AQA GCSE Combined Science: Biology

Revision Notes

6.1.7 Inherited Disorders

Inherited Diseases

  • 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, downloadable IGCSE & GCSE Biology revision notes

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, downloadable IGCSE & GCSE Biology revision notes

Inheritance of polydactyly if only one parent is a carrier

Impact of Inherited Disesase

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

Arguments for and against embryo screening_1, downloadable IGCSE & GCSE Biology revision notes

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

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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