GENERAL ZOOLOGY 



It has been established by careful studies on the fruit fly, maize, and cotton 

 that linear clusters of hereditary units known as pseudo-alleles occur. These 

 have distinct but related functions. They are assumed to have arisen by 

 duplication and reduplication of an original limited region of the chromo- 

 some, an assumption which rests on both cytological and genetical evidence. 

 The repeated units may undergo mutation in different ways, thus making 

 possible evolution of the functional units of heredity. 



An entirely different type of abnormal chromosomal behavior sometimes 

 occurs during the meiotic divisions. Homologous chromosomes may fail to 

 separate from one another and, therefore, will pass together into one cell. 

 ITiis is known as non-disjunction and can involve any pair of chromosomes. 

 One resulting cell will lack a whole chromosome of the typical haploid set, 

 whereas the other will have a haploid set plus one chromosome. Fertilization 

 will result in some individuals which have only one chromosome of a particu- 

 lar pair and in others which have three such chromosomes. These individuals 

 will lack one complete gene string or have an extra one. In Drosophila non- 

 disjunction of X-chromosomes and of the smallest chromosomes (IV) is known 

 to occur (Fig. 6.26). Non-disjunction of the X-chromosome gives rise to 

 females and males with unexpected characteristics, since some females get 

 both X-chromosomes from their mother and some males get an X-chromo- 

 some from their father. Non-disjunction of chromosome pair IV produces 

 haplo-IV and triplo-IV individuals difl'ering from one another and from the 

 normal diplo-IV fly in appearance. Entire sets of chromosomes may fail to 

 disjoin, so that a gamete will contain the diploid rather than the usual 

 haploid number of chromosomes. If fertilization adds a haploid set to such a 

 diploid gamete, the zygote has three chromosomes of each kind and gives rise 

 to a triploid individual. In Drosophila such flies are conspicuously different 

 from the diploid or normal type. 



Individuals that have four or more chromosomes of each set are also known. 

 They may arise as a result of an incomplete mitosis at the time of the first 

 cleavage of the zygote. The chromosomes undergo reduplication but do not 

 separate, so that the number is doubled and a tetraploid individual develops. 

 Such organisms usually are conspicuously larger than their diploid relatives. 

 Chromosome doubling sometimes occurs when gametes from two different 

 species of plants have united. Subsequent synapsis between pairs of homol- 

 ogous chromosomes is thus made possible, and functional gametes may be 

 formed. This usually does not occur in hybrids between different species, 

 such as the mule. Several entirely new species of plants are known to have 

 been established by chromosome doubling in a hybrid, but so far no com- 

 parable cases have been discovered among animals. 



Since gene mutations and chromosomal aberrations offer very interesting 

 material for the study of the mechanism of heredity, many investigators have 

 attempted to increase the rate at which such alterations occur. Drosophila 

 was subjected to high and low temperatures, various nutritional modifications, 

 and treatment with a great many chemicals, but with no appreciable change 



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