When the genotypes of the parents are known, you know what genotypes you may expect in the offspring. These expectations are based on the Mendelian laws, but due to random effects, the real world may deviate from the expectations. E.g. when you mate two parent dogs, one with the genotype Z/Z and the other with Z/z you will get offspring where on average 50 % has the genotype Z/Z and 50 % the genotype Z/z. But in a litter of 4 you even may get 4 puppies with Z/Z and 0 with Z/z. The reason is that by the creation of an embryo each time the chance for the genotype Z/Z is as large as for the Z/z genotype (each 50 %). This is a consequence of the Mendelian sampling: each offspring gets 50 % of the genetic package of a parent, but you do not know which half. Then the chance for 4 Z/Z puppies in a litter of 4 is 0.54 = 0.0625. Thus it is still possible to calculate the average expectation for the genotypes of the offspring when you mate two parents with known genotypes, but random effects cause deviations from the average expectation.
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