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19.2.219 · Genetic Technology

Genetic Screening — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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19.2.2 Genetic Screening — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe genetic screening, its uses for BRCA1/2, Huntington's disease and cystic fibrosis, and the role of counsellors.Genetic screening — testing an embryo, foetus or adult to analyse the DNA…

  • Common reasons screening is advantageous
  • Chorionic villus sampling
  • Amniocentesis
  • Genetic counsellor
  • Autosomal dominant
  • Recombinant

Section 1 of 10What genetic screening is

Genetic screening
  • Genetic screening testing an embryo, foetus or adult to analyse the DNA and identify carriers of an allele for a disorder.
It is needed in some situations, e.g. pregnancy in an older woman or a family history of a genetic disease.
DNA is obtained from:
  • Tissue samples from adults or embryos made by IVF.
  • Chorionic villus sampling or amniocentesis for embryos and foetuses in the uterus.
Methods of detecting alleles include PCR, labelled DNA probes, gel electrophoresis of restriction fragments, microarrays and sequencing.

Section 2 of 10Genetic counsellors

Genetic counsellors
Counsellors read and explain the results.
They can also be seen before screening to discuss:
  • The chances of having a child with a certain disease.
  • Termination of the pregnancy.
  • Possible treatments for the child.
  • Financial implications and the effect on siblings.
  • Ethical issues.

Section 3 of 10Breast cancer (BRCA1 and BRCA2)

Breast cancer (BRCA1 and BRCA2)
BRCA1 and BRCA2 produce tumour suppressor proteins that regulate cell growth.
Faulty alleles raise the lifetime risk of breast and ovarian cancer and can be inherited from either parent.
Advantages of screening a person with a family history:
  • They may take preventive measures, e.g. elective mastectomy.
  • Screening can start earlier or be more frequent, with more clinical examinations of the ovaries.
  • They can take part in research and clinical trials.

Section 4 of 10Huntington's disease

Huntington's disease
Huntington's is a progressive inherited brain disease.
  • Signs appear after the 40s: uncontrolled movements, lower cognitive ability and emotional problems.
  • There is no cure, only treatments for symptoms.
  • It is autosomal dominant, so one allele causes the disease.
Advantages of screening:
  • Planning for the future, including care.
  • Informed reproductive decisions, since children have a 50% risk.
  • Participation in research and clinical trials.

Section 5 of 10Cystic fibrosis

Cystic fibrosis
Cystic fibrosis is autosomal recessive, caused by mutation of the gene for the CFTR transport protein.
Faulty CFTR no longer transports chloride ions across the membrane, so water does not move by osmosis and mucus becomes thick and sticky.
  • Mucus builds up in the lungs and pancreas; a common cause of death is bacterial lung infection.
  • There is no cure, only symptom treatment.
Advantages of screening:
  • Couples who may both be carriers can make informed reproductive decisions.
  • People can take part in research and clinical trials.

Section 6 of 10Inheritance risks

Inheritance risks
Two carriers of a recessive allele have a 25% chance of an affected child and 50% chance of a carrier each pregnancy.
An offspring of a heterozygous parent with a dominant disorder has a 50% chance of inheriting it.
Each pregnancy is independent of previous ones.

Section 7 of 10Newborn, prenatal and predictive screening

Newborn, prenatal and predictive screening
Prenatal testing of a foetus; newborn screening, e.g. for phenylketonuria; carrier testing; PGD of IVF embryos; predictive testing for late-onset conditions.
Reliability: false positives and false negatives are possible, and a positive risk test does not mean disease is certain.
Common reasons screening is advantageous
  • Common reasons screening is advantageous informed reproductive decisions and participation in research and clinical trials.

Section 8 of 10More from the specification

1Recombinant
  • Offspring with a new combination of characteristics formed by crossing over.
2Insulin
  • A pancreatic hormone that lowers blood glucose.
3Somatic
  • Body cells rather than gametes
4Medicine
  • A drug used to treat illness.
5Adenosine
  • Adenine joined to ribose.
6Adenosine deaminase
  • Enzyme missing in ADA-deficiency SCID.
7Genetically modified
  • Having had genes from another organism inserted.
8Recombinant protein
  • Protein made from recombinant DNA in host cells.

Section 9 of 10Key terms

TermMeaning
Genetic screeningTesting DNA to find whether a person carries a particular allele.
Chorionic villus samplingTaking a small placental sample at 10-13 weeks.
AmniocentesisTaking amniotic fluid at 15-16 weeks.
Genetic counsellorPerson who explains results and options.
Autosomal dominantInheritance in which one allele causes the disorder.
CFTRChloride ion transport protein faulty in cystic fibrosis.

Section 10 of 10Exam technique

1
Saying a positive BRCA result means cancer is certain — it raises the risk.
2
Saying Huntington's is recessive — it is autosomal dominant.
3
Saying screening treats disease — it only gives information.
4
Thinking an earlier affected child changes the risk for the next — each pregnancy is independent.

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