Genetics (other half of 2.4)

• Monohybrid cross

  Genetic diagrams consisting of one characteristic controlled by one gene with two alleles
• Recessive/dominant alleles


  • Dominant alleles will be expressed even if a recessive allele is present
  • Recessive alleles will be overridden by the presence of a dominant allele
  • Two recessive alleles (no dominant) need to be present for the recessive characteristic to be expressed in the phenotype
• Phenotype


  The outward expression of a gene (i.e. the physical appearance)
• Genotype


  A set of paired symbols used to represent the alleles present. The genotype can be homozygous (the same, BB or bb) or heterozygous (different, Bb)
• Meiosis


  • During meiosis, gametes with only one gene for each characteristic are produced, meaning parents can only pass on one allele for each characteristic to their offspring
• Mendel concluded that characteristics are determined by factors within the organism (now known to be genes), the factors (genes) can be present in 2 different forms (now known as alleles), and the 2 factors (alleles) in an individual separate during gamete formation (now known as meiosis)
• Punnett Square


  A grid used to determine genotype frequencies
• Large numbers of offspring need to be produced to provide an accurate ratio in a Punnett Square because fertilization is random
• Test (back) cross


  1. The organism in question is crossed with a homozygous recessive individual (tt)

  2. If no short plants (tt) are produced in the offspring the unknown parent was homozygous (TT)

  3. If any short plants (tt) are present in the offspring the unknown parent was heterozygous (Tt)
• Pedigree Diagram


  Used to show how a genetic condition is inherited in a family
• Sex determination in humans

  Male = XY, Female = XX
• Mutations


  Random changes in the structure of a gene or in the number of chromosomes
• Haemophilia


  A sex-linked, inherited condition caused by a recessive allele on the X chromosome
• Haemophilia


  • Sufferers are almost exclusively males because they only need one recessive allele
  • Females with two normal alleles will also be 'normal'
  • Only in the rare case of a female having two recessive alleles will they show the condition
  • Women with one normal and one recessive allele are called carriers
• Cystic fibrosis


  Mainly affects the lungs and digestive system, which become clogged with mucus leading to frequent infections. It is caused by a recessive allele, which means only homozygous recessive individuals (cc) will be affected.
• Huntington's disease


  Affects nerve cells in the brain, leading to brain damage. It usually becomes apparent in middle age. There is no cure and it is eventually fatal.
• Down syndrome

  This condition is caused by the presence of an extra chromosome – a sperm cell with 23 chromosomes fertilises an egg cell with 24 chromosomes. This results in an individual with 47 chromosomes rather than the 'normal' 46. An individual with Down syndrome has three copies of the 21st chromosome rather than two.
• Genetic screening


  Testing people or groups of people for the presence of a particular allele or other genetic abnormality
• Amniocentesis


  1. A needle is inserted into the amniotic fluid that surrounds the foetus and fluid is withdrawn

  2. Foetal cells in the fluid are then examined for the presence of genetic abnormalities such as Down syndrome and cystic fibrosis
• Amniocentesis carries a 1% risk of miscarriage, so it is usually only offered to pregnant women who are at a higher risk of having a child with a genetic abnormality
• Factors that would make a fetus more likely to be born with a genetic abnormality

  • Those who have previously carried a foetus with a genetic abnormality
  • Those who have a family history of a genetic condition
  • Older mothers
  • Those where possible problems have been identified in an earlier medical examination, for example a blood test
• Blood test vs Amniocentesis


  While not as accurate as amniocentesis, the blood test carries no risk to the foetus or the mother; it simply identifies those women who may wish to take the riskier amniocentesis procedure
• Genetic engineering can be done by transferring a desirable gene from a donor organism into the genome of a recipient organism
• Genetic engineering


  • It is useful in agriculture and technology and has helped further scientific research
  • The recipient organism is often a bacterial cell because bacterial DNA is easily manipulated and bacteria can reproduce very rapidly
• Producing human insulin for treatment of diabetes

  1. The human insulin gene is removed using a restriction enzyme

  2. A bacterial plasmid is cut open using the same restriction enzyme

  3. Restriction enzymes leave 'sticky ends', where one of the two DNA strands is longer than the other

  4. Using the same restriction enzyme to cut both the human DNA and bacterial plasmid results in complementary sticky ends that join by base pairing

  5. A different enzyme is used to join the insulin gene and the bacterial plasmid

  6. The bacterial plasmid containing the insulin gene is placed into a bacterial cell

  7. The bacterial cell is placed in a fermenter to allow reproduction under perfect conditions (warmth, moisture and oxygen)

  8. Downstreaming occurs – this is when insulin is extracted, purified and packaged

  9. The pure insulin produced can be used to treat diabetes
• Advantages of genetically engineered insulin

  • Not limited by the slaughter of animals
  • Large quantities can be made quickly
  • No risk of transferring infections
  • More effective at treating diabetes as animal insulin is different from human insulin
  • No ethical issues concerning the use of animals