Page:Popular Science Monthly Volume 85.djvu/435

Rh the latter is completely dominant. Black and white appear only when they are pure (homozygous), blue only when both black and white are present (heterozygous).

Again, a cross of red and white cattle produces roan offspring, but the latter when interbred give rise to reds, roans and whites in the proportion of 1 : 2 : 1, showing that the roans are heterozygotes in which red is not completely dominant over white, while the reds and whites are homozygotes and consequently breed true.

Lang found that when snails with uniformly colored shells were crossed with snails having bands of color on the shells the hybrids were faintly banded, thus being more or less intermediate between the two parents; but when these hybrids were interbred they produced banded, faintly banded and uniformly colored snails in the ratio of 1:2:1, thus proving that Mendelian segregation takes place in the F2 generation, and that dominance is incomplete in the heterozygotes. Many other similar cases of incomplete dominance are known.

Sometimes dominance is incomplete in early stages of development, but becomes complete in adult stages. Davenport found that when pure white and pure black Leghorn fowls are crossed the chicks are speckled white and black, but in the adult fowl dominance is complete and the plumage is black. Similar conditions of delayed dominance are well known in the color of the hair and eyes of children, though dominance may become complete when they have reached adult life.

In a few instances a character may be dominant at one time and recessive at another. Thus Davenport found that an extra toe in fowls is dominant under certain circumstances and recessive under others. Tennent found that characters which are usually dominant in hybrid echinoderms may be made recessive if the chemical or physical nature of the pea water is changed. Such cases seem to show that dominance may sometimes depend upon environmental conditions, sometimes upon a particular combination of hereditary units.

Sex and sex-limited inheritance may be considered here, since they involve questions of dominance. There is good evidence, as was shown in the last lecture, that sex is a Mendelian character, in which the female has a double dose of the determiner for sex, whereas the male has only a single dose. Consequently in the formation of the gametes every egg receives one sex-determiner, while only one half of the spermatozoa receive such a determiner, the other half of them being without it. If. then, an egg is fertilized by a sperm without one of these determiners, a male results; but if an egg is fertilized by a sperm with one of these determiners, a female is produced. This is graphically represented in diagram 60, in which X represents the sex determiner, which is duplex