228 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Mendel's Law of Segregation and Purity. 



In the second generation, when the hybrid dominants were self-fertilised, 

 Mendel found that segregation or splitting of the characters took place in 

 the ratio of three dominants (D) to one recessive (R) ; e.g. the hybrid 

 yellow peas, self -fertilised, gave on the average 75 per cent, yellows and 

 25 per cent, greens, both types often appearing in the same pod. The 

 law of segregation was also confirmed in the above experiments with 

 various plants and animals ; the recessive character, after " skipping a 

 generation," re-appeared apparently unchanged. 



In the third generation Mendel found that the extracted recessives 

 bred true, without reversion to their dominant parents or ancestors ; e.g. 

 the green peas extracted from the hybrid yellows bred true greens, with 

 no trace of yellow. Mendel also tested the dominants of the second 

 generation and found them to be of two kinds — in the ratio of one pure to 

 two hybrid. 



The pure dominants bred true, while the hybrid dominants segregated 

 again into 3D : IR ; e.g. the yellows bred from the hybrid yellows were 

 one- third pure yellows which bred true with no trace of green, and two- 

 thirds hybrid yellows, which gave 75 per cent, yellows and 25 per cent, 

 greens. 



Mendel continued these experiments through several generations with 

 the same result. The Mendelian formula for segregation is therefore 

 IDD : 2DR : IRR, i.e. 25 per cent, pure dominants, 50 per cent, hybrid 

 dominants, and 25 per cent, pure recessives. 



The law of purity was also confirmed in the above experiments with 

 various plants and animals, the extracted recessives in all cases breeding 

 true, with no trace of the dominant parents and ancestors. 



The Mendelian Theory. 



Mendel did not apparently offer any explanation of the facts of 

 dominance, nor has any satisfactory theory of dominance yet been pro- 

 pounded. 



Mendel, however, brought forward a simple theory, which fully 

 accounts for both the facts of segregation and purity, viz. the segregation 

 of the gametes (germ-cells) of the hybrid, so that on the average one-half 

 of the gametes carry the dominant character and one half the recessive 

 character. No gamete carries both. 



This is known as the theory of gametic segregation or gametic purity. 

 On this hypothesis the gametic constitution of the offspring of the hybrid 

 dominants (D x R) will be on the average (DR x DR = IDD : 2DR : IRR) 

 25 per cent, pure dominants, 50 per cent, hybrid dominants, and 25 per 

 cent, pure recessives. This result is in close agreement with the actual 

 numbers Mendel obtained in his experiments. 



Practical Application of the Mendelian Laws. 



When the Mendelian characters coincide with the " points " desired by 

 the breeder, as they do in many cases, a knowledge of Mendel's law will 

 enable him to get what he wants in the shortest possible time. If the 

 desired character is a recessive, it will breed true at sight. 



