Assume that each characteristic is controlled by, not one, but two units of heredity (now known as alleles). Assume that one of these units/alleles is dominant over the other. For example, let the characteristic (gene) for seed shape be represented by S. Let the capital S represent the allele for smooth; small s represent winkled.
Considering the two alleles: plants with SS, Ss, or sSwill have smooth seeds; only plants with ss will have wrinkled seeds.
Further assume, Mendel said, that in reproduction an egg or sperm carries just one of the two alleles, which one is random chance. On fertilization the two alleles come together again and affect the characteristic as noted above.
Spelling this out in more detail, Mendel assumed the pure bred lines were SS—smooth, and ss—wrinkled. When these were crossed all the resulting seeds would be Ss or sS and be smooth because they all had the dominant gene. When these Ss/sS plants were cross in turn, however, 75 percent would be SS, or Ss; only 25 percent would be ss and show the recessive trait, wrinkled seeds in this case.
Show kids how a Punnett square is used to predict the result of such a problem. See following video.
Mendel published his findings in 1866. However, at the time there was nothing to connect it to. Mitosis (cell division) and meiosis (cell divisions leading to gametes) were not described until about 10 and 20 years later. Therefore, “interesting but meaningless theorizing” was the response of the scientific community; Mendel himself moved on and his work was put was put aside.