over time.
The following pedigree shows inheritance of Marfan syndrome in a multigenerational
family.
a) How is this syndrome inherited?
b) Can you determine individual II4’s genotype? Explain.
c) Individual II1 and II2 are considering having another child. What is the probability
that this child will have Marfan syndrome? Explain using a Punnett square.
2. Examine the following pedigree showing the inheritance of straight hair in a fourgeneration family.
a) Is straight hair a dominant or recessive trait? Explain.
b) Identify the genotype of each individual in the pedigree. Whose genotype can you
not be certain of?
c) If individual V3 marries a man who is heterozygous, what is the probability that
they will have a girl with straight hair? Explain using a Punnett square.
3. Humans may have a peaked or smooth hairline. If a man and a woman both have a
smooth hairline, none of their children will have peaked hairlines. How is a peaked
hairline most likely inherited? Draw a pedigree for a family where one parent and two
of three children have a peaked hairline. One of the children with a peaked hairline
marries an individual with a smooth hairline. Their children both have a peaked
hairline. Identify the genotypes and phenotypes of each individual.
4. As a top research scientist, you and your colleagues have discovered a new sex-linked
recessive condition. In the course of your research, you have come across a pedigree
(see below) for a family in which the condition occurs.
a) On which chromosome is the allele for the condition found?
b) Which individuals in the pedigree can you be certain are carriers of the allele?
Explain your reasoning.
c) If individual III3 has a son with a woman who is not a carrier of the allele, what is
the probability that the son will have the condition? Explain.
5. In the pedigree below, different blood types are identified by the letters A, B, AB, and
O.
a) Individuals II4 and II5 have just had identical twin girls. List the possible blood
types these infants may have based on the information provided in the pedigree.
b) Individuals II6 and II7 have a second child with blood type O. What does this tell
you about II6’s genotype?
c) Could I1 and I2 have a child with the AB blood type? Explain why or why not.