遗传分析-第十八章Chromosome Mutation II Changes in Chromosome Number.doc

Chapter 18 Chromosome Mutation II: Changes in Chromosome NumberKey Concepts Organisms with multiple chromosome sets (polyploids) are generally larger than diploid organisms, but meiotic pairing anomalies make some polyploid organisms sterile. An even number of polyploid sets is generally more likely to result in fertility. Then the single-locus segregation ratios are different from those of diploids. Crosses between two different species followed by the doubling of the chromosome number in the hybrid produces a special kind of fertile interspecific polyploid. Variants in which a single chromosome has been gained or lost generally arise by nondisjunction (abnormal chromosome segregation at meiosis or mitosis). Such variants tend to be sterile and show the abnormalities attributable to gene imbalance. When fertile, such variants show abnormal gene segregation ratios for the misrepresented chromosome only. Introduction The second major type of chromosome mutation is change in chromosome number. Few aspects of genetics impinge on human affairs quite so directly as this one. Chromosome numbers change spontaneously as accidents within cells, and this process has been going on for as long as life has existed on the planet. In fact, changes in chromosome number have been instrumental in molding genomes during evolution. For examples of such changes, we have to look no farther than the food on our dining tables because many of the plants (and some of the animals) that we eat arose through spontaneous changes in chromosome number in the course of the evolution of those species. Today, breeders emulate this process by manipulating chromosome number to improve productivity or some other useful feature of the organism. But perhaps the main relevance of chromosome numbers is for members of our own species in that a large proportion of genetically determined ill health in humans is caused by abnormal chromosome numbers. In this chapter, we shall investigate the processes that p