MODliRN THEORIKS OF lIERFiDITV. 385 



" i'ainted lady " lias a red standard and white wings; " Picotee " 

 and " Tinged white " appear as " dilnted " forms of the Purple 

 invincible and " Painted lady " respectively. The latter are 

 dominant over the former, owing to the presence of a 

 " strengthening " factor which we may call. S. All the coloured 

 forms are due to the meeting of two dominant factors which 

 may be called X and Y, one borne by one parent and one by the 

 other. This gives the ratio 9XY : 3Xy : 3xY : ixy; that is, 

 9 coloured : 3 + 3 -h ^ white. To explain the two primary 

 colours we require a fourth pair of allelomorphs, namely, a 

 ■' purple "' factor P, and its absence. Thus PXY gives purple 

 flowers, but pXY gives red flowers. The relation between the 

 shape of the pollen grains and the purple and red flow^ers has 

 already been, described, as an example of "gametic coupling." 

 This last example is one of the most complex of those worked 

 out by the Mendelian school, and has been chosen as one which 

 appears at first sight to be quite of a difiFerent nature. Very 

 few cases of heredity in hybrids, however, have been definitely 

 proved to be non-Mendelian. 



One of the recent developments of Mendelism is the idea 

 that sex may be a hereditary character, inherited according to 

 Mendelian laws. Owing to the nature of the case, the inheri- 

 tance of sex cannot be investigated in the ordinary manner, but 

 quite a number of carefully devised experiments have now been 

 <:arried out, most of which indicate that females are really 

 Jieterozygotes in which femaleness is dominant and maleness 

 recessive, while males are homozygous recessives. This, at any 

 rate, explains the approximate equality of males and females, 

 since all the male gametes bear only the male character, w^hile 

 equal numbers of " male " and " female "* gametes (i.e.. gametes 

 beariiig the characters of '' maleness " and " femaleness " respec- 

 tively) are produced by the female. These, then, mating at 

 random, will produce equal numbers of heterozygotes (females) 

 and homozygous males. It would take too long to explain the 

 experiments on which this idea is based. 



It is known that some diseases and certain peculiarities in 

 man are inherited according to Mendelian laws — as, for instance, 

 colour blindness and red hair — but the difficulty of collecting 

 data and the impossibility of experiment have made progress in 

 this direction very slow. 



In another direction Mendelian experiments have given 

 exceedingly valuable practical results, namely, in the breeding of 

 7vheats which combine many useful characters. By the use of 

 Mendelian methods we can choose almost any characters 

 occurring in dififerent varieties of wheats and combine these in 

 one breed. Two of the most important of these are " strength " 

 (a quality very important in baking the flour), which is dominant 

 over " weakness," and immunity to the attacks of " rust," which 

 is recessive to susceptibility. By the introduction of an immune 



