Sex-linked Lethal Factors in Man. hi 



The rats on the ordinary bread grew very poorly, gaining on 

 the average only 18 grams in 9 weeks. The rats on extra-yeast 

 bread grew much better, gaining 59 grams on the average in 

 9 weeks. 



The superior nutritive value of the extra-yeast bread was 

 ascribed to its high content of water-soluble B and to the supple- 

 mental action of the complete protein of the yeast. 



53 (1635) 



Evidence for sex-linked lethal factors in man. 



By C. C. LITTLE and MARION GIBBONS. 



[From the Carnegie Station for Experimental Evolution, Cold 

 Spring Harbor, N. Y.] 



Among the points brought to light during the investigations 

 which followed the rediscovery of Mendel's Law of Heredity were 

 two of especial interest to medical men. These were lethal factors 

 and sex-linked inheritance. 



A lethal factor is a Mendelian unit which can be carried by a 

 normal individual as can any recessive, but which when present 

 without its normal allelomorph to balance it, causes the death of 

 the individual possessing it. Among those lethal factors demon- 

 strated for mammals is that for yellow coat color in mice. The 

 color of the wild house mouse is called by geneticists "black 

 agouti." It has as an alternative condition or allelomorph, a type 

 in which almost if not all, black and brown pigment has disap- 

 peared from the coat leaving only the yellow pigment unmodified. 

 When black agouti mice are crossed together yellows are never 

 produced. When yellows are crossed together however, yellows 

 and black agoutis are produced in a ratio of 2 to 1. If the yellows 

 had been ordinary mendelian heterozygotes, the ratio should have 

 been 3 to i,but it is clear that the 2 to 1 ratio is the one involved. 



The yellows so produced are never homozygous, as they should 

 be in 33 per cent, of the cases, were unmodified mendelian inheri- 

 tance involved. The obvious hypothesis is that the homozygous 

 yellow individuals start their development, but perish in early 



