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

• 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 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