 These and similar observations were explained by Mendel as follows. He hypothesised that each plant contains a pair of ‘factors’ that together determine some aspect of its phenotype—in this case, seed shape. A plant inherits one factor from each of its parents. Suppose that there is one factor for round seeds \((R)\), another for wrinkled seeds \((W)\). There are then three possible types of plant: \(RR,\) \(RW\) and \(WW\). An \(RR\) plant will have round seeds, a \(WW\) plant wrinkled seeds. What about an \(RW\) plant? Mendel suggested that it would have round seeds—the \(R\) factor is ‘dominant’ over the \(W\) factor. The observations could then be easily explained. The initial pure-breeding lines were \(RR\) and \(WW\). The F1 plants were formed by \(RR \times WW\) crosses, so were all of the \(RW\) type and thus had round seeds. The F2 plants were formed by \(RW \times RW\) crosses, so contained a mixture of the \(RR, RW\) and \(WW\) types. If we assume that each \(RW\) parent transmits the \(R\) and \(W\) factors to its offspring with equal probability, then the F2 plants would contain \(RR, RW\) and \(WW\) in approximately the ratio 1:2:1. (This assumption is known as Mendel’s First Law or The Law of Segregation.) Since \(RR\) and \(RW\) both have round seeds, this explains why three quarters of the F2 plants had round seeds, one quarter wrinkled seeds.