ass 1

HUMAN MOLECULAR GENETICS
(MMED3935)

2011

Assignment 1
Worth 15% of the total topic assessment




Written answers to the questions are required
outlining the reasoning for your answers




DUE DATE: 4.00 pm Monday 29th August 2011



Please use Assignment Cover Page and submit Assignment 1
to:
School of Medicine Teaching and Learning Support Unit
Reception Desk, adjacent to the Medical Library,
Level 5, Flinders Medical Centre



Late submissions should be handed directly to Dianne Luke,
Department of Haematology and Genetic Pathology,
Room 6D201, Level 6, Flinders Medical Centre

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Question 1
(30 marks)

Depending on the location and function of the gene involved, human genetic
disorders can generally display one of the following modes of inheritance:

Autosomal dominant inheritance

Autosomal recessive inheritance

X-linked dominant inheritance

X-linked recessive inheritance

Mitochondrial inheritance

However, in real life, human pedigrees often display patterns of inheritance
that do not neatly fall into classical Mendelian patterns. This can be because
there is insufficient evidence to unambiguously assign one particular mode of
inheritance, or because other factors intervene.

For each of the following 3 pedigrees, and considering each of the above
modes of inheritance, explain

1) which features of the pedigree fit with the mode of inheritance;
2) which features of the pedigree don't fit with the mode of inheritance, and
what genetic phenomenon or event could explain those features which don't
fit; and
3) the effect the sex of the fetus (shown on each pedigree as ) has on the
risk to the fetus.

Briefly justify your answers.


Example partial answer

Pedigree 1

Autosomal dominant
1) Both sexes are affected and approximately half the children are affected.
2) Individual II4 should be affected, as she has an affected child. This could
be explained by non-penetrance in individual II4.
3) Sex of the fetus has no effect on risk if this is an autosomal disorder.

Autosomal recessive
1) Both sexes are affected, which fits with autosomal recessive inheritance.
Also 50% of children in generation II are affected, which is expected from a
cross between a homozygous affected individual and a heterozygous carrier.
2) If this was autosomal recessive, II5 would need to be a carrier. If the
disease allele was common in the population, this would explain a high carrier
frequency.
3) Sex of the fetus has no effect on risk if this is an autosomal disorder.

X-linked dominant
1) ...etc.

2

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Pedigree 1


I
1
2
II
1
2
3
4
5
III
1
2





Pedigree 2



I
1
2
II
1
2
3
III
1





3

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Pedigree 3




I
1
2
II
1
2
3
4
5
6
III
1
2
3
4
5
6









4

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Question 2
(10 marks)


The following five karyotypes have all resulted from chromosomal non-
disjunctional events.

45, X
45, Y
47, XXX
47, XYY
47, XXY


For each of the above karyotypes :

a) In which parent (mother, father, or either parent) did the non-disjunctional
event occur? Explain your reasoning.

b) How many Barr bodies would be observed in cells with this karyotype?
Explain your reasoning.

c) Is this karyotype viable? If viable, what would be the phenotype for this
karyotype?





Question 3
(10 marks)



An individual was found to have the following karyotype


45, XX, -21, -21, + i(21q,21q)


a) Explain in words what this karyotype means.

b) What are the possible outcomes from a pregnancy arising from a union
between this person and a person who has a normal karyotype?





End of Assignment

5

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