Introduction to Genetic Testing

Genetic tests examine DNA, the chemical in our cells which provides our bodies with instructions about how to grow, develop and function. DNA is composed of two strands of four nucleotide bases, or “letters” (Adenosine (A), Thymine (T), Guanine (G), Cytosine(C)), arranged in a particular order. Each gene produces a specific protein, determined by the order of these bases. Humans have ~25,000 different genes, arranged on a number of thread-like structures, called chromosomes. We inherit our chromosomes from our parents, 23 from our mother and 23 from our father, so we have two sets of 23 chromosomes, or 23 ‘pairs’. A good description is if you think of genetics as the book of life, then the DNA are the letters, the genes are words, and the chromosomes are the chapters.

Often times, people living with the same inherited retinal disease (IRD), for example Retinitis Pigmentosa (RP) have varying mutations in the same genes or different mutations in different genes which causes the condition. Genetic testing allows for these specific mutations to be identified and can influence the health care and management of the individuals with these conditions into the future.

A mutation occurs when one or more of these bases are replaced by another base-pair. Some mutations don’t change the type of protein produced (synonymous mutation) and everything continues to function as normal. Other mutations called non-synonymous mutations alter the protein produced and can have a negative impact on how our bodies function. If you think of a mutation as a spelling mistake or a series of words changed in a sentence, then this causes a problem in the meaning and the sentence no longer makes sense. Retinal conditions caused by a genetic mutation are called inherited retinal diseases (IRDs).