Mutations (WJEC GCSE Biology)

• Mutations are random genetic changes, that can sometimes result in the creation of brand-new alleles or genes
• Most mutations have no effect on the phenotype as the protein that a mutated gene produces may work just as well as the protein from the non-mutated gene
• Rarely, mutations lead to the development of new alleles and new phenotypes. If they do, most will have a small effect on the organism
• Occasionally, the new allele gives the individual a survival advantage over other members of the species
• For example:
  • A bird develops a mutation leading to a change in beak shape
  • This gives the bird access to a wider range of food sources
  • This increases the chances that the bird will survive and reproduce
  • Therefore giving more chances of passing on the mutated allele, and therefore phenotype, to the next generation
• Mutations can also lead to harmful changes that can have dramatic effects on the body - for example, sickle cell anaemia in humans
• Mutations happen spontaneously and continuously but their frequency can be increased by exposure to the following:
  • Gamma rays, x-rays and ultraviolet rays - all types of ionising radiation which can damage bonds and cause changes in base sequences
  • The greater the exposure to ionising radiation, the more chance there is of mutations in DNA
• Increased rates of mutation can cause cells to become cancerous, which is why the above are linked to increased incidence of different types of cancer

Cystic fibrosis

• Cystic fibrosis is a genetic disorder of cell membranes
• It results in the body producing large amounts of thick, sticky mucus in the air passages which blocks the bronchioles
• Over time, this may damage the lungs and stop them from working properly
• Cystic fibrosis arises from a mutation and can be inherited as a recessive allele (f)
• This means:
  • People who are heterozygous (only carry one copy of the recessive allele) won’t be affected by the disorder but are ‘carriers’
  • People must be homozygous recessive (carry two copies of the recessive allele) in order to have the disorder
  • If both parents are carriers, the chance of them producing a child with cystic fibrosis is 1 in 4, or 25%
  • If only one of the parents is a carrier (with the other parent being homozygous dominant), there is no chance of producing a child with cystic fibrosis

Inheritance of cystic fibrosis genetic diagram

Inheritance of cystic fibrosis if both parents are carriers or if only one parent is a carrier

Family trees

• The patterns of inheritance can be shown using family trees
• This may be useful in determining the risks to other members of the family
  • They key shows that individuals with the shaded blue shapes do not have the disorder whereas those with red borders and no shading are sufferers

Family tree diagram

A family tree allows us to trace the inheritance of cystic fibrosis

Gene therapy

• Modern medical research is exploring the possibility of genetic modification to overcome some inherited disorders, such as cystic fibrosis
• As these inherited genetic diseases are caused by faulty genes, it may be possible to treat these by inserting working versions of these genes into people with the genetic disease
• This is called gene therapy
• Unfortunately, treatment by gene therapy is not straightforward
  • To treat cystic fibrosis sufferers, genes can be introduced into the body via an inhaler which is breathed into the lung tissue, but there is no guarantee that they will work
  • It is difficult to ensure that the correct cells are targeted
  • Gene therapy is not a cure, as it does not alter all the cells of the body, this means that any relief of symptoms is temporary and treatment must be repeated