Biology of Disease

X]VeiZg&*/ GENETIC DISEASES

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duplications, inversions and translocations. Changes to the chromosomal

complement include aneuploidy and euploidy. Down syndrome is the

commonest aneuploid condition and is characterized by trisomy of the

whole, or part of, chromosome 21. Genetic disease can be diagnosed using

molecular techniques, including the use of the polymerase chain reaction

and DNA probes in microarrays. Once a genetic defect has been diagnosed,

genetic counseling can be offered to affected individuals as part of the

treatment process.

QUESTIONS

1. WhichONEof the following statements about phenylketonuria is
   INCORRECT?

a) PKU sufferers should be placed on a diet low in phenylalanine.

b) PKU has an autosomal dominant mode of inheritance.

c) Individuals with untreated PKU may suffer from mental retarda-

tion.

d) PKU has an incidence of around 1:10 000 in the UK.

e) PKU sufferers have insufficient melanin in their skin and hair.

2. Which of the following has an aneuploid karyotype?

a) A male with a balanced Robertsonian translocation involving

chromosomes 13 and 21.

b) A male sufferer of phenylketonuria.

c) A female sufferer of fatal familial insomnia.

d) A female with a balanced translocation involving chromosome

arms 11q and 22q.

e) A male with Klinefelter’s syndrome.

3. Duchenne muscular dystrophy is an X-linked recessive trait. On aver-
   age, what proportion of the children of a normal father and carrier
   mother would be affected?


4. A female is heterozygous for fragile X syndrome. Describe the inherit-
   ance pattern you would expect to see in her children, given the father
   is a normal male.


5. Autosomal dominant and X-linked dominant diseases can both be
   passed on to children. Suggest characteristics that would help identify
   whether a particular syndrome was inherited in an autosomal or X-
   linked fashion.


6. Examine the three accompanying karyotypes (A)–(C). In each case,
   name the syndrome caused by these karyotypes.

Further Reading

Bahado-Singh, RO, Oz, U, Kovanci, E, Cermic, D, Flores, D, Copel, J, Mahoney,

M and Cole, L (1998) New triple screen test for Down syndrome: combined

urine analytes and serum AFP. J. Maternal-Fetal Med.7: 111

 114.

Y

A)

1 2 3 4 5

6 7 8 9 10

11 12 13 14 15

16 17 18 19 20

21 22 X Y

1 2 3 4 5

6 7 8 9 10

11 12 13 14 15

16 17 18 19 20

21 22 X

1 2 3 4 5

6 7 8 9 10

11 12 13 14 15

16 17 18 19 20

21 22 X

C)

B)