608 



CONTINUITY OF LIFE 



mutations are referred to as parallel muta- 

 tions. 



Mutation rate 



Obviously, genes must be very stable 

 molecules, because if they were not it 

 would be difficult to understand how a 

 species could maintain itself for any great 

 length of time. If changes occurred readily 

 in response to minor alterations in the en- 

 vironment, the race would be very unstable. 

 Under normal conditions, mutations occur 

 in Drosophila cultures about once in every 

 million individuals. Even then, some genes 

 mutate more frequently than others; indeed, 

 some have never been known to mutate. 

 Since mutations are the tools with which 

 the geneticist works, it is understandable 

 that in the early days of modern genetics 

 a continued effort was made to find some 

 means of bringing about these gene 

 changes artificially. For a long time the 

 work was fruitless until Muller, a Nobel 

 prize winner in 1946, found that by expos- 

 ing fruit flies to a blast of x-rays just short 

 of the lethal dose he was able to increase 

 the normal mutation rate 150 times. Appar- 

 ently the particles were able to penetrate 

 to the genes in the sex cells and bring about 

 a change in the structure of their molecules. 

 Many of the mutations that appeared in 

 Muller's cultures were no different from 

 those appearing spontaneously. They were 

 also recessive and usually harmful or lethal. 

 Here, then, was a technique of bringing 

 about gene changes artificially, which was 

 a boon to genetic research. Other radiations 

 such as ultra violet and radium have since 

 been successfully employed. Even high 

 temperatures may be effective if used at 

 particular times in the Iffe cycle of the 

 animal. 



CHROMOSOME ALTERATIONS 



Mutation refers to changes within the 

 gene itself, that is, chemical changes of 

 the nucleoprotein that makes up ( or is ) the 



gene. Not all changes in hereditary pattern 

 are due to such changes — some are caused 

 by a physical change in the chromosome 

 itself. For example, during meiosis a small 

 segment of a chromosome may break off 

 and become lost, thus rendering that chro- 

 mosome short in those genes that were con- 

 tained in the missing fragment. This kind of 

 alteration is called deletion. Such gametes, 

 deficient in certain genes, often cannot 

 function at all or ff they do the resulting 

 offspring may be deficient in certain visible 

 traits. Occasionally a broken fragment be- 

 comes attached to another chromosome 

 where it does not belong, thus duplicating 

 certain genes in such a gamete. This condi- 

 tion is referred to as duplication. The dupli- 

 cate set of genes may have the effect of 

 exaggerating that particular trait in the 

 offspring. It has been possible in some in- 

 stances actually to verify these breaks by 

 examining the chromosomes under a micro- 

 scope. When this has been successful, gene 

 locations on chromosome maps have been 

 definitely ascertained. 



Another peculiar aberration that some- 

 times occurs amonsf chromosomes is inver- 

 sion, a condition that results when one 

 portion fragments and then reattaches it- 

 self but in reverse order, that is, end for 

 end. Inverted chromosomes seem to have 

 no effect on the offspring because all of 

 the genes are present and they can perform 

 their function even though in reverse or- 

 der. However, in gamete formation, trouble 

 is encountered during synapsis when the 

 homologous genes must lie opposite one 

 another, and since they are arranged in a 

 linear fashion on the chromosome, this be- 

 comes a physical impossibility. 



Variations in chromosome number 



One might expect that occasionally dur- 

 ingr meiosis the number of chromosomes 

 going to the gametes might vary in number. 

 This happens when a pair of homologous 

 chromosomes fails to separate at reduction 

 division. One gamete will tlien have one 



