Edexcel International A Level Biology

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2.16 Cystic Fibrosis

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Cystic Fibrosis

  • Genes can affect the phenotype of an organism
    • A gene codes for a single polypeptide
    • The polypeptide can affect the phenotype, e.g. it could form part of an enzyme or a membrane transport protein
  • Genetic disorders are often caused by a mutation in a gene that results in a differently-functioning or non-functioning protein that alters the phenotype of the individual

Cystic fibrosis

  • Cystic fibrosis is a genetic disorder of cell membranes caused by a recessive allele of the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene located on chromosome 7
    • This gene codes for the production of chloride ion channels required for secretion of sweat, mucus and digestive juices
    • A mutation in the CFTR gene leads to production of non-functional chloride channels
    • This reduces the movement of water by osmosis into the secretions
    • The result is that the body produces large amounts of thick, sticky mucus in the air passages, the digestive tract and the reproductive system
  • Because cystic fibrosis is determined by a recessive allele, this means
    • People who are heterozygous won’t be affected by the disorder but are carriers
    • People must be homozygous recessive 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, as the recessive allele will always be masked by the dominant allele

Inheritance of cystic fibrosis

Cystic fibrosis is a genetic disorder caused by a recessive allele

The respiratory system

  • Mucus in the respiratory system is a necessary part of keeping the lungs healthy
    • It prevents infection by trapping microorganisms
    • This mucus is moved out of the respiratory tract by cilia 
  • In people with cystic fibrosis, due to the faulty chloride ion channels, the cilia are unable to move as the mucus is so thick and sticky
  • This means microorganisms are not efficiently removed from the lungs and lung infections occur more frequently
  • Mucus builds up in the lungs and can block airways which limits gas exchange
    • The surface area for gas exchange is reduced which can cause breathing difficulties
  • Physiotherapy can support people with cystic fibrosis to loosen the mucus in the airways and improve gas exchange

The digestive system

  • Thick mucus in the digestive system can cause issues because
    • The tube to the pancreas can become blocked, preventing digestive enzymes from entering the small intestine
      • Digestion of some food may be reduced and therefore key nutrients may not be made available for absorption
    • The mucus can cause cysts to grow in the pancreas which inhibit the production of enzymes, further reducing digestion of key nutrients
    • The lining of the intestines is also coated in thick mucus, inhibiting the absorption of nutrients into the blood

The reproductive system

  • Mucus is normally secreted in the reproductive system to prevent infection and regulate the progress of sperm through the reproductive tract after sexual intercourse
  • The mucus in people with cystic fibrosis can cause issues in both men and women
    • In men the tubes of the testes can become blocked, preventing sperm from reaching the penis
    • In women thickened cervical mucus can prevent sperm reaching the oviduct to fertilise an egg

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Naomi H

Author: Naomi H

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.