14 Scientific Proceedings, Royal Dublin SocietiJ. 



But breed now with the lialf-bred progeu}', and what will liaitpen ? It is 

 a question of chances. The following diagram will show that there is one 

 chance of the two red half-determinants uniting, one of the two white, 

 and two of red uniting with white. 



•<---->• . • • o o 



Thus X "^vill give 



,/\, • o • o 



0,^—^0 



That is to say, if a sufBcient number of roan sliort-horns are mated 

 together, their progeny will work out in the proportion of one red to two 

 roans to one white. 



Another diagram will show that, when roans are mated witli whites, tlie 

 progeny will be one half roan and one half wliite. 



• <r — >o 



rpi \/ • ••00 



Thus X gives 



,/ ^^ 0000 



Still another will show that when roans are mated with reds, the progeny 

 must be one half red and the other half roan. 



•< — ->• 



\ / • • • • 



Thus /^ gives 



o<--^. ..00 



But all colour determinants do not work so simply. Black and red do 

 not blend like the red and the white of the Shorthorns. When black 

 cattle are crossed witli red, the progeny are not a mixture of red and black : 

 they are black. The black colour is dominant and gets its way. And 

 when these black hybrids are bred together, their progeny do not come out 

 like the progeny of roan Shorthorns, but in the proportion of three black 

 to one red. Only one, however, of the three blacks is pure : the other two 

 are hybrids, like tlieir parents, in which the black colour dominates the red. 



Let us put it diagrammatically as before, but using letters instead of 



circles, with black, the dominant colour, represented by the capital letter B, 



and red, the subdued' colour, by the small letter r. A pure black animal is 



B r ■ • 



represented by t, and a pure red by ,, while a hybrid between them is 



represented by , • If these hybrids are bred together, their progeny come 

 out thus : — • 



\/ • B B r r 



X gives n „ 



/\, ^ B r B r 



' Mendel used the word " recessive." 



