2.4

• Antenatal and postnatal screening

  A variety of techniques used to monitor the health of the mother, developing foetus and baby
• Allele


  One of a number of forms of a particular gene
• Amniotic fluid


  Liquid that protects the foetus in the uterus
• Autosomes


  Chromosomes that control the characteristics of an organism but do not determine its sex
• Carrier


  Someone who does not suffer from a condition but carries the allele and can pass it to their offspring
• Chromosome


  Genetic structures that usually occur in functional pairs in the nucleus of cells (except gametes and bacterial)
• Clot


  A clump of platelets and blood cells that forms when a blood vessel is damaged
• Cystic fibrosis


  A disorder that mainly affects the lungs, pancreas, liver and intestine. The main symptom is difficulty breathing
• Dominant


  An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself
• Down's syndrome


  A chromosomal condition caused by the presence of a third copy of chromosome 21
• Gene


  A section of DNA which codes for a protein
• Genotype


  The alleles that an organism has for a particular characteristic, usually written as letters
• Heterozygous


  A genotype in which the two alleles for a particular characteristic are different
• Homozygous


  A genotype in which the two alleles for the characteristic are identical
• Huntington's disease


  An inherited disease of the brain that causes damage to certain brain cells
• Karyotype


  Shows an individual's chromosomes arranged as a homologous pair
• Miscarriage


  The loss of a pregnancy during the first 23 weeks
• Phenotype


  The visible characteristics of an organism which occur as a result of its genes
• Placenta


  The organ in the uterus of pregnant mammals that allows the transfer of nutrients and waste products between the mother and the foetus through the umbilical cord
• Pre-eclampsia


  A medical condition that can occur in mothers during the second half of pregnancy or shortly after giving birth. Early symptoms include high blood pressure and protein in the urine. Although often mild, pre-eclampsia can lead to serious complications for the health of both mother and baby
• Recessive


  An allele that will only show in the phenotype if there is no dominant allele present (i.e. when two recessive alleles are present)
• Antenatal screening


  1. Identify any possible risks to the mother or foetus during pregnancy or to identify health issues that will affect the baby

  2. If a risk is identified, further tests and prenatal diagnosis can be offered
• Antenatal screening techniques


  • Ultrasound imaging
  • Blood and urine tests
• Ultrasound imaging


  1. Dating scans take place between 8 and 14 weeks to determine pregnancy stage and due date

  2. Anomaly scans take place between 16 and 20 weeks to detect serious physical abnormalities in the foetus
• Blood and urine tests


  Routine tests carried out throughout pregnancy to monitor the concentrations of marker chemicals which vary normally during pregnancy
• Measuring a chemical at the wrong time could lead to a false positive result, which is why blood and urine tests are used in conjunction with scans
• Diagnostic testing


  • Amniocentesis
  • Chorionic villus sampling (CVS)
• Amniocentesis


  Uses a small amount of amniotic fluid from the amniotic sac surrounding the foetus to check for genetic disorders
• Chorionic villus sampling (CVS)


  Tests a sample of cells from the mother's placenta for genetic disorders. CVS can be carried out earlier in pregnancy than amniocentesis, although it has a higher risk of miscarriage
• Cells from samples can be cultured to obtain sufficient cells to produce a karyotype to diagnose a range of conditions such as Down's syndrome or Turner's syndrome
• In deciding to proceed with these tests, the element of risk will be assessed, as will the decisions the individuals concerned are likely to make if a test is positive
• Postnatal screening


  1. New-born babies are regularly checked for health conditions during the first two months of life

  2. Diagnostic testing for phenylketonuria (PKU)
• Phenylketonuria (PKU)


  An autosomal recessive inherited metabolic condition where a substitution mutation means the enzyme which converts phenylalanine to tyrosine is non-functional. Individuals with high levels of phenylalanine are placed on a restricted diet
• Analysis of patterns of inheritance


  1. Doctors can use a pedigree analysis chart to show how genetic disorders are inherited in a family

  2. They can use this to work out the chance that someone in a family will inherit a condition
• A pedigree analysis is usually undertaken if referred to a genetic counsellor following the birth of an affected child
• Autosomal recessive


  Traits are expressed rarely, sufferers require two recessive alleles to be affected so are homozygous recessive. Recessive traits may skip generations and will affect both genders equally. An example is cystic fibrosis
• Autosomal dominant


  Any sufferers from autosomal dominant conditions only require one dominant allele so can be either homozygous dominant or heterozygous. This affects both genders equally. An example is Huntington's disease
• Incomplete dominance


  The dominant allele may not fully express itself. Both incomplete dominance alleles are shown with upper case letters, but the letters are different
• Incomplete dominance in chickens

  • Genotypes: WW, BB, BW

  Phenotypes: White, Black, Speckled
• Sex-linked recessive inheritance


  The sex chromosomes, although mainly responsible for determining gender, carry some genes that code for a number of body functions. A recessive allele on the X chromosome will show in the phenotype as there is no dominant allele to override it. An example is haemophilia