SECTIONAL TRANSACTIONS.— D. 435 



lipochrome is absent, as in 1 birds, there is a series of blue forms. The 

 various tones of green and blue depend upon S and s. In birds with S the 

 feathers are modified in structure so that blue is less vividly reflected. 

 Heterozygotes are intermediate, being olive in the green series (L), and 

 cobalt in the blue series (1). 



Prof. R. R. Gates, F.R.S.— Colour inheritance in Man. 



(i) Eye-colour. — Simplex blue eyes are due to the absence of brown 

 (melanin) pigment from the anterior surface of the iris, the blue appearance 

 resulting from the posterior purple pigment of the choroid being reflected 

 through the muscle fibres in the iris. In European populations and in 

 crosses with American Indians, Eskimos and other races blue is a simple 

 recessive to brown, with certain complications not fully understood (see 

 Gates, Heredity in Man). Dominantly inherited blue eyes have recently 

 been observed in natives of Ceylon beUeved to be of pure Singhalese 

 descent. It may be noted that the intense ' black ' eyes of American Indians 

 and of most Singhalese as well as various other races is probably due to an 

 intensifying factor. Some Singhalese have pale brown eyes. The yellowish 

 sclerotic coat of the eye in negroes is due to dilute melanin pigment. 



(2) Hair Colour. — A recent statistical study of the post-natal develop- 

 ment of hair and eye colour in 2,670 schoolboys of Sheffield (MacConaill 

 and Ralphs, Ann. Eugenics, 7, 218-225, 1936) classifies those with blond 

 hair and blue eyes as leucochromes. This type falls steadily in numbers 

 from five years of age to a steady ratio of 17 per 1,000 at puberty. The 

 leucochrome is regarded as a Mendelian recessive, the other types of pig- 

 mentation, allochromes (i.e. dark hair and/or eyes) showing delayed domin- 

 ance. This is true of European peoples in general ; but in more pigmented 

 races the babies have intense ' black ' eyes and hair from a very early age. 

 Blue eyes and blond hair probably arose through mutations from dark hair 

 and eyes. Red hair is due to a lipochrome pigment independently inherited. 



(3) Skin colour. — In a study of skin pigmentation in parents and offspring, 

 the Davenports (Amer. Nat. 44, 641 and 705, 1910) found that in Caucasian 

 families blond X blond have only blond children and two albino parents 

 only albino children. Brunet is epistatic to blond, and some intermediates 

 at least are heterozygous. From negro X white crosses, Davenport 

 (Carnegie Inst. Wash. Publ. No. 188, 1913) concluded that two factors for 

 skin colour were present in the negro. In Ojibway Indian X white crosses 

 Gates concluded that two or more factors for skin colour are present, 

 one of which also affects eye-colour or is closely linked to a factor for 

 eye-colour. 



Friday, August 19. 



Joint Discussion with Section K (Botany), on The mechanism of evolution 

 (lo.o). 



Chairman : Prof. D, M. S. Watson, F.R.S. 



Dr. J. 8. Huxley, F.R.S. — Character gradients (lo.o). 



It has been found that many characters of organisms vary in a graded 

 way, apparently always in direct or indirect correlation with variations in 

 the external environment. Cline has been suggested as a technical term for 



