592 



COLLEGE ZOOLOGY 



{Rr) and the other half Rh-negative {rr). 

 This is because the chances are equal that 

 either the R or r gene from the Rh-positive 

 [Rr] parent will combine with the r gene 

 of the Rh-negative (rr) parent. 



In an earher discussion (p. 528), the 

 causes of Rh incompatibility were discussed. 



The first and the second child born of 

 parents with Rh incompatible blood groups 

 usually escape harm, but by the time the 

 third child is conceived there may be suf- 

 ficient antibodies in the mother's blood to 

 produce serious effects. The mechanism for 

 Rh incompatibility is illustrated in Fig. 420. 



Rh positive erythrocytes of fetus enter maternal circulation 

 and stimulate production of Rh antibodies 



Rh \ / Rh V::.::.-.--- ' f ^^. 



positive ) X I "^9°*'^® ) .1 P°*'*'^® 



Father y \Mother/ — ':^'\ Fetus 



Antibodies produced by mother enter fetal circulation and 

 cause erythroblastosis 



Figure 420. Diagram of the Rh mechanism in pregnancy. This illustration shows what may 

 take place in pregnancy when an Rh-positive man is mated with an Rh negative woman. The 

 Rh-positive erythrocytes of the fetus cause the production of Rh-antibodies in the mother. 

 The Rh-antibodies from the mother pass through the placenta and cause destruction of the 

 red blood corpuscles in the fetus. 



It should be remembered that if the 

 mother has had a blood transfusion of Rh- 

 positive blood this would serve as a stimulus 

 for the formation of anti-Rh antibodies. In 

 such a case, even the first child might suffer 

 the effects of Rh incompatibility. Finally, 

 for reasons as yet unexplained, the transfer 

 of antibodies and red blood corpuscles across 

 the placenta occurs only in a small per- 

 centage of cases. Thus in only one mating 

 out of 28 between Rh-positive males and 

 Rh-negative females is there any sign of the 

 effects of Rh incompatibility. 



Lethal genes 



One factor that may complicate the study 

 of heredity is the presence of lethal genes. 

 A lethal gene is one which causes the death 

 of an organism. It may be dominant or 

 recessive and may affect the organism in any 

 state of development. A lethal recessive 

 gene is one that in the homozygous condi- 

 tion kills the organism. If this gene is sex- 

 linked, all males will die in early develop- 

 mental stages, and the offspring will be all 



females, since the male receives only one 

 X chromosome. Those females also will die 

 that possess two X chromosomes containing 

 homologous lethal genes. If a dominant 

 lethal gene is located in the autosomes, only 

 the one gene must be present in both males 

 and females to bring about their death. 

 Lethal gene traits have been demonstrated 

 for many animals, including cattle, horses, 

 sheep, swine, poultry, cats, dogs, guinea pigs, 

 mice, rats, rabbits, and man. Infantile 

 amaurotic idiocy is a late-acting lethal gene 

 trait. There is a degeneration of the neurons 

 of the central nervous system which results 

 in death when the child is 2 to 3 years of 

 age. This is a recessive trait. 



Genes concerned 

 with defects 



Of particular interest to human society 

 are the genes of man that are concerned 

 with defects. There are a great many of 

 these, including genes for hemophilia, blind- 

 ness, deafness, insanity, and feeble-minded- 

 ness. These genes are segregated in matura- 



