154 MENDEL'S LAWS 



One of each of these characters is always dominant over its 

 alternative, for example yellow in the cotyledons is dominant over 

 green, purple in flowers over white, round shape over wrinkled 

 shape in seeds. 



258. But, if a variety has one character which is dominant over 

 its alternative in another variety, it does not follow that the rest 

 of its characters are also dominant. One or more characters in the 

 second variety may be dominant. Consequently, if two varieties 

 are crossed, the first mongrel generation (all impure dominants] may 

 exhibit some of the characters of one variety and some of the other. 



259. Moreover, if a variety having a certain set of Mendelian 

 characters is crossed with a variety having another set, then the 

 descendants are apt to exhibit the characters in new combinations 

 even after they have become * pure.' Thus, if a variety with 

 inflated pods, wrinkled seeds, and green cotyledons be crossed 

 with one having constricted pods, round seeds, and yellow 

 cotyledons, the ' pure ' descendants may exhibit every combination 

 of the alternative characters, for example contracted pods with 

 round seeds and yellow cotyledons, or inflated pods with wrinkled 

 seeds and yellow cotyledons. It appears, therefore, that unlike 

 sexual characters, the alternative or Mendelian characters do not 

 necessarily hang together in sets, but are * inherited ' independently 

 of one another. When, therefore, we study heredity in terms of 

 the Mendelian theory, we must think, as always, not of the in- 

 dividual as a whole, but of his characters separately. Obviously, 

 the hypothesis that characters are represented in the germ-plasm 

 by units, which pair in the fertilized ovum but separate in its 

 germinal descendants, is an inference, not an observed fact. These 

 units cannot be seen in the germ-plasm, or, if seen, recognized. 

 It is this inference which is especially the Mendelian hypothesis 

 and on which stress is laid. 1 



260. The points to be especially noted when studying the 

 Mendelian theory are (i) the alternative units are supposed not to 

 blend or unite in the fertilized ovum (zygote) with their ' opposite 

 numbers,' but to remain distinct and ultimately to segregate, so that 

 the germ-cells (gametes) of the individual that arises from the 

 zygote contain each only one unit for each character ; hence the 

 terms ' unit segregation ' and ' gametic purity.' (2) The different 



1 " This purity of the germ-cells and their inability to transmit both of the 

 antagonistic characters is the central fact proved by Mendel's work. We thus 

 reach the conception of unit characters existing in antagonistic pairs." (First 

 Report to the Evolution Committee of the Royal Society, by Miss Sanders and Mr 

 Bateson, p. 126, 1902.) 



