EVOLUTION, HEREDITY, EUGENICS S3S 



Sex-Linked Characters. — Along with the accessory chromosome 

 go certain " sex-linked " characters. Morgan has shown that there 

 are more than loo such characters in the fruit fly. In man, we 

 know of five which appear in the male when simplex and in the 

 female when duplex with reference to accessory chromosomes. 

 These are Gowers muscular atrophy, haemofhilia (slow clotting of 

 blood), color blindness or Daltonism (red from green), nightblindness^ 

 and neuritis optica (progressive atrophy of the optic nerve). These 

 sex-linked characters require two determiners for t\\Q female, and but 

 one for their appearance in the male. It is interesting to recall the 

 fact that Charles Darwin long ago noted that male albino cats with 

 blue eyes are usually deaf. 



Identical vs. Fraternal Twins. — It has long been known that 

 there are two kinds of twins, one type like ordinary sisters, brothers, 

 or brother and sister, coming from different eggs, and c^W^d fraternal 

 twins; while the other kind, produced by the division of one egg cell, 

 always appear to have the same characteristics and are always of 

 the same sex. These last are called identical twins and furnish 

 evidence that goes far towards substantiating the theory of sex 

 determination before cell division. Likewise, studies on the arma- 

 dillo, which produces from one ovum four identical offspring of the 

 same sex, would seem to clinch the matter as to the time of sex 

 determination. 



Earlier workers were wont to rely upon the types of fetal mem- 

 branes in determining the origin of twins. The assumption that 

 all monochorional twins are monozygotic and all dichorial twins are 

 dizygotic has met with considerable opposition, since several remark- 

 able exceptions to the rule have been reported. In a paper by 

 Curtius (1930), three strikingly identical sets of dichorial twins are 

 reported as monozygotic. In these cases, it is probable that the 

 twinning took place during early cleavage, possibly as far back as 

 the four-cell or the eight-cell stage. It could not have occurred at 

 the two-cell stage, as this is not equational. Newman (1931) has 

 discussed such remarkable cases, and pointed out that studying the 

 finger prints of twins should be regarded only as a final check on 

 diagnoses. 



References on Twins 



Curtius, Fr. 1930- Nachgeburtsgefunde bei Zwilligen und Ahnlich- 



keite-diagnosis. Arch. f. Gynakologie, Bd. 140, Heft 2 u. 3. 

 Newman, H. H. 1917. The Biology of Twins. Chicago Press. 



