83 PRESIDENTIAL ADDRESS SECTION D. 



brings in the red factor from the mother, while the Y chromosome 

 does not bring in any dominant factor. At the ripening of the 

 germ cells in the F ± female the number of chromosomes is reduced 

 to half. There result two kinds of eggs, half with the red-bearing 

 and half with the white-bearing X. Similarly in the male there 

 will be two classes of sperm, half with the red-bearing X chromo- 

 some, half with the indifferent Y chromosome. Random meeting 

 of eggs and sperm will give ... a 3 : 1 ratio, as in other Mendelian 

 crosses, but the white individuals in F 2 will be males. The factor 

 for red in the F x male will always stay in the X chromosome, so 

 that all the female-producing spermatozoa will carry red, and con- 

 sequently all F 2 females will be red. The males will have red 

 eyes if they receive the red-bearing chromosome from their mother 

 and white eyes if they receive the white-bearing chromosome from 

 their mother. 



The reciprocal cross is made by mating a white-eyed female to 

 a red-eyed male. The daughters will have red eyes and the sons 

 white eyes. If these are inbred their offspring will be red and 

 white in equal numbers, and not the visual three reds to one white." 



In Lepidoptera and birds there is a surprise as regards sex 

 chromosomes, for the usual conditions are reversed. The eggs are- 

 male and female-determining, not the spermatozoa, as the sperm 

 are alike and the eggs are different as regards accessory chromo- 

 somes. 



The sex-chromosome hypothesis explains sex-limited in- 

 heritance, such as colour blindness (commoner in males) and 

 haemophilia. 



Turning now to man, it is of interest to note that in the negro, 

 according to Guyer and to Montgomery,* there is only half the 

 number of chromosomes found in the white race. In the latter the 

 somatic number is 48 in the female and 47 in the male. 



Most interesting work has been done by Alan S. Parkes on 

 "Sex heredity with special reference to the abnormal numerical 

 inequality between the sexes." Investigations on the subject have 

 taken two lines, first, breeding experiments and pedigrees of families 

 showing sex-limited characteristics, and second, cytological research 

 where "a physical factor was looked for as an elaboration of the 

 chromosome theory." The supernumerary pair of chromosomes, or 

 a pair in one sex and one — or one and a vestigial one — in the other 

 sex, provided the explanation. The result has been to establish 

 that man is grouped with the higher animals and with the Droso- 

 phila fly in possessing heterozygous males and homozygous females. 

 The inclusion of man Is based on the study of sex-limited charac- 



* Since writing the above my attention has been drawn to a letter by 

 T. S. Painter in "Science," May 27. 1921, who accepts the results 

 of H. von Winiwarter as regards the number of chromosomes in 

 man (approximating to 48), and apparently finds no difference in 

 number tor whites and American negroes. Unfortunately Painter 

 does not appear to have followed up his work by a more complete 

 publication, and it is hardly a correct procedure to rush into print 

 with a mere letter on such an interesting matter, without stating his 

 evidence more fully. 



