Two different study approaches will be considered here:
Both these study approaches have been of value in enriching our understanding of the contribution of genetic and environmental factors in disease causation.
Twin studies use the presence or absence of disease in twins as an indication of the level of disease causation that can be attributed to an individual’s genetic make-up as opposed to their environmental exposures.
Genetically, twins can be either monozygotic or dizygotic.
Monozygotic twins are considered to be genetically identical as they developed from the same fertilised egg, that divided into two shortly after conception.
In contrast dizygotic twins result from two different fertilised eggs, and therefore on average share around 50% of their genes, the same as any other pair of siblings who share the same mother and father.
Twins are said to be concordant for a condition, if both develop the disease of interest, and are said to be discordant, if only one twin develops the disease of interest.
If a condition is found to have a greater degree of concordance in monozygotic as opposed to dizygotic twins, the condition is assumed to have a greater genetic component as monozygotic twins share a greater degree of their genes in common compared to dizygotic twins.
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