File:EB1911 Mendelism - tall and dwarf pea breeding conceptualization.jpg

English: Conception of what occurs when a cross is made between two individuals may be illustrated by the diagram shown. Zygotes are here represented by squares and gametes by circles. The dominant and recessive characters are indicated by small plain and black rectangles. Each zygote must contain two and each gamete but one of these unit-characters. Zygotes such as the original parents which breed true to a given character are said to be homozygous for that character, and from their nature such homozygotes must produce identical gametes. Consequently when a cross is made only one kind of zygote can be formed, viz. that containing both the dominant and recessive unit-characters. When the germ-cells of such a hetero zygote split to form gametes, these, as indicated in the diagram, will be of two sorts containing the dominant and recessive characters respectively, and will be produced in equal numbers. If we are dealing with a hermaphrodite plant such as the pea the ovules will consist of one half bearing only the dominant character and one half bearing only the recessive character; and this will be true also of the pollen grains. Consequently each dominant ovule has an equal chance of being fertilized by a dominant or by a recessive pollen grain, and the dominant ovules must therefore give rise to equal numbers of dominant homozygous and of heterozygous plants. Similarly the recessive ovules must give rise to equal numbers of recessive homozygotes and of heterozygotes. Hence of the total offspring of such a plant one quarter will be pure dominants, one quarter recessives, and one half heterozygotes as indicated in the diagram.