a quick word on types of twins
Fraternal twins (Dizygotic)
Before getting into each type of twin studies, it’s essential to know what types of twins there are. One type of twin is referred to as monozygotic, or MZ. These are commonly known as identical twins. To form MZ twins there must be fertilization of one egg which later splits into two. Secondly there are dizygotic, or DZ. These are known as fraternal twins. These involve fertilization of two separate eggs. MZ twins share 100% of their genetic material; DZ twins share 50%, (Taylor, Jeanette). Another way to look at it is that MZ twins are composed of 100% of the same material, while DZ twins can be considered similar to regular non-twin siblings, only having been born at the same time.
Classical mz-dz comparisonThis type of twin study compares MZ twins to DZ twins. For the most part, MZ twins are more so similar than DZ twins. This correlates with the fact that MZ twins have a much higher similarity genetically (Twin Study Wikipedia).
This is where the combination of genetics and environment are studied. For instance, a pair of MZ twins may be exactly alike genetically, but if they grew up in different environments from each other (i.e. living with abusive parents vs living with nurturing ones), the two twins are most likely going to be different in more than likely many ways, focusing more on the phenotypical in this study. This is considered the most common twin study (Santrock). |
Multivariate analysisThe purpose of this type is to examine multiple variables at once. In this case, these variables are genetically correlated traits.
"For example, consider two traits - dark skin and black hair. These two traits may individually have a very high heritability (most of the population-level variation in the trait due to genetic differences, or in simpler terms, genetics contributes significantly to these two traits), however, they may still have a very low genetic correlation if, for instance, these two traits were being controlled by different, non-overlapping, non-linked genetic loci (Genetic Correlation Wikipedia)." |
co-twin control sCo-twin control studies involve "case-control studies of MZ twins who are perfectly matched for genes and family background (Nature.com)."
They also measure gene expression in discordant twins. Discordant twins are usually MZ, but can sometimes be DZ. One twin of such description is born larger than the other one, with a difference of at least 10% in weight. |
Extended Twin studyLike the Classical MZ-DZ Twin Study, the Extended Twin Study takes environment into consideration, but also studies the parents of the twins. Focusing more on the families of the twins, an example of such a study could include the prenatal environment and how it affects the unborn twins (Nature.com).
This takes into account the sensitivity of genetics in response to environments, giving study to parents as well. On a similar topic, there is the adoption study, which seeks to discover whether behavior and psychological characteristics of adopted children are more like those of their adoptive parents, who provide a home environment or are more like those of their biological parents, who have contributed to their heredity. |
genotyping at candidate lociFor MZ twins this involves looking at the genotypes in order to detect genetic variability (Nature.com).
For DZ twins this involves looking at the genotypes to "estimate associations within and between families" (Nature.com). |
genotyping at Marker LociThe marker locus (loci is the plural form) is part of a chromosome that can be used for identification and for use in linkage analysis (Marker Loci).
This twin study type looks at the genotypes of DZ twins along with their parents to study the marker loci, basically finding the linkage to the QTL, which is the quantitive trait loci (Nature.com). |
Importance of twin studies
By studying twins we can gain a deeper understanding in how genetics work. Not only do we learn how genes are inherited, but we also learn how factors such as the environment take effect on genes. This gets into the idea of Nature vs Nurture, a classic topic in psychology. We learn how the environment affects individuals and to what degree. Not every set of twins is the same as the next set of twins, so we see examples of variability.
Works cited
"Genetic Correlation." Wikipedia. Wikimedia Foundation, 19 July 2012. Web. 13 Sept. 2012. <http://en.wikipedia.org/wiki/Genetic_correlation>.
"Marker Locus." - Definition from Biology-Online.org. N.p., n.d. Web. 13 Sept. 2012. <http://www.biology-online.org/dictionary/Marker_locus>.
Nature.com. Nature Publishing Group, n.d. Web. 13 Sept. 2012. <http://www.nature.com/nrg/journal/v3/n11/fig_tab/nrg932_T1.html>.
Santrock, John W. Children. New York: McGraw Hill, 2012. Print.
Taylor, Jeanette, Ph.D. "Research With Twins." Twin Research. Taylor Research Lab, n.d. Web. 13 Sept. 2012. <http://www.psy.fsu.edu/~taylorlab/TwinRsch.htm>.
"Twin Study." Wikipedia. Wikimedia Foundation, n.d. Web. 13 Sept. 2012. <http://en.wikipedia.org/wiki/Twin_study>.
"Marker Locus." - Definition from Biology-Online.org. N.p., n.d. Web. 13 Sept. 2012. <http://www.biology-online.org/dictionary/Marker_locus>.
Nature.com. Nature Publishing Group, n.d. Web. 13 Sept. 2012. <http://www.nature.com/nrg/journal/v3/n11/fig_tab/nrg932_T1.html>.
Santrock, John W. Children. New York: McGraw Hill, 2012. Print.
Taylor, Jeanette, Ph.D. "Research With Twins." Twin Research. Taylor Research Lab, n.d. Web. 13 Sept. 2012. <http://www.psy.fsu.edu/~taylorlab/TwinRsch.htm>.
"Twin Study." Wikipedia. Wikimedia Foundation, n.d. Web. 13 Sept. 2012. <http://en.wikipedia.org/wiki/Twin_study>.