Types of Genetic Mechanism
(A) Trinucleotide repeats
Approximately 10% of the human genome is composed of repeti-tive DNA repeats which are usually inherited in a stable form. Certain trinucleotide repeats become unstable and prone to dra-matic expansion affecting the expression of neighbouring genes.
Trinucleotide repeat expansion diseases
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
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:
- Uniparental disomy: each chromosome of a pair has been in-herited exclusively from one parent, usually by non-disjunction.