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!