A WORKING MODEL OF MENDELISM 



W. Burns 



Economic Botanist to the Government of Bombay; Agricultural College, Poona, India 



ONE of the first things told to a 

 student of Mendehan inher- 

 itance is that the distribution 

 of characters is according to the 

 law of chance: but to those who have 

 not learned to think mathematically, 

 chance in the abstract means little. I 

 have, therefore, used the following 

 method for the last five years to demon- 

 strate the 3:1 ratio in the simplest case 

 of Mendelian inheritance, and the way 

 in which it is controlled by chance. 



Take two packs of playing cards 

 without jokers, each containing the 

 usual fifty-two cards only. Put the 

 red cards (hearts and diamonds) from 

 both packs into one heap, and the black 

 cards (spades and clubs) into another 

 heap. These two heaps are now looked 

 on as masses of male and female 

 gametes, or germ-cells, of two plants 

 (the supposition would hold equally for 

 animals), one having a red character, 

 and the other a black character. 



If these two hypothetical plants are 

 crossed, we have a black gamete uniting 

 with a red gamete to make the zygote 

 or fertilized egg-cell from which the 

 new individual develops. To represent 

 the cross, we take a card at random from 

 each heap, as represented in Fig. 11. 



As red and black cannot be well 

 distinguished in a photograph, I have 

 represented the red cards by black vnth 

 white bars. These cards represent the 

 parental generation. 



Black is dominant in this cross; the 

 hybrid produced from such a union 

 therefore shows only the dominant 

 character (black), the red being hidden 

 behind it — recessive, to use the technical 

 term. This is shown in Fig. 12; the red 



Fig. 11 



Fig. 12 



has been allowed to stick up, in order 

 that its existence may be seen. In the 

 actual plant, the individual will show 

 only the black, the red being wholly 

 latent. This generation is designated 

 as the first filial or Fi generation. 



We now assume that this hybrid 

 plant of ours, showing the black dom- 

 inant character, but still carrying the 

 red as recessive, is self -fertilized. To 

 illustrate the process of self-fertilization, 

 make up both packs properly again, and 

 shuffle each very thoroughly. Each 

 pack will then consist of twenty-six 

 red and twenty-six black cards, mixed 



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