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578 BOTANY OF THE LIVING PLANT 



then it will be seen at once that the plants resulting from 



Combinations 1, 2 and 5 must be Crimson, Normal ( T s ff ths) 



those from „ 3, 4 , 7, 9, 10 and 13 „ Pink, Normal ( T %ths) 



6 „ Crimson, Peloric (tV^ 3 ) 



8 and 14 „ Pink, Peloric (yVths) 



11, 12 and 15 „ Ivory, Normal ( T Vths) 



16 ,, Ivory, Peloric dVth) 



If all red-flowered plants, whether crimson or pink, are grouped together the 



ratio is : 



9 Red, Normal : 3 Red, Peloric : 3 Ivory, Normal : 1 Ivory, Peloric. 



This 9:3:3:1 ratio is the typical F 2 ratio for a " dihybrid " cross. It is the neces- 

 sary consequence of random distribution of two independent pairs of allelo- 

 morphs by meiosis followed by random mating between the four resultant 

 types of nuclei. Naturally, a close approach to the theoretical ratio, in this 

 and in all other cases of segregation, will be realised in practice only when a 

 sufficiently large F 2 generation is grown. In the present instance there are 

 six F 2 phenotypes, because of the incomplete dominance of the crimson char- 

 acter. If both crimson flower-colour and normal flower-form had been incom- 

 pletely dominant, there would have been nine phenotypes ; and if both had 

 been fully dominant, there would have been only four. The number of distinct 

 genotypes is in any case nine. Four of these, Nos. 1, 6, 11 and 16, are homo- 

 zygous and when selfed will perpetuate their respective phenotypes ; the 

 remainder are heterozygous in one or in both characters and, on being selfed, 

 will show further segregation. The combinations of particular interest are 

 No. 16, the homozygous Ivory Peloric and No. 1, the homozygous Crimson 

 Normal. In these it will be seen that the characters of the two parents have been 

 recombined. This must necessarily arise in F 2 whenever the original parents 

 differ in two or more independent Mendelian characters. 



The inheritance of fruit-colour in the Tomato furnishes another example of 

 the genetical behaviour of plants differing in two pairs of genes, when crossed 

 with one another. Tomatoes are most commonly bright red with a shiny 

 surface ; but races are also known with matt red, golden yellow or pale yellow 

 fruits. The fruit-colour depends upon two pairs of genes. " R " gives rise to 

 red flesh (mesocarp) ; " r " to yellow flesh. " Y " produces a deep yellow 

 opaque skin (epicarp) ; " y " a faintly coloured translucent skin. The geno- 

 type of the ordinary bright red tomato is RRYY (giving red flesh plus deep 

 yellow skin). An RRyy tomato (red flesh plus pale translucent skin) is matt 

 red ; an rrYY tomato is golden yellow (yellow flesh plus deep yellow skin) ; 

 and an rryy tomato is pale yellow (yellow flesh plus pale translucent skin). 

 This case is of special interest because it illustrates a very wide-spread pheno- 

 menon, namely the production of a particular phenotype through the inter- 

 action of the effects of different genes. 



In the Tomato, the interaction is of an unusually simple and obvious nature, 

 the outward appearance of the fruit being determined by the various combina- 

 tions of two different skin-types with two different flesh-colours. In most 

 cases, the mechanism of the interaction is far more complex and recondite. 

 In the Snapdragon a very considerable number of genes are involved in the 

 production of the wide range of flower-colours found among the cultivated 



