Ascribing, say, the uniqueness of the Windsor face to nurture as opposed to nature goes beyond the realm of credibility. In his monastery gardens he created, by self-breeding, strains of peas that bred true for a given character like pea color or pod shape. Then he crossed his inbred strains with each other and observed how the various traits assorted in the progeny pea plants. Most importantly, he showed that each pea has two sets of these factors, one coming from the male parent, the other from the female. Mere examination of a plant's physical appearance need not reveal its genetic composition.
Recessive genes present in only one copy can be identified only by further genetic crosses.
Mendel further made the equally important observation that genes do not necessarily stay together when the male and female sex cells are formed. Instead, they often independently assort from each other, giving rise to progeny with sets of features very different from those of either parent. Mendel's work, done before the behavior of chromosomes during cell division was understood, almost had to lay unappreciated until the turn of the century, when three plant breeders working on the European continent, Correns, De Vries, and Tschermak, independently rediscovered the basic rules for hereditary transmission, which today we call Mendel's Laws. It was not until 1890 that the sex cells were found to possess only half the number of chromosomes present in adult cells. Fertilization through combining the haploid N number of chromosomes of the sperm with the haploid N number of the egg restores the 2N diploid chromosome number of adult plants and animals.