Page:Popular Science Monthly Volume 85.djvu/335

Rh

When a pure dominant is crossed with a mixed dominant-recessive all the offspring show the dominant character, though one half are pure dominant and the other half dominant-recessives. Thus if a pure round-seeded variety of pea is crossed with a hybrid between a round and a wrinkled seeded one, all the progeny are round-seeded, though one half of them carry the factor for wrinkled seed; this may be graphically represented as follows:

In subsequent generations the progeny of the pure round (RR) breed true and produce only round-seeded peas, whereas the progeny of the hybrid round and wrinkled (RW) split up into pure round, hybrid round and wrinkled, and pure wrinkled in the regular Mendelian ratio of 1RR : 2R(W) : 1WW (Fig. 52, C).

When a pure recessive is crossed with a mixed dominant-recessive another typical ratio results. Thus if a wrinkled-seeded variety of pea is crossed with a hybrid between a round and wrinkled seeded one, round-seeded and wrinkled-seeded peas are produced in the proportion, of 1 : 1. This is due to the fact that the hybrid produces two kinds of germ cells, the pure-bred but one, and the possible combinations of these are as follows:

This ratio of 1 : 1 is approximately the ratio of the two sexes in many animals and plants, and there is good reason to believe that sex is a Mendelian character of this sort, in which one plant is heterozygous for sex and the other homozygous.

2. Results of Crossings where there is more than one Contrasting Character.—It rarely happens that two individuals differ in a single character only; more frequently they differ in many characters and this leads to a great increase in the number of types of offspring in the F2 generation. But however many pairs of contrasting characters the parents may show each pair may be considered by itself as if it were the only contrasting pair, and when this is done all the offspring may be classified according to the regular Mendelian formula given above.