Eugene M. McCarthy, PhD Genetics
It is hard to assess just how often new forms of life arise via the addition or deletion of individual chromosomes. Series of related aneuploid forms are known among plants. For example, in the genus Crepis, four different chromosome numbers (n = 3, 4, 5, and 6) occur. Distinct forms within the genus therefore often differ with respect to chromosome number.The members in such series are believed to be derived from each other by individual steps. Kim (1992) says aneuploids resulting from a decrease in chromosome number are more common among plants than are those resulting from an increase.
But among animals, viable aneuploids seem more often to result when chromosomes are added and when the chromosomes involved in the change are small. Viable autosomal trisomies have long been known in the mouse. Chickens with extra copies of certain small chromosomes are viable. Such is the case, too, with Down's syndrome in which affected individuals have three copies of Chromosome 21 ("trisomy 21"), one of the smallest human chromosomes. However, Shoffner et al. (1979) describe a viable F2 hybrid, from the cross Anser rossii × A. canagicus (Emperor goose × Ross's goose), that was trisomic for its largest chromosome. Also, Mayr et al. (1985) report that a calf (Bos taurus) trisomic for chromosome 21, one of the medium-sized bovine chromosomes, was viable.
Not every type of organism can produce new forms by simply adding or deleting individual chromosomes. Nor, in the case of those that can, is the production of viable aneuploid offspring possible for every chromosome. In the human case, addition or deletion of most chromosomes is lethal. Individuals with only a single copy of any human autosome die before birth. Down's syndrome is the only human trisomy involving an autosome that results in viable individuals who normally reach maturity. Recall that an autosome is any chromosome other than a sex chromosome (viable sex-chromosome aneuploids also occur in humans ). But the overall frequency at which new forms of life are produced by such processes is poorly known.
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