Inherited Disorders (AQA GCSE Biology: Combined Science)

• Some disorders are inherited (passed from parents to offspring)
• These disorders are caused by the inheritance of certain alleles
• For example, cystic fibrosis and polydactyly are two genetic disorders that can be inherited:

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
• Over time, this may damage the lungs and stop them from working properly
• Cystic fibrosis is caused by 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 if both parents are carriers or if only one parent is a carrier

Polydactyly

• Polydactyly is a genetic disorder that causes someone to be born with extra fingers or toes
• Polydactyly is caused by a dominant allele (D)
• This means:
  • Even if only one parent is a carrier, the disorder can be inherited by offspring

Inheritance of polydactyly if only one parent is a carrier

Embryo screening

• In vitro fertilisation (IVF) is the process by which embryos are fertilised in a laboratory and then implanted into the mother’s womb
• A cell can be taken from the embryo before being implanted and its genes can be analysed
• It is also possible to get DNA from the cell of an embryo that’s already in the womb and analyse its genes in the same way
• Genetic disorders (eg. cystic fibrosis) can be detected during this analysis
• This has led to many economic, social and ethical concerns:
  • An IVF embryo (ie. a potential life) might be destroyed if alleles causing a genetic disorder are found in its genes
  • Pregnancy might be prematurely terminated if an embryo already in the womb (also a potential life) is found to have alleles causing a genetic disorder within its genes

Arguments for & against embryo screening

Gene therapy

• Gene therapy is the process by which normal alleles are inserted into the chromosomes of an individual who carries defective alleles (eg. those that cause a genetic disorder)
• It is a developing technology and is not always successful
• The process raises similar economic, social and ethical concerns to embryo screening:
  • Many people believe that gene alteration is unnatural
  • Many believe it is a good idea as it can help to alleviate suffering in people with genetic disorders