HEREDITY AND VARIATION 



gene, and those resulting from what are known as chromosomal aberrations 

 which give rise to changes in the numbers of genes or in their relationships 

 to one another in the linkage groups. 



Gene mutations are ordinarily thought of as changes in the quality of a 

 given part of the gene string. Hundreds of such mutations have been dis- 

 covered in the many thousands of Drosophila that have been examined, 

 although the number of times that any particular mutation has occurred is 

 very small (Fig. 6.25). One gene has been recorded as mutating 4 times in 

 the formation of 500 gametes, another 2 times in 1800 gametes. When all 

 the genes in chromosome II of Drosophila were considered, it was found that 

 only 30 mutations occurred in 5000 chances. Not all genes mutate with the 

 same frequency, and very few, apparently, change often under normal con- 

 ditions. Mutations produce changes in the structure of organisms and in the 

 way they function. Many mutations that tend to alter function result in the 

 death of the organism; they are called lethal mutations. 



Chromosomal aberrations involve parts of chromosomes, whole chromosomes, 

 and even entire haploid sets of chromosomes. The fact that losses or addi- 

 tions of genes in the cell produce observable changes in the individual reminds 

 us that genes ordinarily interact with one another in what must be thought of 

 as a balanced condition. If this balance is shifted by adding or subtracting 

 groups of genes, the effect may be to kill the individual; with other aberrations 

 the individual may live but be unable to produce functional germ cells. 

 Sometimes such aberrations can be handed on from one generation to the 

 next. The various types of alterations in linkage groups can be described 

 briefly. 



Alteration in a linkage group may be brought about by loss of a certain 

 region of a chromosome. This is known as deletion, and an individual in 

 which a deletion occurs will contain only one of each of the genes located in 

 that region of the chromosome (Figs. 6.20 and 6.23E and F). Sometimes 

 breeding results indicate that inversion has taken place; that is, a portion 

 of the gene string has been reversed in postion (Fig. 6.23G and //). It is of 

 considerable interest that alteration of the linear order may produce a herit- 

 able eflfect different from that resulting from the same genes in their typical 

 sequence. Occasionally, a part of one chromosome becomes attached to the 

 other member of a pair in such a way that one chromosome has a given region 

 duplicated, whereas its homologue is deficient for the same region and the 

 genes it carries (Fig. 6.23/ and /). This is, in effect, an unequal crossing 

 over. Another type of shift in linkage relations occurs when pieces of non- 

 homologous chromosomes become interchanged (Fig. 6. 23 A" and L). This is 

 known as translocation. What cytological evidence there is concerning the 

 ("hromosomal behavior responsible for these altered linkage relationships 

 indicates that, when the chromosomes are in the form of long threads during 

 the early growth period, they may come into contact with one another, stick 

 together, and sometimes be broken when the contractions of the threads 

 occur. 



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