GENETICS 679 



presence or absence of a different agglutinogen, the Rh factor, first 

 found in the blood of rhesus monkeys. There are actually several alleles 

 at the rh locus, but to simplify matters we shall consider just two: Rh, 

 which produces the rh positive antigen, and the recessive rh, which does 

 not produce the antigen. Genotypes RhRh and Rhrh are phenotypically 

 rh positive and genotype rhrh is phenotypically rh negative. An rh 

 negative woman married to an rh positive man may have an rh positive 

 child. If some blood manages to pass across the placenta from the fetus 

 to the mother it will stimulate the formation, in her blood, of antibodies 

 to the rh factor. Then, in a subsequent pregnancy, some of these rh 

 antibodies may pass through the placenta to the child's blood, and react 

 with the rh antigen in the child's red cells. The red cells are agglutinated 

 and destroyed and a serious, often fatal, anemia, called erythroblastosis 

 fetalis, ensues. This is now treated by massive blood transfusions, so that 

 essentially all of the blood of the newborn is replaced. 



Extensive surveys have shown that 41 per cent of native white 

 Americans are type O, 45 per cent are type A, 10 per cent are type B 

 and 4 per cent are type AB. The frequency of the blood groups in other 

 races may be quite different; American Indians, for example, have a 

 low frequency of group A and a high frequency of group B. No one 

 blood type is characteristic of a single race; the racial differences lie in 

 the relative frequency of the several blood types. Studies of the relative 

 frequencies of the blood groups found in different races living today and 

 in mummies and skeletons have provided valuable evidence as to the 

 relationships of the present races of man. 



283. Lethal Genes 



Certain genes produce such a tremendous deviation from normal 

 development that the organism is unable to survive. Many such genes 

 will escape detection, for their action is usually evident only in special 

 circumstances in which the usual genetic ratios are altered because one 

 of the expected classes is completely missing. One of the first such lethal 

 genes to be discovered was found when the inheritance of yellow coat 

 color in mice was investigated. It proved impossible to establish a true- 

 breeding strain of vellow mice; breeding two yellow mice resulted ni off- 

 spring in the ratio of two yellow to one nonyellow (gray, black or brown). 

 Breeding yellow mice with nonvellow ones produced equal numbers of 

 yellow and nonyellow offspring. This indicated that the yellow mice 

 were heterozygous, Yy, and that in the mating of two yellow mice, Yy x 

 Yy the ratio of 2 yellow to 1 nonyellow (rather than the expected 5 

 yellow to 1 nonyellow) was obtained because the homozygous yellow YY 

 animals died. Investigators then noticed that the number of offspring 

 produced by a yellow X yellow mating was indeed smaller, only about 

 three-quarters as large as the average mouse litter. Later research showed 

 that these homozygous yellow mice do begin development but die and 

 are resorbed. If the uterus of the mother is dissected open early in preg- 

 nancy, the abnormal embryos are visible. 



The "creeper" gene in chickens provides an exactly comparable 



