5.1.1- Inheritance of Human Genetic Disease

Contains revision cards for OCR Human Biology


Genetic Terms

  • Gene- A length of DNA which codes for the production of a polypeptide.
  • Allele- One of the different forms of a gene that occupy the same locus on homologous chromosomes.
  • Locus- The position of a gene on a chromosome.
  • Phenotype- A person's observable characteristics.
  • Genotype- The genetic make-up of a person.
  • Dominant- A dominant allele always shows its effect on the phenotype.
  • Recessive- A recessive allele only shows its effect on the phenotype when the dominant allele is absent.
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Phenylketonuria & Huntington's Disease


  • PKU
  • Recessive gene mutation on chromosome 12.
  • Lack of phenylalanine hydroxylase.
  • Results in high levels of phenylalanine.
  • Toxic to young children.

Huntington's Disease

  • Autosomal dominant gene mutation on chromosome 4.
  • CAG codes for glutamine.
  • Mutation causes a repetition of CAG so a series of them produces Huntington's proteins.
  • People with Huntington's have 40 or more repeats. 
  • Symptoms- 
    • memory loss
    • changes in personality & mood
    • uncontrolled muscle movements
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Cystic Fibrosis

  • CF
  • Recessive gene mutation on chromosome 7.
  • Deletion of three base pairs results in loss of an amino acids.
  • Makes the cystic fibrosis transmembrane regulator protein (CFTR).
  • Blocks the movement of chloride ions and water across membranes.
    • Causes the secretion of abnormally thick mucus.
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When both alleles affect the phenotype.

  • Sickle cell anaemia and ABO blood groups caused by co-dominance.
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Malaria & Sickle-Cell Anaemia

In areas where malaria is endemic, sickle cell anaemia has a heterozygous advantage.

  • HbA HbA
    • Disadvantageous
    • Malaria affects the person normally
  • HbA HbS
    • Advantageous
    • Causes malaria infection to stay in body longer
    • Immune system has longer to react
    • Composition of RBC changed, parasite can't reproduce as much.
  • HbS HbS
    • Disadvantageous
    • Person has sickle cell anaemia

Causes an increase in frequency of HbA HbS.

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Fyzah :p


OMG!!! This is so amazing :D

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