PHENOMENA OF INHERITANCE 431 



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 propor- 

 tion 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 F 2 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 domi- 

 nance 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 



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 



