Genetic conditions

Some genetic conditions are inherited – passed from parent to child.

Genetic conditions are caused by mutations.

Mutations are random changes in the structure of a gene or in the number of chromosomes.


  • Sufferers are unable to clot their blood resulting in excessive bleeding, even from small cuts or bruises.
  • It is a sex-linked, inherited condition caused by a recessive allele on the X chromosome.
  • Sufferers are almost exclusively males because they only need one recessive allele.
  • However, if his X chromosome carries the dominant normal allele, he will be normal.
  • Females with two normal alleles will also be normal.
  • Only in the rare case of a female having two recessive alleles will they show the condition.
  • Those women with one normal and one recessive allele are called carriers. Although they themselves are normal, they can pass the recessive allele to half of their children.

Cystic fibrosis

  • Mainly affects the lungs and digestive system, which become clogged with mucus leading to frequent infections.
  • It is caused by a recessive allele, which means only homozygous recessive individuals (cc) will be affected.

Huntington’s disease

  • Affects nerve cells in the brain, leading to brain damage.
  • It usually becomes apparent in middle age.
  • There is no cure and it is eventually fatal.
  • This condition is caused by the presence of one dominant allele.

Down syndrome

  • Individuals have reduced muscle tone and cognitive development.
  • It is caused by the presence of an extra chromosome – a sperm cell with 23 chromosomes fertilises an egg cell with 24 chromosomes.
  • This results in an individual with 47 chromosomes rather than the ‘normal’ 46.
  • An individual with Down syndrome has three copies of the 21st chromosome rather than two.