Genetic Effects of Radiations 139 



' plants. However, in a normal diploid organism no more than 

 2 allelomorphs of a gene may be present together in the same 

 individual. Moreover, each of the gametes produced by a given 

 individual will contain only one allelomorph and, where the 

 individual is heterozygous, half its gametes will possess one 

 allelomorph and half the other. For example, the rare nervous 

 disease Huntington's chorea is transmitted, on the average, to 

 half the afifected person's children. The particular genes of the 

 affected persons may be symbolized as // for the abnormal gene 

 responsible for the manifestation of the disease and h for its 

 normal allelomorph. The affected person would be Hh and his 

 (or her) gametes half H and half Ji. Since the disease is so rare, 

 the spouse would normally be hh and the children therefore, on 

 an average, half Hh (capable of developing the disease) and the 

 other half /i/z (normal). 



The gene, H, for Huntington's chorea is usually spoken of as 

 being dominant to the normal gene // which is recessive. In 

 fact, the term dominant implies that there is no difference in the 

 appearance (phenotype) of HH and Hh individuals. In man 

 this particular information is lacking, so the use of the term 

 "dominant" in this connection is convenient rather than correct. 

 Probably a majority of genes producing abnormalities in man 

 are strictly recessive, the homozygous and heterozygous normals 

 being alike, or else intermediate in their dominance, the heter- 

 zygous being more like the homozygous normals than the homo- 

 zygous abnormals, which may be very extreme in their char- 

 acter. 



Gene segregation is orderly and dependent upon the regular 

 pairing together and separation of the chromosomes at meiosis. 

 This precedes gamete formation and is constituted by two special 

 nuclear divisions, in the course of which the number of chromo- 

 somes contributed to the daughter nuclei becomes half that in 

 the parent nucleus. The orderliness is such that each daughter 

 nucleus receives one each of the n different homologous chromo- 

 somes. IMoreover, in any particular gamete, a given homologue 

 may be compounded of complementary parts of the two homo- 

 logues present in the parent. Thus, a parent which in one of a 



