434 SCIENCE PROGRESS 



of mating them, of mixed parentage. Your anticipation would 

 not be fulfilled. Your Andalusians would again produce 

 blacks, Andalusians, and splashed whites in the proportion 

 1:2:1. But if you were acquainted with Mendelian phenomena 

 you would immediately suspect that the Andalusian was a 

 heterozygote form, and that the two homozygote forms which 

 by their union gave rise to it were the blacks and the splashed 

 whites. This is indeed the case. Mate the black with the 

 splashed white, and you get nothing but Andalusians. The 

 obvious plan, therefore, instead of selling the blacks and whites 

 to the poulterer and keeping the Andalusians to breed from, 

 and then only getting 50 per cent, of what you want, is to sell 

 the Andalusians to fanciers, keep the blacks and whites, and 

 get 100 per cent, of Andalusians by crossing them, making 

 occasional matings of Andalusians to keep up the supply of 

 blacks and whites. 



This case is summarised in the following diagram : 



Black Splashed white 



1 Black 2 Andalusians 1 Splashed white 



Let us now return to the Peas. We have referred in detail 

 to only one pair of characters, namely, Tall and Dwarf. Mendel 

 dealt with six other pairs of characters, namely : 



1. The shape of the ripe seeds, which was either round or 



wrinkled. 



2. The colour of the ripe cotyledons in the seed, which 



were either yellow or green. 



3. The colour of the seed-coat, which was either white or 



grey (with or without purple spots). 



4. The texture of the ripe pods, which was either hard and 



inflated or soft and constricted between the seeds. 



5. The colour of the unripe pods, which was either green or 



yellow. 



6. The arrangement of the flowers on the stem. The flowers 



either came off normally at the nodes along the stem, 

 or they were situate in a bunch at the top of the stem ; 

 this latter is the so-called Mummy habit. 



