Data from a foster feeding experiment with rat mothers and litters of
  four different genotypes: A, B, I and J.
  Rat litters were separated from their natural mothers at birth and
  given to foster mothers to rear.
genotypeThe data frame has the following components:
Littergenotype of the litter.
Mothergenotype of the foster mother.
WtLitter average weight gain of the litter, in grams at age 28 days. (The source states that the within-litter variability is negligible.)
Venables, W. N. and Ripley, B. D. (2002) Modern Applied Statistics with S-PLUS. Fourth Edition. Springer.