Indessen wird es nach den Regeln der Wahrscheinlichkeit im Durchschnitte vieler Fälle immer geschehen, dass sich jede Pollenform A und a gleich oft mit jeder Keimzellform A und a vereinigt; es wird daher eine von den beiden Pollenzellen A mit einer Keimzelle A, die andere mit einer Keimzelle a bei der Befruchtung zusammentreffen, und eben so eine Pollenzelle a mit einer Keimzelle A, die andere mit a verbunden werden.
However, according to the rules of probability it will always occur on the average of many cases that each pollen form A and a unites equally often with each germ form A and a; one of the two pollen cells A will therefore come together with a germ cell A, the other with a germ cell a in fertilisation, and in the same manner a pollen cell a will be joined with a germ cell A, and the other with a.

However = Indesssen Introducing this sentence by way of contrast reveals that Mendel assumes that his readers are not familiar with the rules of probability, as the somewhat circuitous explanation of the combinatorial events also indicates.

rules of probability = Regeln der Wahrscheinlichkeit Bateson uses the singular. Mendel invokes what Siméon Denis Poisson (1781–1840) called the “law of large numbers”; see Joseph Hain, Handbuch der Statistik des österreichischen Kaiserstaates (Vienna: Tendler, 1852), vol. 1, p. 16. On the concept of “rule”, see p. 23, s. 4. According to Vítězslav Orel, Gregor Mendel: The First Geneticist (Oxford: Oxford University Press 1996), p. 76, Mendel learned the basics of probability theory from Joseph Johann von Littrow, Die Wahrscheinlichkeitsrechnung in ihrer Anwendung auf das wissenschaftliche und praktische Leben (Wien: F. Beck, 1833). Littrow states the law of large numbers in the following way: “that relationships of natural events, which initially are mostly variable and appear quite fortuitious, tend more and more towards a more constant relationship (Verhältniss), the more numerous these appearances are” (op. cit., p. 12). Just like Mendel himself (see p. 26, s. 3), Littrow was convinced that this is due to an underlying natural “tendency” (Neigung; op. cit., p. 11).

pollen form … germ form = Pollenform … Keimzellform See p. 3, s. 8.

germ cells = Keimzelle See p. 24, s. 4.

unites = vereinigt See p. 41, s. 6.

joined = verbunden Bateson has “unite”, Sherwood “associated”; see p. 10, s. 14.