Topic 3

• Human Genome Project


  Project to find every single human gene
• Human Genome Project


  • Thousands of scientists from all over the world collaborated
  • Project officially started in 1990
  • Complete map of the human genome, including the locations of around 20,500 genes, was completed in 2003
  • Helped to identify about 1800 genes related to disease
• There are lots of medical applications for the Human Genome Project's research
• Prediction and prevention of diseases

  Many common diseases like cancers and heart disease are caused by the interaction of different genes, as well as lifestyle factors. If doctors knew what genes people have, they could give advice on the best diet and lifestyle to avoid likely problems. Doctors could also check people regularly to ensure early treatment if they do develop the diseases they're susceptible to.
• Testing and treatment for inherited disorders


  Inherited disorders (e.g. cystic fibrosis) are caused by the presence of one or more faulty alleles in a person's genome. Thanks to the Human Genome Project, scientists are now able to identify the genes and alleles suspected of causing an inherited disorder much more quickly than they could do in the past. Once an allele that causes an inherited disorder has been identified, people can be tested for it and it may be possible to develop better treatments or even (eventually) a cure for the disease.
• New and better medicines

  Genome research has highlighted some common genetic variations between people. Some variations affect how our individual bodies will react to certain diseases and to the possible treatments for them. Scientists can use this knowledge to design new drugs that are specifically tailored to people with a particular genetic variation. They can also determine how well an existing drug will work for an individual. Tests can already identify whether or not someone with breast cancer will respond to a particular drug, and what dosage is most appropriate for certain drugs in different patients. More generally, knowing how a disease affects us on a molecular level should make it possible to design more effective treatments with fewer side-effects.
• There could also be drawbacks to the Human Genome Project
• Potential drawbacks


  • Increased stress for people who know they are susceptible to a disease
  • Gene-ism - people with genetic problems could come under pressure not to have children
  • Discrimination by employers and insurers - life insurance could become impossible to get (or expensive at least) if you have any genetic likelihood of serious disease, and employers might discriminate against people who are genetically likely to get a disease
• Mutations are changes to the base sequence of DNA
• Alleles


  Different versions of a gene
• Mutations don't always have a big effect on the phenotype of an organism
• Some mutations do have a small effect on the phenotype - they alter an individual's characteristics, but only very slightly
• Rarely, a single mutation will have a big effect on phenotype, such as in cystic fibrosis where a mutation causes a protein that controls the movement of salt and water into and out of cells to stop working properly
• New combinations of alleles may also interact with each other to produce new phenotypes
• Over 20,000 genes were mapped in the Human Genome Project
• Organisms of the same species have differences, which is called variation within a species
• Genetic variation


  Caused by organisms having different alleles (versions) of genes which can lead to differences in phenotype (the characteristics an organism displays)
• Acquired characteristics


  Environmental variations in phenotype - characteristics that organisms acquire (get) during their lifetimes
• Most variation in phenotype is determined by a mixture of genetic and environmental factors
• Multiple alleles


  More than two possible alleles for a single gene
• Codominant alleles


  When two alleles are both expressed, neither is dominant over the other
• Recessive allele


  An allele that is only expressed when two copies are present
• Predicting blood groups using genetic diagrams

  1. Work out the potential genotypes

  2. Remember how the different alleles interact to produce the phenotype
• Genetic disorders can be sex-linked, meaning they are more likely to affect males
• Sex-linked characteristic


  A characteristic where the allele that codes for it is located on a sex chromosome (X or Y)
• Colour blindness is a sex-linked genetic disorder
• X chromosome


  Present in male and female
• Some Genetic Characteristics Are Sex-Linked
• Sex-linked characteristic


  Allele that codes for it is located on a sex chromosome X or Y
• Y chromosome


  • Smaller than the X chromosome
  • Carries few genes
  • Most genes on the sex chromosomes are only carried on the X chromosome
• Men


  • Only have one X chromosome
  • Often only have one allele for sex-linked genes
• Recessive characteristics for sex-linked genes

  More common in men than women
• Sex-linked genetic disorders


  Disorders caused by faulty alleles located on sex chromosomes
• Colour blindness


  Sex-linked disorder caused by a faulty allele carried on the X chromosome
• Women need two copies of the recessive allele to be colour blind, while men only need one copy</b>
• Colour blindness is much rarer in women than men
• A woman with one copy of the recessive allele is a carrier of colour blindness
• Genetic cross between a carrier female and an unaffected male

  1. Unaffected female (carrier)

  2. Unaffected male

  3. Unaffected female carrier

  4. Colour blind male
• Ratio of unaffected: colour blind offspring is 3:1
• There's a 1 in 4 (25%) chance of a child being colour blind