CHROMOSOMES IN MAN 299 



most susceptible are persistently weeded out : and thus, if the 

 infection persists long enough as a common mode of elimination, 

 a race may become relatively immune. No one doubts tKe 

 heritability of natural immunity, though there is still great 

 uncertainty as to what the mechanism of immunity is. 



10. Note on Chromosomes in Man 



In a very interesting paper (1906), H. E. Ziegler has illustrated 

 the modern doctrine of the material basis of inheritance with 

 particular reference to man. He takes the number of chromo- 

 somes in man as 24 (see page 46) ; but the argument is not 

 affected by the particular number. 



Let us take two parents, P 1 J and P 1 ? ; in each body-cell there 

 are 24 chromosomes, and in each mature germ-cell there are 

 12 chromosomes. Thus the fertilised ovum has again 24, and in 

 each cell of the offspring (F 1 ) there are 12 chromosomes of paternal 

 origin (from P 1 d) and 12 of maternal origin (from P 1 ?). 



In the mature sperm-cell or egg-cell of the parent (P 1 3 or 

 P 1 ?) there are 12 chromosomes, but it does not necessarily 

 follow that 6 of these must be from a grandfather (P 8 c?), and 



6 from a grandmother (P 2 ?). Why not ? Simply because in 

 the reduction of chromosomes from 24 to 12, which occurs in 

 maturation, it does not necessarily follow that the parental (P 2 ) 

 contributions are retained in equal number. The total number 

 12 always results, but it may be made up of 5 from P 2 3 and 



7 from P 8 ?, or of 8 from P 2 <$ and 4 from P* ?, and so on. 

 Suppose the mature sperm-cell had 9 from P 2 3 and 3 from P a ? , 

 then, as far as the paternal inheritance goes, we should expect 

 the offspring (F 1 ) to be very like its grandfather. 



The chances are that the grand-paternal and grand-maternal 

 contributions in any mature germ-cell will approximate to 

 equality, but the numerous possibilities enable us to see one 

 reason at least why there is often great diversity in a family 



