Page:Popular Science Monthly Volume 85.djvu/337

Rh But the plants raised from these seeds, when self-fertilized, yielded seeds of four types, round and yellow (RY), wrinkled and yellow (WY), round and green (RG), and wrinkled and green (WG) in the proportion of 9:3:3:1 as shown in figure 54.

In this case also this ratio may be explained by assuming that the germ cells (ovules and pollen) are pure with respect to each of the contrasting characters, round-wrinkled, yellow-green, and therefore any combination of these may occur in a germ cell except the combinations RW and YG. Accordingly there are four possible kinds of germ cells as follows: $$ \begin{matrix} Y &         &         &  G  \\ \mid &\diagdown& \diagup &\mid \\ \mid & \diagup &\diagdown&\mid \\ R &         &         &  W \end{matrix} $$ i.e., YR, YW, GR, GW. Each one of these four kinds of pollen may fertilize each one of the same four kinds of ovules giving rise to sixteen combinations, no two of which are alike, as shown in Fig. 54. The dominant characters are in this case round and yellow, and only when one of these is absent can its contrasting character, wrinkled or green, develop. Accordingly the sixteen possible combinations yield seeds of four different appearances and in the following proportions: 9RY:3RG:3WY:1WG. Only one individual in each of these four classes is pure (homozygous) and continues to breed true in successive generations; in Fig. 54 these are found in the diagonal from the upper left to the lower right corner. All other individuals are heterozygous and show Mendelian splitting in the next generation.

When parents differ in three contrasting characters a much larger number of combinations are possible in the F$2$ generation. Thus if a pea with round (R) and yellow (Y) seeds, and with tall (T) stem is crossed with one having wrinkled (W) and green (G) seeds, and dwarf (D) stem all the progeny of the F$1$ generation have round and yellow seeds and tall stem, R, Y and T being dominant to W, G and D. But in the F$2$ generation there are sixty-four possible combinations (genotypes) of these six characters; but since a recessive character does not develop if its contrasting dominant character is present there are only eight types which come to expression (phenotypes) and in the following numbers: 27RYT:9RYD:9RGT:3RGD:9WYT:3WYD:3WGT:1WGD. Of these sixty-four genotypes only eight are homozygous and breed true (those lying in the diagonal between upper left and lower right corners in Fig. 55), while only one is pure dominant and one pure recessive (in the upper left and lower right corners of Fig. 55).

When the parents differ in one character only, the offspring formed by their crossing are called monohybrids, when there are two contrasting characters in the parents the offspring are dihybrids, when three, trihybrids, and when the parents differ in more than three characters the offspring are called polyhybrids. There are certainly few cases in which parents actually differ in only a single character, but since each contrasting character may be dealt with separately, as if it were the only