Chapter 16: Chromosome Mutations: Variation in Number and Arrangement

Concepts

• Specific terminology describes variations in chromosome number
• Variation in the number of chromosomes results from nondisjunction
• Monosomy, the loss of a single chromosome, may have severe phenotypic effects
• Trisomy involves the addition of a chromosome to a diploid genome
• Polyploidy, in which more than two haploid sets of chromosomes are present, is prevalent in plants
• Variation occurs in the structure and arrangement of chromosomes
• A deletion is a lost portion of a chromosome
• A duplication is a repeated part of the genetic material
• Inversions rearrange the linear gene sequence
• Translocations alter the location of chromosomal segments in the genome
• Fragile sites in humans are susceptible to chromosome breakage

Structural Changes in Chromosomes

Chromosomal aberrations that involve a change in the structure of a chromosome are most often the result of chromosome breakage and reunion of chromosome parts.   Breaks can occur spontaneously through errors in replication or recombination.  Structural alterations can include deletions,which are the loss of a chromosome segment; duplications, which are extra copies of a chromosome segment; translocations, which move a segment from one chromosome to another; and inversions, which reverse the order of a chromosome segment.  A number of mutagens, including chemicals and X-rays can also induce breaks along the DNA strand.  These structural changes in chromosomes are often deleterious, but are also a powerful force for evolutionary change.

Structural Changes in Chromosomes, Deletions/Duplications

In humans, deletions involving relatively small amounts of chromosomal material underlie several genetic disorders, including Prader-Willi syndrome and cri-du-chat syndrome. Deletions of large amounts of chromosome material are usually lethal. Duplications may have evolutionary significance. Duplicated genes can take on new functions, providing the organism with new adaptive abilities. Many gene families have arisen by duplication followed by evolutionary divergence. For example, the alpha and beta chains of hemoglobin arose following a duplication event that occurred 500 million years ago.

Structural Changes in Chromosomes,
Inversions 

 Inversions are important in the evolution of species. During meiosis in inversion heterozygotes, a single recombination event within the inverted segment produces dicentric and acentric chromosomes that are eliminated, leaving only a chromosome with the normal gene order, and a chromosome carrying the inversion. Because no crossover chromatids are produced, the inversion acts as a crossover suppressor, preventing the genes within the inverted segment from being separated by recombination. If this set of genes increases fitness, it will provide a selective advantage for inversion heterozygotes. In the evolution of species in the genus Drosophilia, it has been estimated that over ten thousand inversions have become fixed in different species, and prevent the formation of fertile hybrids.

Structural Changes in Chromosomes, Translocations

Chromosome structural changes have significant implications for evolution by generating genetic diversity, and are of interest to geneticists for the phenotypic effects they have on organisms.  Chromosome rearrangements cause several notable genetic disorders in humans.  A deletion of the short arm of chromosome five results in cri-du-chat syndrome.  Unequal crossing over between  chromosomes within the segment carrying multiple copies of the globin genes often results in deletions that cause thalassemias.  Approximately five per cent of all cases of Down syndrome involve a translocation of chromosome 21.  In all these cases, the chromosomal rearrangements have a phenotypic impact on the individual.