Heredity — Genetics 695 



];)lants, lethal genes may prevent the development of chlorophyll so the 

 young plant is unable to photosynthesize food. Lethal genes may arise 

 naturally, or they may be induced by irradiations by x-rays, radium, etc. 

 About 80 per cent of all mutations induced by radiations are lethals. 

 Possibly many of the lethal mutations may actually result from some type 

 of chromosomal aberration, although some are true gene mutations. 

 Some lethals have been discovered which are dominant, while others 

 behave as recessives. 



10. Mutations. — A mutation (mu -ta' shun) (L. mutare, to change) 

 is an inheritable trait which appears suddenly not as a result of environ- 

 mental influences, but has originated spontaneously in the hereditary 

 mechanism; hence, it may be transmitted to future ofTspring. Morgan, 

 in 1910, discovered the sudden appearance of a white-eyed mutant in a 

 stock of true-breeding red-eyed fruit fly, Drosophila melanogaster. When 

 the white-eyed mutant was crossed with red-eyed flies, the white-eye trait 

 was inherited as a recessive gene in the X chromosome. Hundreds of 

 plants and animals have mutated with greater or lesser frequency. Muta- 

 tions may aff'ect any part of an animal or plant or any of their functions. 

 They may occur periodically and frequently and be unobserved in 

 Nature. The same mutation may arise simultaneously in diff^erent indi- 

 viduals. A mutation results in a new trait which is inheritable, and, if 

 sufficiently extensive, may result in a new type or even a new species of 

 that organism. Mutations may occur naturally, although some may be 

 induced by irradiations such as x-rays, radium, etc. About 80 per cent 

 of all induced mutations are lethal. In general, mutations are usually 

 changes for the worse, although a few may be valuable. Some of our 

 important plants and animals have arisen as mutants which possessed 

 desirable traits. In the broad sense, changes in traits are the result of 

 (1) actual, sudden change in a gene (gene mutation), (2) changes in 

 the number of chromosomes or from (:hromosomal aberrations (anomo- 

 zygous mutations), or (3) recombinations of previous genes in an en- 

 tirely new arrangement for that particular organism (recombination). 

 In a strict sense, the term mutation is reserved for those inheritable 

 traits which arise abruptly from changes in a gene. If an organism 

 mutates in a certain manner, it may now possess a trait which will 

 benefit it in the future, or it may mutate in such a way as to possess 

 undesirable traits which will interfere with its normal existence. 



11. Linkage and Crossing Over. — Genes are considered to be asso- 

 ciated with each other in a linear order within chromosomes. All the 

 many genes in each chromosome tend to be inherited as a group and 



