Mendel developed his genetic laws in 1866, using pea plants, but they were not rediscovered in the scientific literature until 1900. Mendel stated his laws in terms of "chance" or probability. In modern terminology, Mendel's First Law states that for the pair of alleles an individual has of some gene (or at some genetic locus), one is a copy of a randomly chosen one in the father of the individual, and the other if a copy of a randomly chosen one in the mother, and that a randomly chosen one will be copied to each child. He also said that each allele has an equal chance to be the one copied, and that the copyings of alleles to different offspring or from different parents are independent. This very basic set-up underlies all of genetics.
Since a parent has two alleles of each gene, the parent has 0.5 chance of passing one of the alleles to the offspring. For example, if a parent has a normal CF gene, and a mutant CF gene, he or she has a 0.5 chance of passing the mutant gene to the offspring. Likewise, he or she has a 0.5 chance of passing the normal gene to the offspring.
Segregation of the sex chromosomes works the same way. In the case of X-linked genes, the mother has two alleles for each X-linked gene, therefore she has 0.5 chance of passing one of them to an offspring. The father, on the other hand, only has one X, and he only passes it to his daughters. Therefore, the chance that he will pass an allele of an X-linked gene on to a daughter is 1, and that he'll pass it to a son is 0. He passes his Y chromosome to each son.