THE DEVELOPMENT OF VERTEBRATA. 219 



together a dominant gamete may unite with a dominant or 

 with a recessive ; in the first case the offspring will be a 

 pure dominant, in the second a mixed individual. Similarly 

 a recessive gamete may unite with a recessive or with a 

 dominant; in the first case the result will be a pure 

 recessive, in the second a mixed zygote. The mixed zygotes 

 and the pure dominant zygotes develop into adults or chicks, 

 all showing the dominant character, and therefore the 

 dominants will be to the recessives as three to one, as they 

 are found to be by experiment. The gametes of the cross- 

 bred individuals may be represented by the symbols D and 

 R, and they must be supposed to occur in the gonad in 

 equal numbers nD -f- wR. The fertilisations will be repre- 

 sented by the symbols DD, RR, DR. If we take the Ds in 

 one individual, considering that they have an equal chance 

 of uniting with a D or an R in the other individual of oppo- 

 site sex, they will form an equal number of zygotes of forms 

 DD and DR. Similarly the Rs will form equal numbers 

 of zygotes of forms RR and RD, the latter of which 

 is the same as DR. Thus the total number of offspring 

 will be ttDD + 2wDR-f wRR, and since DD and DR 

 resemble each other in their visible character, the propor- 

 tion of dominants to recessives will be three to one. 



There is a very interesting connection between this 

 Mendelian theory of gametic segregation and the reduction 

 of the number of chromosomes which is known to take 

 place in the maturation of gametes. There is reason to 

 believe that the halving of the number of chromosomes is 

 due to the separation of the somatic chromosomes at one 

 of the reduction divisions into two groups, each containing 

 half the number. The reduction is thus due to segregation 

 of undivided chromosomes. If the chromosomes are the 

 carriers of heredity, the character of the male parent in a 

 cross must have been contained in the male chromosomes. 

 In the development of the offspring each somatic cell con- 

 tains both male and female chromosomes, but when the 

 gametes are formed separation of the chromosomes takes 

 place, so that an ovum may contain the character of one of 

 the parents or the other, but does not contain both ; and 

 thus on the average half of the gametes contain tjie domi- 



