My question can be easily misunderstood, so please allow me to briefly explain the experimental conditions.

I have measured a variable from 120 individuals in total, 60 of one genotype and 60 of another. The distribution of the complete sample of each of the two genotypes is non-normal. Thus, to compare their means I have either used non-parametric tests or transformed the data to have normal distribution in each genotype and then used parametric tests. However, in 20 of the 120 individuals (10 of one genotype and 10 of the other), after I measured the initial response, I added a drug and measured again. To compare their means before and after the addition of the drug I should follow some related measurements test, like repeated measurements ANOVA, using only the values of the 20 individuals.

My question is, should I check again for normality in those 20 values, or assume that they should have non-normal distribution like the complete sample?

If these 20 values have a normal distribution, is it valid to use parametric tests only for these 20 individuals, despite the fact that the larger sample of 120 is clearly non-normal?

Thank you very much in advance.