I was having a discussion with a colleague and this question came up...

It is commonplace in molecular genetics studies to only genotype one of a pair of monozygotic twins (and then implicitly impute the other's genotype by assuming the pair is genetically identical).

My question is this: assuming a quantitative trait study, what should the phenotype be? Should it be the phenotype of the genotyped twin OR should it be an average of the two twins' phenotypes?


  • I genotype Twin A
  • Twin A has a phenotype score of 4
  • Twin B has a phenotype score of 6
  • Should Twin A's phenotype for the genotype-phenotype test be 4 (the individual's phenotype) or 5 (the average phenotype of the genome)?

Answers from experience and from theory welcome!


  • $\begingroup$ I think it's quite unclear what you are asking $\endgroup$
    – rg255
    Nov 26 '14 at 21:14
  • $\begingroup$ Can you suggest what further information you need? Have added an example, if that helps? $\endgroup$
    – Joni
    Nov 27 '14 at 9:24
  • $\begingroup$ Interesting, perhaps it depends what analysis you plan to do later. If the study aims to estimate the heritability of the trait, wouldn't you want to record a separate datum for both twins, so that you can compare the variation present with different degrees of relatedness or genetic similarity. Can you explain what sort of quantitative trait study you mean and how it is done? Thanks! $\endgroup$
    – Teige
    Feb 4 '15 at 0:32
  • $\begingroup$ Hi Teige, what I meant by quantitative trait was that the phenotype was continuous rather than dichotomous, and so doing an average would make sense (if the test is YES/NO, then it makes little sense to be half a YES!) I'm assuming there is no genetic variance between identical twins, so there's no new information gained by having both twins' genotypes. I'm talking about an analysis where you're not using the fact they're twins per se, but rather just using one of them as an individual - do you randomly select one twin, or do you combine their phenotypes into one? $\endgroup$
    – Joni
    Feb 4 '15 at 17:13

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