4i: 



THE NEW INDIVIDUAL 



Part IV 



The genes in the egg and developing embryo may mutate independently 

 of fertilization by the male cell. This has been observed in certain partheno- 

 genetic animals such as waterfleas (Cladocera) by Arthur M. Banta who 

 reared these through 850 generations and observed many mutations. 



Effects of Mutations. More mutations are harmful than helpful. Their 

 character shows that any desirable ones that appear are selected by the natural 

 conditions inside and outside the organism. Otherwise they must be preserved 

 by human selection, e.g., the valuable platinum or silver blue mink, the 

 seedless grape. 



Some of the most striking effects of environment on the expression of genes 

 are produced by differences in temperature. At 27.5° C. the gene for "short 

 wing" in Drosophila has a more marked effect than at a lower temperature. 

 In Siamese cats the dark pigment is produced only in cooler parts of the body 

 (Fig. 20.21). 



Fig. 20.21. Dark pigment of Siamese cats, 

 produced in the extremities of the body 

 which are below a certain level of tempera- 

 ture. (Courtesy, Boyd: Genetics. Boston, 

 Little, Brown and Co., 1950.) 



Giant Plants — Giant Cells. Polyploidy is a type of mutation in which the 

 whole number of chromosomes, diploid or haploid, is increased two to sev- 

 eral times. Such increased numbers are hereditary and are accompanied by 

 marked changes. Polyploid plants are very large. Various garden flowers and 

 vegetables, crop plants and fruit trees are polyploids. Plant polyploids are 

 frequently found in nature, animal polyploids rarely if ever. 



Many plant polyploids have been produced experimentally, largely by 

 means of colchicine, a solution derived from the bulbs of the autumn crocus 

 (Colchicum). The buds are bathed in colchicine solution which penetrates 

 into the developing reproductive organs and affects the cells. The chromo- 

 somes double their number but the rest of the cell fails to divide. In most 

 plants, fertilization goes on as usual except that, for example, instead of 4 



