Genetic Similarity

• The only truly DNA-identical people are monozygotic (MZ) twins whose birth is the result of one egg splitting after fertilization. MZ twins inhabit two separate embryos but share 100% of their dna
• Dizygotic (DZ) twins are the result of two separate eggs being fertilized by two separate sperm and hence share 50% of their dna
• MZ twins are useful for genetic similarity research since they have 100% shared DNA and hence act as each other's control group.
• Twin studies start by identifying a proband (the first participant) and the second twin as a comparison.
• Studies that use concordance rates (a large scale comparison between twins in which one variable like depression is measured, concordance occurs when both twins have depression) usually look at specific behaviors being inherited
• Genetic similarities are useful for determining whether there's a biological explanation for behavior or if the environment is also a key contributor to the behavior.
• Testing twins reared apart is extremely difficult to implement though, because not only are twins rare, twins raised separately are even rarer
• Kendler et al 2006 was a large scale Swedish twin study with over 15000 twin pairs that investigated the heritability of major depression
• Twin studies are limited in that factors like common age, sex, and appearance might account for similarities in behavior, not just genetics alone. So it's a bit difficult to disentangle environmental and genetic factors.
• Kinship or family studies investigate the heritability of a behavior by looking at its incidence over a number of generations. It's usually limited to three generations.
• Fernandez-Pujals et al 2015 was a correlational kinship study that investigated the heritability of MDD (major depressive disorder).