Edexcel A (SNAB) A Level Biology:复习笔记2.4.6 Chi-squared Test

Genetic Screening: Uses

  • Some circumstances, e.g. in a pregnancy where there is a family history of a genetic disorder, may require individuals to determine if they have a particular allele present in their genome
  • This can be determined by genetic screening
  • There are three main uses of genetic screening
    • Identifying individuals who are carrying an allele at a gene locus for a particular disorder
    • The screening of embryos prior to implantation during fertility treatment; this is Preimplantation Genetic Diagnosis (PGD)
    • Testing a foetus before birth; this is prenatal testing

Identification of carriers

  • Carrier testing is offered to individuals with a history of genetic disorders in their family
  • It can show whether people who have no symptoms carry the allele for particular disorders, such as cystic fibrosis
  • Couples can be tested prior to having children to determine the probability of future children inheriting the disorder
  • Benefits
    • Families can make informed decisions before having children
    • Women can decide whether to have prenatal testing during pregnancy

Preimplantation Genetic Diagnosis

  • IVF, or in vitro fertilisation is a type of fertility treatment during which fertilisation is carried out in the lab; embryos produced in this way can be implanted into the uterus where they develop into a foetus
  • PGD involves analysis of the DNA of an embryo prior to implanting it into the uterus
    • The sample of DNA to be analysed can be obtained by taking cell samples from embryos produced during IVF
  • Benefits
    • Reduces the chances of having a baby with a genetic disorder
    • It avoids abortion as it is carried out before implantation of the embryo

Prenatal testing

  • Prenatal testing is offered to pregnant women with a family history of genetic disorders
  • It involves testing the foetus for genetic diseases
  • The DNA can be obtained by chorionic villus sampling or amniocentesis of embryos and foetuses in the uterus
  • Chorionic villus sampling
    • This involves removing and testing a small sample of cells from the placenta using a fine needle
    • The cells contain foetal DNA which can be analysed for genetic disorders, allowing parents to make informed decisions about the pregnancy and foetus
    • It is carried out at around 11-14 weeks of pregnancy
    • The process has a  1-2 % risk of miscarriage
  • Amniocentesis
    • This involves removing and testing a small sample of cells from amniotic fluid using a fine needle
      • The amniotic fluid is the fluid that surrounds the foetus within the uterus
    • The fluid contains foetal cells which contain DNA to be analysed
    • It is carried out at around 15-20 weeks of pregnancy
    • The procedure has a 1% risk of miscarriage
  • Benefits
    • The tests allow parents to make informed decisions
    • Results can help parents prepare for the future care of the child, including medical treatment

Genetic Screening: Social & Ethical Issues

  • Genetic screening comes with a range of different social and ethical issues
  • As genetic screening can leave future parents with many questions, genetic counsellors are available to help
    • Counsellors can be seen before screening has occurred; they may discuss the following with the prospective parents
      • The probability of the couple having a child with a genetic disorder
      • Termination of the pregnancy
      • Therapeutic treatments possible for the child
      • Financial implications of having the child
      • Effect on existing siblings
      • Ethical issues
    • After screening the counsellors will read the results and explain them to the future parents
  • Each use of genetic screening brings potential concerns that should be considered
    • Identification of Carriers
      • Finding out you are a carrier can cause emotional stress and upset
      • It may impact your ability to find a partner
      • Other genetic disorders may be identified
      • Employers and insurance companies may use the data against an individual
      • The tests are not always 100 % accurate; false positives or negatives may be given
    • Preimplantation Genetic Diagnosis
      • There are concerns it could lead to "designer babies" where parents select embryos with certain traits such as eye colour or sex
      • False results could lead to inaccurate information and poor decisions being made about the fate of embryos
    • Prenatal testing
      • Each procedure brings risk of miscarriage
      • False results could provide inaccurate information and poor decisions being made
      • There are ethical implications involved if parents decide to abort the fetus