Twin studies are a beneficial tool when studying genetic heritability, especially in today's world, because today these external findings can be combined with molecular genetics research. Identical twins would be best to use for this type of study, as they are monozygotic (two beings created from the same egg and sperm) and therefore share all the same genes.
DNA samples can be taken from each twin to determine if there are differences in their actual genetic makeup. Researchers can also use the DNA to determine which genes contribute to the traits that manifest.
By studying the development of the twins, a researcher can estimate which traits are most inheritable. Using the DNA samples, they can identify which particular genes are causing those traits to manifest.
One issue that arises when using twins for this type of study is that, although they are identical, each twin will often exhibit different traits. Some twin studies failed to consider that environmental factors may have a part to play in why certain features manifest. It is crucial to select twins who interact with the same people and share in the same family environment.
Even so, one twin may experience exposure to different allergens than the other. Perhaps they meet different people in social environments that cause them to react emotionally to stress factors the other may never experience. These things can trigger traits to emerge in one twin that may not have under different circumstances.
Another problem is in the genetic mechanism itself. Genes have dominant and recessive traits; a dominant trait will overpower a recessive trait, which causes the dominant feature to manifest. Some genes have an additive effect and could interact with several different genes to determine the outcome of a single trait. The dominant and recessive gene mechanisms can also influence this additive effect, so often it is difficult to determine which genes are working in conjunction.
Designing a twin study cannot depend on a set methodology. The researcher must always look for factors that could influence the results. The analysis must include external factors in the form of foreign bodies, emotional reactions, environmental conditions, and genetic interactions to determine the reasons why variations in genetic traits manifest.
In the past, these issues made studies much more difficult, but combined with molecular genetics research, it will become easier to pinpoint the specific genes that actively associate with the traits that manifest and then determine whether they do so from external triggers, genetic mechanisms, or both.