GENETICS: THE HERITABLE TRANSMISSION OF CATALYSTS 155 



Mendel's First Law: Segregation 



The F 2 families always gave an average ratio of three tails to 

 one dwarf. The F 2 dwarfs always bred true to type, and so did 

 one-third of the tails. The other two-thirds of the tails behaved 

 just like the original F x hybrids; that is, they gave, in F 3 , three 

 tails to one dwarf. Mendel termed tallness a dominant charac- 

 ter, and dwarfness a recessive character, since it reappeared in sub- 

 sequent generations raised from the hybrid seed. Several other 

 pairs of contrasting characters gave similar results, which are 

 shown in the following diagram, where R indicates plants with a 

 recessive character, D dominants which breed true, and D dom- 

 inants which carry the recessive particular producer or gene, as it 

 is now called: 



Parents DXR 



i 

 Family Fi D 



1 



Family F 2 D D D R 



1 



Family F 3 D DDDK DDDR R 



Although each cell of the hybrid Fi evidently developed under 

 the joint influence of a dominant and a recessive gene, in both 

 the ova and the pollen grains these factors separate cleanly from 

 each other, or segregate, so that, according to the laws of chance, 

 each ovum or pollen grain would get a pure dominant gene, D, or 

 equally often a pure recessive gene, R. Thus with random or hap- 

 hazard fertilizations, we would get 



Pollen D R 



JVM 



Ova D R 



leading to an F 2 family averaging D/D; 2DR; R — that is, three 

 dominants to one recessive. 



Mendel tested this hypothesis by what is known as a backcross: 

 he fertilized dwarf flowers with pollen from F x hybrids, and ob- 

 tained, as he expected, equal numbers of tall- and dwarf- producing 

 seeds. The following diagram indicates what happened: 



Pollen of Fx hybrid (tall) D R 



Ova of recessive dwarf R R 



Progeny of backcross D/R, D/R ; R/R, R/R 



2 tall 2 dwarf 



