CIE A Level Biology复习笔记19.2.2 Genetic Screening

• In certain circumstances (eg. in the pregnancy in an older woman, or pregnancy where there is a family history of a genetic disease) may require individuals to determine if they have a particular allele present in their genome. This can be determined by genetic screening
• Genetic screening can help identify individuals who are carrying an allele at a gene locus for a particular disorder
• Genetic screening is the testing of an embryo, fetus or adult to analyse the DNA
• The sample of DNA to be analysed can be obtained by:
  • Taking tissue samples from adults or embryos produced by in-vitro fertilisation
  • Chorionic villus sampling or amniocentesis of embryos and fetuses in the uterus
• As genetic screening can leave future parents with many questions, genetic counsellors are available to help. The counsellors will read the results and explain them. Counsellors can also be seen before screening has occurred. They may discuss the following with the prospective parents:
  • The chances of the couple having a child with a certain disease
  • Termination of the pregnancy
  • Therapeutic treatments possible for the child
  • Financial implications of having the child
  • Effect on existing siblings
  • Ethical issues

Breast cancer (BRCA1 and BRCA2)

• BRCA1 and BRCA2 are genes that produce tumour suppressor proteins and thus they play an important role in regulating cell growth
• Faulty alleles of these particular genes exist which increase the risk of an individual developing breast and ovarian cancers during their lifetime
• Faulty BRCA1 and BRCA2 alleles can be inherited from either parent
• The advantages of genetic screening for an adult who has a family history of BRCA1 and BRCA2 gene mutations are:
  • That the person may decide to take preventative measures (e.g. by having an elective mastectomy – breast removal – to reduce the risk of developing cancer)
  • Screening for breast cancer may begin from an earlier age or more frequently, and the individual (if female) will have more frequent clinical examinations of the ovaries
  • That it enables the person to participate in research and clinical trials

Huntington’s disease

• Huntington’s is a progressive (gets worse with time) inherited disease that affects the brain
• Signs of the disease typically appear in affected individuals after reaching their 40’s and include uncontrolled movements, lower cognitive (thinking) ability and emotional problems
• There is no cure for the Huntingdon’s disease, with treatments available only alleviating the symptoms but not curing it
• Huntington’s is an autosomal dominant disease (therefore if the person has an allele for Huntington’s they will get the disease)
• The advantage of genetic screening for Huntington’s is it enables:
  • People to plan for the future (how they will live and be cared for)
  • Couples to make informed reproductive decisions (as the risk that their children may inherit the disease is 50%)
  • People to participate in research and clinical trials

Cystic fibrosis

• Cystic fibrosis is an autosomal recessive genetic disorder that is caused by a mutation of the gene that codes for a transported protein called CFTR
• It is a progressive disease that causes mucus in various organs (lungs, pancreas, lungs) to become thick and sticky. This is because the faulty CFTR protein no longer transports chloride ions across the cell plasma membrane and therefore water does not move by osmosis across the membrane either (the presence of water would normally make the mucus thinner enabling cilia to remove it)
• There is no cure for cystic fibrosis, although there are many different treatments that help alleviate symptoms. The common cause of death is bacterial infection in the lungs
• The advantage of genetic screening for cystic fibrosis is:
  • It enables couples to make informed reproductive decisions (as both may be carriers and therefore not display any symptoms)
  • That people can participate in research and clinical trials