Types of Genetic Mechanism

(A) Trinucleotide repeats

Approximately 10% of the human genome is composed of repetitive DNA repeats which are usually inherited in a stable form.  Certain trinucleotide repeats become unstable and prone to dramatic expansion affecting the expression of neighbouring genes.

Interestingly, to date, trinucleotide repeat expansion has been implicated only in conditions with a neurological component, for example:

• Huntington’s disease
• Myotonic dystrophy
• Fragile X syndrome
• Friedrich’s ataxia
• Spinocerebellar ataxia

(B) Imprinting

Genomic imprinting is a genetic mechanism by which genes are selectively expressed from the maternal or paternal homologue of a chromosome.  Imprints are erased during the early development of the male and female germ cells and then reset prior to germ cell maturation.  The imprint remains throughout life.

Imprinting defects are more common following IVF, with Beckwith-Wiedemann syndrome being particularly more common.

Imprinted genes can be altered in different ways to produce a non-functioning copy of a gene with a resultant phenotype:

• Mutated
• Silenced
• Deleted
• Uniparental disomy: each chromosome of a pair has been inherited exclusively from one parent, usually by non-disjunction.