CROSSING. 61 



hybrid. In other words, in such cases of testmating coloured 

 hybrids, we find mono-factorial segregation. If we are growing 

 Emily Henderson sweet-peas, and find an xmexpected colour- 

 ed plant, this may be a hybrid from pollen of a coloured plant, 

 but equally as well from pollen of another white. Mated back 

 into our Emily Henderson strain it will probably show to have 

 one gene more, here responsible for pigmentation. If we are 

 lucky enough to lay our hands upon the original F, hybrid, we 

 can in the case of the sweet-pea test it by self-fertilization. In 

 this case the ratio between its coloured and white-flowered 

 off-spring will be 9 : 7 in the case its unknown parent was a 

 white, and 3 : 1 if it was a coloured. 



But it is evident, that if we did not discover coloured-plants 

 in our strain till the next generation, we might find that a 

 coloured plant in a white strain gave a 3 : 1 ratio of coloured 

 to white off-spring, even if its other unknown grand-parent 

 were a white. 



The most important fact we think, is this, that if we discover 

 a dominant novelty, we can ordinarily not decide whether 

 it owes this new dominant character to the spontaneous crea- 

 tion of a new gene, or to a combination of genes which each 

 <io not determine the novelty. The third possibility, namely, 

 the acquisition of the gene from a form in which it determined 

 the same character, can as a rule be excluded by inspection 

 of related forms. 



This case of the two white sweet-peas giving a dominant 

 coloured-form was soon followed by a great number of instan- 

 ces in which two parents, having a similar recessive character 

 produced hybrids with a corresponding dominant character. 

 In most instances the recessive character was not quite the 

 same in both parents, such as it was in the sweet-peas. The two 

 white chickens in Bateson's work which gave coloured young 

 had a different down-colour. The white Viennese rabbit, which 

 Baur found to give pigmented young with pink-eyed albinos 

 has itself blue eyes. The two yellow rats which originated in 

 England a few years ago, and which, when crossed, give wild- 



