HEREDITY IN MAN 361 



are chromosomal determiners which arc capable of a certain 

 response if th(> right conditions prevail. 



Unit Characters. The complexity of human responses is so 

 baffling that it is often impossible to discover all of their under- 

 lying causes. In simpler conditions the behaviour of heritable 

 characters can often be traced through several generations, so 

 that the occurrence of unit characters in man is well established. 

 Eye color, hair color, pigmentation of the skin, curhness of hair, 

 polydactj^ly and symphalangism are among the well known struc- 

 tural characters in this category. 



The chromosomes of man are also similar to those of other 

 animals and furnish a basis for heredity of the same type. Various 

 investigators have studied the cytology of human cells with the 

 result that the chromosome number is commonly accepted as 

 forty-eight. Of these, forty-six are autosomes and two allosomes. 

 There are an x and a y chromosome in the male and two x chromo- 

 somes in the female. The gametes therefore contain either 

 23+x or 23+y. This is only a moderately large number but con- 

 sidering only one determiner to a chromosome it affords the 

 possil)ility of 4^^ or more than two hundred thousand billion 

 recombinations. The diversity of human beings is not surprising! 



Eye Color. The color of the eyes depends upon the presence 

 of two pigments, brown and blue. Brown pigment varies greatly 

 in quantity, so that brown-flecked blue eyes are common, but 

 when distributed through the entire area of the iris it masks the 

 blue because it lies in front. It is dominant over lack of 

 brown, which is, under ordinary conditions, equivalent to domi- 

 nance over blue. The two are not allelomorphic, but the aflelo- 

 morph of brown permits the blue pigment to show and only 

 in albinos can the absence of blue be seen. Brown-eyed parents 

 may be heterozygous and are therefore able to produce blue-eyed 

 children. 



Hair Color. Hair color is also due to two pigments. Its be- 

 haviour is not thoroughly understood because of the occurrence 

 of various modifying conditions, but darker colors are dominant 

 over light hair and inheritance is in general similar to that of 

 eye color. Black-haired parents may produce blond children but 

 blond parents cannot produce brunettes.^ 



'A recent article by Hausman {Am. Nat. LXI, 545-554, 1927) contains many 

 interesting facts on the pigmentation of iiuraan hair. 



