218 ZOOLOGY, 



5. Reduction Divisions and Heredity. The behaviour of 

 the spermatozoon in fertilisation, the fact that the chromatin 

 of the male nucleus alone seems to be the essential element, 

 together with the fact that the offspring inherits as much 

 from the male parent as from the female, lead to the con- 

 clusion that the chromosomes are the carriers of heredity. 

 It was discovered by Mendel in 1865 that when two indi- 

 viduals of different varieties, differing in some one definite 

 character, were bred together, the character of one parent 

 appeared in all the offspring. For example, if a Dorking 

 fowl, which has an additional toe, five instead of the normal 

 number four, is crossed with a fowl of another breed 

 having only four toes, all the cross-bred chicks have the 

 extra toe. But when these cross-bred offspring are bred 

 together some of the chicks of the second generation have 

 the extra toe and some have the normal number ; that is to 

 say, some resemble one grandparent and some the other. 

 .The character which appears in all the individuals of the 

 'first generation is said to be dominant and the other 

 recessive ; and in the second generation the proportions of 

 the individuals possessing the dominant character to those 

 without it is three to one. Further, the recessives if bred 

 together breed true, and the dominant character does not 

 reappear ; they behave as though they were pure-bred and 

 had never been crossed Of the dominants, on the other 

 hand, one-third breed true or are pure dominants, and two- 

 thirds produce dominants and recessives again. Now these 

 curious facts would be explained if the gametes behaved in 

 certain ways ; or in other words we may say that from 

 these facts certain conclusions may be drawn concerning 

 the history of the gametes in the process of reproduction in 

 such a Mendelian experiment. In the first cross each 

 fertilised ovum or zygote consists of two gametes, one from 

 each parent, and in development the dominant character 

 conquers the other in every individual. But when the 

 gametes of these compound or mixed individuals are formed, 

 the characters of the two parents separate, or, as it is 

 technically termed, segregate, so that half the gametes 

 carry the dominant character and half only the recessive. 

 When two individuals of this cross-bred generation breed 



