Brent Cornell

    
Genetic diseases are caused when mutations to a gene (or genes) abrogate normal cellular function, leading to the development of a disease phenotype

• Genetic diseases can be caused by recessive, dominant or co-dominant alleles

An autosomal recessive genetic disease will only occur if both alleles are faulty

• Heterozygous individuals will possess one copy of the faulty allele but not develop disease symptoms (they are carriers)
• An example of an autosomal recessive genetic disease is cystic fibrosis

An autosomal dominant genetic disease only requires one copy of a faulty allele to cause the disorder

• Homozygous dominant and heterozygous individuals will both develop the full range of disease symptoms
• An example of an autosomal dominant genetic disease is Huntington’s disease

If a genetic disease is caused by co-dominant alleles it will also only require one copy of the faulty allele to occur

• However, heterozygous individuals will have milder symptoms due to the moderating influence of a normal allele
• An example of a genetic disease that displays co-dominance is sickle cell anaemia

Inheritance Patterns for Dominant and Recessive Genetic Disorders

    
Cystic Fibrosis

• Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7
• Individuals with cystic fibrosis produce mucus which is unusually thick and sticky
• This mucus clogs the airways and secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts
• Heterozygous carriers who possess one normal allele will not develop disease symptoms

Huntington’s Disease

• Huntington’s disease is an autosomal dominant disorder caused by a mutation to the Huntingtin (HTT) gene on chromosome 4
• The HTT gene possesses a repeating trinucleotide sequence (CAG) that is usually present in low amounts (10 – 25 repeats)
• More than 28 CAG repeats is unstable and causes the sequence to amplify (produce even more repeats)
• When the number of repeats exceeds ~40, the huntingtin protein will misfold and cause neurodegeneration
• This usually occurs in late adulthood and so symptoms usually develop noticeably in a person’s middle age (~40 years) 
• Symptoms of Huntington’s disease include uncontrollable, spasmodic movements (chorea) and dementia

    
There are over 4,000 identified single gene defects that lead to genetic disease, but most are very rare

• Any allele that adversely affects survival and hence the capacity to reproduce is unlikely to be passed on to offspring

Recessive conditions tend to be more common, as the faulty allele can be present in carriers without causing disease

Dominant conditions may often have a late onset, as this does not prevent reproduction and the transfer of the faulty allele