Severe combined immunodeficiency (SCID) is a group of life-threatening diseases. SCID is
caused by mutations that prevent the normal function of the immune system. Infants born with
SCID are at very high risk of infectious diseases.
One feature of SCID is that T-lymphocytes do not develop normally.
In the development of normal T-lymphocytes, the production of circular pieces of DNA called
T-lymphocyte receptor excision circles (TRECs) is an important event.
It is possible to use the polymerase chain reaction (PCR) to detect TRECs in DNA extracted from
a sample of blood. The results of this reaction can be used to identify children with SCID.

Q: What topic does this question test?

This question tests Genetic technology in A-Level Biology (syllabus code 9700). It is worth 16 marks.

Q: Which exam paper is this question from?

This question appeared in the Cambridge A-Level Biology Feb/Mar 2020 examination, Paper 4 Variant 2 (9700_m20_qp_42.pdf).

Question

Severe combined immunodeficiency (SCID) is a group of life-threatening diseases. SCID is
caused by mutations that prevent the normal function of the immune system. Infants born with
SCID are at very high risk of infectious diseases.
One feature of SCID is that T-lymphocytes do not develop normally.
In the development of normal T-lymphocytes, the production of circular pieces of DNA called
T-lymphocyte receptor excision circles (TRECs) is an important event.
It is possible to use the polymerase chain reaction (PCR) to detect TRECs in DNA extracted from
a sample of blood. The results of this reaction can be used to identify children with SCID.

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