CONTINUITY OF THE RACE 



587 



"dopa" ( dihydroxylphenylalanine ) , but this 

 oxidation process will not proceed with- 

 out the enzyme, dopa oxidase. In the white 

 pig this enzyme is not produced because 

 the gene responsible for its production is 

 absent. Without it, the oxidation of dopa 

 and the production of melanin cannot take 

 place, hence the pig has no pigmentation 

 and is white. 



Once this theory was established, an ef- 

 fort was made, naturally, to find character- 

 istics in man that could be explained the 

 same way. Of several, the one concerned 

 with the abnormality called alcaptonuria is 

 the best known. In this anomaly the urine 

 turns black upon standing, owing to the 

 presence of homogentisic acid which is oxi- 

 dized to carbon dioxide and water in the 

 blood of normal people by an enzyme. 

 Without the enzyme no oxidation takes 

 place, and the acid appears in the urine 

 where it eventually turns black. The enzyme 

 is apparently produced by a gene and those 

 people who have this anomaly do not pos- 

 sess the gene. Whether or not the action of 

 all protoplasmic activities can be explained 

 by this one-gene, one-enzyme, one-reaction 

 principle awaits further research. 



GENE COMBINATIONS 



As was learned in an earlier chapter, each 

 body cell contains a pair of genes for each 

 trait, a paternal gene from the father and a 

 maternal gene from the mother. The genes 

 lie in the same position in homologous 

 chromosomes and these points on the 

 chromosomes are called loci (singular — 

 locus ) . At the time of meiosis they separate 

 and are distributed to the sex cells, but 

 upon fertilization the pair is restored once 

 more. Obviously, if the gene in each of the 

 homologous chromosomes carries the po- 

 tentialities for producing the identical trait 

 in the offspring there is no evidence of 

 their existence. It is only when the genes 

 produce opposite effects that the mechan- 

 ics of inheritance can be studied. Two such 



contrasting traits are called allelomorphs or 

 alleles and they are always inherited in a 

 way that the offspring can possess one but 

 not both of them. For example, the gene 

 that controls color in guinea pigs is located 

 at one locus on the chromosome and it may 

 possess the ability to produce black coat or 

 brown coat in the offspring, but it cannot 

 produce both because only the one gene 

 can exist in one locus. There may be genes 

 that alter the hair in other ways such as 

 producing a rough coat, but these genes 

 would occupy other loci on the same or 

 other chromosomes and would therefore 

 have no effect on those that produce color. 

 Note that although the genes produce the 

 effect, they are not colored themselves nor 

 do they carry the actual trait in any way 

 except that the production of the trait is 

 bound up in the chemistry of the gene. 



When the homologous genes are alike, 

 the stock is said to be "pure" or, better, 

 homozygous, that is, it always breeds true 

 by showing only the one character. If, on 

 the other hand, it carries a gene for one 

 trait in one of its chromosomes and a gene 

 for a contrasting trait in the homologous 

 chromosome the individual is said to be 

 heterozygous or hybrid. This is best illus- 

 trated by the use of letters as symbols for 

 genes. For example, a homozygous black 

 guinea pig may be represented thus, WW; 

 likewise, a homozygous white pig may be 

 represented thus, ww. By convention, the 

 first letter of the trait that has appeared 

 most recently in evolution is used. In guinea 

 pigs, colored pigs probably preceded white 

 ones, therefore the letter w is employed. 

 Capital letters indicate the condition of one 

 gene whereas lower case letters represent 

 its allele or contrasting trait. An animal 

 carrying both genes, that is, a heterozygote, 

 would be represented by the letters Ww. 

 With this scheme it is a simple matter to 

 follow the genes from one generation to 

 the next. 



It is convenient to designate an indi- 

 vidual both in respect to its appearance and 



