epistatic与interaction之间的区别与联系？
genetic上的interaction是如何定义的？
Epistasis is the phenomenon where the effect of one gene (locus) is dependent on the presence of one or more 'modifier genes', i.e. the genetic background.
Epistasis上位效应，是指一个基因或者基因座在表型上的体现，依赖其他基因或基因座。换句话说就是一个基因的效果被其他基因mask了。
In classical genetics, if genes A and B are mutated, and each mutation by itself produces a unique phenotype but the two mutations together show the same phenotype as the gene A mutation, then gene A is epistatic and gene B is hypostatic. For example, the gene for total baldness is epistatic to the gene for brown hair.
在经典遗传学中，如果基因A和B都突变了，理论上它们都有各自独特的表型，但是如果A和B都突变了的表型与单独A突变的表型一样，那么就可以称A是epistatic，B是hypostatic。举一个极端的例子：比如控制秃顶的基因A就是控制发色的基因B的epistatic。
这时就会出现有趣的悖论：显性隐性的规律不起作用了。
sample size极大地影响了power，对于rare variant的影响更大，最大的就是interaction了。
如下：我们想看rare RET CD variants对表型的影响是否被common的RET risk alleles改变了。这里的common的就相当于秃顶基因座，rare就是发色基因座。
RET Common and Rare Variants Epistasis
To assess if the effects of rare RET protein-alternating variants changed with the dosage of common HSCR associated risk alleles (T for rs2435357 and A for rs9282834), we stratified samples into 3 groups: carrying zero, 1, or at least 2 common HSCR-associated risk alleles. Samples were further subdivided into 3 subgroups (totaling 9 , 3x3 combinations), according to the presence of mutations and their predicted pathogenicity (damaging and benign).
Epistasis：The effects of an allele at one gene hide the effects of alleles at another gene;
Recessive epistasis: The recessive allele of one gene hides the effect of another gene;
Dominant epistasis: The dominant allele of one gene hides the effect of another gene;