E. R. Saunders 337 



viz. 1 in 4. This appears undoubtedly to be the case in the great 

 majority of families, but there are a certain number of cases in 

 which a considerably lower percentage of doubles was obtained, while 

 occasionally the proportion was in excess of this amount. The cases in 

 which the proportion of doubles is vei-y small are so marked and, in 

 some unions, of such frequent occurrence, that it seems clear that they 

 cannot be regarded as other than genuine — that they must in fact 

 represent a distinct ratio and not an extreme variation from the usual 

 3 s. : 1 d. On the other hand it seems highly probable that in the one 

 or two cases where the proportion of doubles recorded is distinctly 

 higher than 1 in 4 the excess is accidental. 



It is the frequent occurrence of numbers approximating to the ratio 

 3 s. : 1 d. which suggests that, in the ?io-ei-strains, the factors for 

 singleness (X and F) are so coupled that re-combination with x and y 

 in the garaetogenesis of Fi cannot occur, in the manner described 

 above for matings between two eversporting forms, where ovules with 

 X and Y (mcoupled meet xy pollen grains. This condition of single- 

 ness which is typical of non-double-throwing forms is, as stated above, 

 conveniently represented thus XY: and since as regards singleness 

 and doubleness, reciprocal heterozygotes of similar composition give 

 similar results, we may write XY for both the ovules and the pollen 

 of a typical no-d-lorm. But, as explained above (p. 33.5), a single 

 might breed true and yet not be pure-bred, a fact which should 

 not be overlooked in considering any unexpected result in Fn. For in 

 any cross between an eversporting and a true-breeding type, made 

 in the form d '^ x no-d ^, a certain number of the F^ singles will have 

 the composition XY XY \ they will breed true to singleness, and on 

 self-fertilisation will be indistinguishable from a pure-bred true-breeding 

 single having the composition XY XY. But the cross-bred true-breed- 

 ing single will presumably behave differently from a typical ^«re-bred 

 single, when crossed with an eversporting form; since in the one mating 

 the xy germ cells of the c^-type will unite with certain germ cells in 

 the wo-d-type carrying an uncoupled XY group, and in the other, not. 

 We have already seen that of the double-throwing plants assumed at 

 first to be pure-bred eversporting, some were probably cross-bred', and 

 it may well be that now and again the same may be found to be the 

 case with a supposed type single. From such a true-breeding but 



' See pp. 313 — 317 where an account is given of the behaviour of commercial samples 

 of the wliite and cream strains. 



