688 



GENETICS AND EVOLUTION 



Trra.dia.te to 

 produce. mutaJbionS 



"Wild type'neurosporcL 



No growth 

 Min im al'ine dium. 



Figure 33.6. The method of producing and testing for biochemical mutants in 

 the mold, Neurospoia. See text for discussion. 



Similar one-to-one relationships of gene, enzyme and biochemical 

 reaction in man were first described by the English physician A. E. 

 Garrod, in 1909. Alcaptonuria is a trait, inherited by a recessive gene, in 

 which the patient's mine turns black on exposure to the air. The urine 

 contains homogentisic acid; the tissues ot normal people have an enzyme 

 which oxidizes homogentisic acid so that it is excreted as carbon dioxide 

 and water. Alcaptonurics lack this enzyme because they lack the gene 

 which produces it. As a result, homogentisic acid accumulates in the 

 tissues and blood and spills over into the urine. Garrod used the term 

 "inborn errors ol metabolism" to describe alcaptonuria and comparable 

 conditions such as phenylketonuria and albinism. 



It has recently been found that when a wing bud from a creeper 

 chick is transplanted onto normal chick blastoderm it will develop into 

 a normal wing, not a creeper wing. Evidently the creeper gene interferes 

 with the production of some substance reqviired for normal wing de- 

 velopment, a substance which can be supplied by the enzyme systems 

 of the normal tissue. If this missing substance could be identified and 

 supplied to a fertilized creeper egg in suitable amount, the egg might 

 develop into a normal rather than a creeper chick. 



The identification of the chemical and biologic mechanisms which 

 underlie differentiation remains one of the major unsolved problems of 



