596 



CONTINUIT 



recessive, there apparently is no enzyme to 

 oxidize phenylpyruvic acid in the blood, so 

 that it appears in the urine. This simulates 

 alcaptonuria mentioned earlier, except that 

 people who have phenylpyruvic acid in 

 their urine are also imbeciles or low-grade 

 morons. It appears, then, that the gene 

 which fails to produce the enzyme that 

 oxidizes this acid has even more profound 

 effects in some other parts of the body. Just 

 what these are remain unknown at present. 



Multiple gene effects 



It was pointed out earlier that several 

 unlike genes may occupy the same locus in 

 a chromosome but not at one time. These 

 are called multiple alleles. By the use of 

 x-rays, hundreds of multiple genes have 

 been produced in Drosophila, so that for 

 one locus, eye color for example, there are 

 fourteen known alleles, all different. Their 

 influence on eye-color production ranges 

 from dark red to white with all shades 

 in between. Coat color in rabbits likewise 

 is controlled by multiple alleles. There is 

 a gene (C) which controls coat color, 

 whereas the homozygous genes (cc) cause 

 no color or albinism. Two alleles, (c^'') and 

 (c''), when homozygous, produce "Chin- 

 chilla" and "Himalayan," respectively. The 

 former is steel gray, while tlie latter is white 

 all over except the tips of the ears, toes, tail, 

 and nose. When arranged in a series, C, 

 ^ch^ c'', c, each gene is dominant to those 



following it and recessive to all of those 



o 



preceding it. For example, a rabbit with the 

 genes c''"c is a "Chinchilla," one with c'^c"^ 

 is also "Chinchilla," whereas one with genes 

 c''c is "Himalayan." 



Another interesting; fact has come from 

 the coat-color stvidies in rabbits with re- 

 spect to physical factors that control genie 

 action. In the "Himalayan" rabbits the in- 

 tense black color that occurs at the tips is 

 controlled by temperature. If the hair is 

 removed from a portion of the body that 

 is white and this region kept cool, the hair 

 grows in black, indicating that the gene for 



Y OF LIFE 



colored tips (which are cooler than other 

 portions of the body ) can produce its effect 

 only at reduced temperatures. This obser- 

 vation probably is a general phenomenon 

 which needs further attention by geneti- 

 cists. 



One of the best-known characteristics in 

 man that is inherited by multiple alleles is 

 the A, B, AB, and O blood groups which we 

 discussed in Chapter 19 (see p. 513). Three 

 alleles are responsible for the various 

 groups: genes A, A^ and a, of which the 

 first two are dominant to the last. Gene 

 A controls the formation of anti-A (agglu- 

 tinogen); gene A^ controls the formation 

 of anti-B; and gene a is without effect in 

 that it causes no antibody formation. 

 Neither A nor A^ is dominant to the other, 

 so when both are present in homologous 

 chromosomes the blood group AB results. 

 Since these genes are inherited in a definite 

 fashion, the knowledge of blood types has 

 some value in addition to that needed in 

 transfusions. In cases of questionable pa- 

 ternity the knowledge of blood types can be 

 used and may rule out certain males as 

 possible fathers of a child. It cannot be 

 employed to determine whether a certain 

 man is the father but only that he could be. 

 The manner in which this operates can be 

 seen from a consideration of all of the 

 genotypes : 



Blood Group Genotype 



A Aa or AA 



B A^fl or AM^ 



AB AA' 



O aa 



A child with blood group A {Aa or AA), 

 for example, and a mother with Group B 

 {A^a or A^A^) must have a father with 

 group A or AB. A male with group O or B 

 could not possibly be the father of such a 

 child. Do you see why? 



Supplementary genes 



Sometimes two pairs of genes interact, 

 so that the expected 9:3:3:1 ratio in the 

 Fo is significantly altered; in fact, an en- 



