Could randomly create inbred lines and do many different crosses to get an estimate for the population, but this is difficult due to inbreeding depression and time and labor expense. If you think about it, there are situations where we can easily separate out genetic and environmental components of variation by studying differences and similarities between individuals who share different proportions of their genes. One of the oldest attempts is the use of various types of twins by Galton. While this is an animal behavior course, I'm going to use mostly human examples since they are more interesting and controversial. Besides, we are animals too.
Monozygotic twins result from one egg fertilized by one sperm.
The zygote splits at an early stage of cell division to produce two identical genetic compliments. There will be minor environmental differences in utero due to position in the uterus, circulation, etc., but all genes are identical in the two offspring. Dizygotic twins result from two eggs ovulated at one time, fertilized by two sperm giving rise to two zygotes. If the sperm are from the same father, they are no more alike genetically than are normal sibs, although they share the uterine environment. Can be same or different sex.