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Modern Genetic Analysis Chapter 8. Chromosome Mutations
Summary

Chromosome sets undergo rare spontaneous changes to produce chromosome rearrangements or changes in chromosome number, collectively called chromosome mutations. When mutations change the number of whole chromosome sets present, aberrant euploids result; when mutations change parts of sets, aneuploids result. Polyploids such as triploids (3x) and tetraploids (4x) are common in the plant kingdom and are even represented in the animal kingdom. An odd number of chromosome sets makes an organism sterile because there is not a partner for each chromosome at meiosis, whereas even numbers of sets can produce standard segregation ratios. In plants, allopolyploids (polyploids formed by combining sets from different species) can be made by crossing two related species and then doubling the progeny chromosomes through the use of colchicine or through somatic cell fusion. These techniques have important applications in crop breeding, because allopolyploids combine the features of the two parental species. Polyploidy can result in an organism of larger dimensions; this discovery has permitted important advances to be made in horticulture and in crop breeding. Aneuploids also are important in the engineering of specific crop genotypes, although aneuploidy per se usually results in an unbalanced genotype with an abnormal phenotype. Examples of aneuploids include monosomics (2n 1) and trisomics (2n+1 Aneuploid conditions are well studied in humans. Down syndrome (trisomy 21), Klinefelter syndrome (XXY), and Turner syndrome (XO) are well-documented examples. The spontaneous level of aneuploidy in humans is quite high and produces a major proportion of genetically based ill health in human populations. Most instances of aneuploidy result from chromosome nondisjunction at meiosis.

The main structural rearrangements are deletions, duplications, inversions, and translocations. Deletions are missing sections of chromosomes. If the region removed is essential to life, a homozygous deletion is lethal. Heterozygous deletions can be nonlethal or lethal and can uncover recessive alleles, causing them to be expressed phenotypically. Deletions in one homolog often allow the expression of recessive alleles in the other. Duplications and deletions can unbalance the genetic material, thereby producing a deleterious phenotypic effect or death of the organism. However, duplications can be a source of new material for evolution. An inversion is a 180-degree turn of a part of a chromosome. In the homozygous state, inversions may cause little problem for an organism unless heterochromatin brings about a position effect or one of the breaks disrupts a gene. On the other hand, inversion heterozygotes have pairing difficulties at meiosis, and an inversion loop may result. Crossing-over within the loop results in inviable products. The crossover products of pericentric inversions, which span the centromere, differ from those of paracentric inversions, which do not span the centromere, but both show reduced recombinant frequency in the affected region and reduced fertility. A translocation moves a chromosome segment to another position in the genome. In the heterozygous state, translocations produce duplicationdeletion meiotic products, which can lead to unbalanced zygotes. New gene linkages can be produced by translocations. Translocation heterozygotes show 50 percent sterility (semisterility).

Chromosomal rearrangements are an important cause of ill health in human populations and are useful in engineering special strains of organisms in pure and applied biology. Extensive chromosomal rearrangement has occurred during evolution.

© 1999 by W. H. Freeman and Company. All rights reserved.