Genetic Similarity as an Explanation for Behaviour
What is genetic similarity?
- Human beings are 99.9% genetically similar to each other i.e. you are almost 100% identical to every other person on the planet
- However, as we know from experience, there is huge variety amongst human beings in terms of physical appearance, skills, abilities, personalities etc.
- The only truly DNA-identical people in the world are monozygotic (MZ) twins whose birth is the result of one egg splitting post-fertilisation (from one sperm)
- MZ twins inhabit two, separate identical embryos that share 100% of their DNA
- Dizygotic (DZ) twins are the result of two separate eggs being fertilised by two separate sperm and thus share 50% of their DNA
Why are twins so useful for genetic similarity research?
- MZ twins - because they have 100% shared DNA, they act as each other’s control group in research which is looking for a biological basis to behaviour
- Twin studies begin by identifying a proband and then using the second twin as a comparison
- Studies which use concordance rates use MZ and DZ twins to look for evidence of specific behaviours being inherited e.g. rates of depression in MZ twins are compared to rates of depression in DZ twins
- Genetic similarities are useful for determining whether there is a biological explanation for a behaviour or whether the environment is also a key contributor to the behaviour being investigated
Is this moo-sic to your ears?...Or are you horrified by the thought?
Which other types of genetic similarity have been investigated?
- Adoption studies look for concordance rates between adopted children and their biological parents compared to their adoptive parents
- Adopted children share no genes with their adoptive parents therefore any similarities between them are thought to be caused by the environment
- Family studies look at inherited traits within the same family i.e. people can inherit conditions which predispose them to particular behaviours or conditions
Which studies investigate genetic similarity?
- McGuffin et al. (1996) – a twin study using concordance rates for depression in MZ and DZ twins
- Brunner et al. (1993) – a family study in which a dysfunctional MAOA gene was linked to anti-social behaviour seen in the affected males of a large family in the Netherlands
The studies by McGuffin et al. (1996) and Brunner et al. (1993) can be found in ‘Two Key Studies of Genetic Similarity’ on this site: just navigate the Genetics & Behaviour topic to find it
Exam Tip
On section A of Paper 1 you may be asked to outline/describe/explain one twin or family study, so make sure you don’t write about both a twin and a family study
Exam Tip
You can use Brunner et al. (1993) to answer a question on the effect of neurotransmitters (serotonin) and also a question on genes and behaviour
Worked example
ERQ (Extended Response Question) - 22 marks
‘To what extent could genetic similarity be said to affect behaviour?’ [22]
The following paragraph addresses the command term ‘To what extent’ by assessing how successful the research is in explaining behaviour:
Twin studies such as Mc Guffin et al. (1996) use concordance rates to measure the key variable of interest, in this case depression, as experienced by MZ and DZ twins. The findings of this study could provide some evidence for the idea that depression has a biological basis i.e. that the outcome of one MZ twin can be determined by the outcome of the other MZ twin due to their identical DNA.
What this study does not do is to provide clear biological evidence alone in support of nature over nurture. The fact that a higher concordance rate for depression in MZ twins was found may instead be based on the fact that MZ twins are more likely to spend greater amounts of time together than DZ twins (who may be of the opposite sex).
