250 The Induction oj Gene Mutations 



Lethal Mutations 



That genes can result in the early death of an individual was 

 mentioned in Chapter 3. Lethal mutations can be induced by 

 irradiation, and they act in the same manner as do those that 

 arise spontaneously. As a matter of fact, about 80 per cent 

 of the mutations induced by irradiation are lethals. Probably 

 a large percentage of the lethal mutations are really chromo- 

 somal deficiencies rather than true gene mutations, and some 

 may be due to other chromosomal aberrations such as transloca- 

 tions and inversions. 



If treatment with X-rays or radium induces a dominant lethal 

 mutation, the effect is immediate. If such a mutation should be 

 produced in one of the chromosomes of the egg or of the sperm, 

 either that gamete would die or, if it were capable of function- 

 ing, the individual formed as the result of its union with a nor- 

 mal gamete would die in a very early stage of development. On 

 the other hand, a recessive lethal mutation will have no lethal 

 effect unless it is homozygous. If irradiation induces the pro- 

 duction of a recessive lethal mutation in a sperm, and if that 

 sperm then fertilizes an egg, the resulting individual will survive 

 because there is no lethal at the same locus of the homologous 

 chromosome which was received from the egg. The lethal from 

 the sperm in a sense is "covered up" by the nonlethal allele in 

 the homologous chromosome. If a heterozygous individual is 

 self-fertilized, or if two heterozygotes bearing a lethal at the 

 same locus are mated, a fourth of the offspring will be homozy- 

 gous for this lethal and will fail to reach maturity. The effect 

 of a recessive lethal, therefore, will not be so immediate in 

 expression as the effect of a dominant. One exception may be 

 made to this statement. A recessive lethal induced in one of the 

 sex chromosomes of the female will be immediately apparent 

 in the male offspring. Most of the induced lethal mutations 

 that have survived and been detected have been recessives. 



Often induced lethal mutations are not gene mutations but 

 are losses of small chromosomal segments, known as deficiencies. 

 Sometimes the deficiencies may be large enough to be detected 

 by examining or measuring somatic chromosomes, but usually 

 they are too small to be observed by such means. In Drosophila, 

 many of these smaller deficiencies may be detected by the loss 



