GENETIC EFFECTS OF RADIATIONS 



D. G. CATCHESIDE, M.A., D.Se. 

 Lecturer in Botany, University of Cambridge, and Fellow of 



Trinity College 



Introduction 



GENETICS is concerned with the mechanism of heredity, 

 with the reasons why offspring resemhle their parents and 

 in some cases differ from them. The characters of the 

 human body, or of any other organism, are controlled by genes 

 present in every cell. The genes are passed from parent to 

 offspring in the gametes. They are situated in and largely, if 

 not wholly, constitute the chromosomes, of which there is a 

 fixed number in a given kind of organism. The gametes con- 

 tain a haploid set (n), the zygote and body cells a diploid set 

 (2n) of chromosomes. Thus each chromosome or homologue is 

 represented once in the gamete and twice in a body cell. 



Each gene occupies a fixed position (locus) in its particular 

 chromosome of the haploid set. The gene present at a given 

 locus may not always be exactly the same one, but may be 

 replaced by a slightly different one, called an allelomorph 

 (or allel). Thus, at a particular genetic locus in two homol- 

 ogous chromosomes, a given body cell may possess the same 

 allelomorphic gene and be homozygous, or may possess two 

 different allelomorphs and be heterozygous, llie number of 

 allelomorphs of a given gene is not limited. Thus 4 allelo- 

 morphs controlling the AB l)lood group series are recognized 

 in man, about 20 allelomorphs of the "a' (white eye) series in 

 the fruit-fly Drosopliila niclanogastcr, and between 40 and 50 

 allelomorphs of the gene concerned with incompatibility reac- 

 tions of pollen grains to style in certain self-sterile flowering 



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