GENETICS 683 



and the number of offspring is rather small, statistical methods based 

 on this law have enabled investigators to determine the method of in- 

 heritance of many traits and to predict the proportion of types of off- 

 spring. For example, albinism, the complete lack of pigment which 

 results in white skin and hair and pink eyes, is a rare condition in 

 man that is inherited by a single pair of genes. The gene a for albinism 

 is recessive to the gene A for normal pigmentation. Surveys have shown 

 that albinos (genetically aa) occur in the population with a frequency 

 of about 1 in 20,000. Substituting this number, 1/20,000, for q^ in the 

 Hardy-Weinberg equation, we can calculate that q, the square root of 

 1/20,000, equals 1/141. Since p + q = 1, then p = 1 _ q or 1 — 1/141, 

 or 140/141. The frequency of heterozygous individuals, Aa, in the pop- 

 ulation is equal to 2 pq, or 2 X 140/141 X 1/141, which equals 1/70. 

 Thus, about 1 person in 70 is heterozygous for albinism— is a "carrier" 

 of the gene for albinism. It is surprising, perhaps, to find that there are 

 so many carriers for such a rare trait. H. J. Muller has calculated that 

 each of us is, on the average, heterozygous for about eight undesirable 

 genes. 



287. Biochemical Genetics 



Since 1911, when the gene theory was formulated by T. H. Morgan, 

 biologists have accepted the idea that genes are the fundamental units 

 of heredity, located in a linear order on the chromosomes, and that these 

 units govern the development of all the characters of the body. Re- 

 search in the field of biochemical genetics has been directed toward 

 providing an explanation of (1) the chemical and physical nature of the 

 gene and (2) the mechanisms by which the genes may control the devel- 

 opment and maintenance of the individual organism. 



Many attempts have been made to observe the genes within the 

 chromosomes but not even electron microscopy has been able to reveal 

 them. By a fortunate coincidence, one of the organisms which has been 

 most extensively used in genetic experiments, the fruit fly Drosophila, 

 has greatly enlarged, giant chromosomes in the cells of its salivary 

 glands. Each of the four giant chromosomes has a distinctive pattern of 

 cross bands by which it can be recognized. The detailed pattern of 

 bands is repeated with extreme fidelity in all the animals of a given 

 strain. C. B. Bridges and others have mapped the pattern of stripes on 

 each chromosome and then compared these cytologic maps with the 

 genetic maps calculated from crossover values. From such studies it has 

 been possible to conclude that the gene for a particular character is 

 located in (or is associated with) a particular band of the chromosome. 

 It appears, however, that the band itself is not the gene; some bands con- 

 tain several genes. 



Chemical Nature of the Gene. It has been possible, by special 

 techniques, to isolate chromosomes from ground-up cells and to show 

 by direct chemical analysis that they contain proteins and nucleic acids. 

 One of the two kinds of nucleic acid, desoxyribonucleic acid (abbrevi- 

 ated DNA), is found only within the chromosomes, nowhere else in the 

 cell. This fact, plus the parallelism between the number of genes and 



