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 the 

 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, Prof. H. E. Ziegler has illustrated 

 the modern doctrine of the material basis of inheritance with 

 particular reference to man ("Die Chromosomen-Theorie der 

 Vererbung in ihrer Anwendung auf den Menschen." Archiv. 

 fiir Rassen- und Gesellschajts-Biologie, iii., 1906, pp. 797-812). 



Let us take two parents, P^ S and P^ $ ; 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^) there are 12 chromosomes of paternal 

 origin (from P^ S) and 12 of maternal origin (from P^ ?). 



In the mature sperm-cell or egg-cell of the parent (P^ S or 

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

 foUow that 6 of these must be from a grandfather (P^ ^), and 



6 from a grandmother (P^ ?). 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^) 

 contributions are retained in equal number. The total number 

 12 always results, but it may be made up of 5 from P^ c^ and 



7 from P2 ? , or of 8 from P^ S and 4 from P^ ? , and so on. 

 Suppose the mature sperm-ceU had 9 from P^ S and 3 from P2 ? , 

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

 the offspring (pi) 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 

 equahty, but the numerous possibilities enable us to see one 

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



