Human molecular diseases

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             Within this broad category of single gene defects, many different types of mutations can be responsible for the disorder. .
             Deletions in genes can be a major cause of many disorders. Deletions can either involve the deletion of the whole gene, as seen in Á-thalassaemia, or partial deletion of the gene. With partial gene deletion some of the encoded protein may still be translated, although often with severely disrupted function. An example of partial gene deletion can be seen with X-linked muscular dystrophies. Small deletions of just a codon, or several codons can lead to a gene lacking in several amino acids, this can affect the function of the protein to differing extents. An example of this type of deletion can be seen in Cystic Fibrosis. .
             Unequal crossing over during meiosis can lead to duplications of genetic material; this can have a large impact if the reading frame is disrupted.
             Another large cause of single gene defects is point mutations. A point mutation is when one base pair is exchanged for another base pair in the double-stranded DNA. Due to the degeneracy of the genetic code, this does not always result in a different amino acid being encoded, and therefore there is not always a change in the gene product. There are several different types of point mutations; missense mutations, nonsense mutations and frameshift mutations. A missense mutation involves the substitution of a different base which results in a different amino acid being encoded. A nonsense mutation also involves the substitution of a different base; however the result is that the codon now specifies a stop codon rather than an amino acid. The polypeptide chain is therefore prematurely terminated; this mutation has varying consequences depending where in the chain it occurs. The reverse of this can also take place; a stop codon can be changed into an amino acid encoding codon, due to mutation.