"Structural chromosomal aberrations," the second category of mutations, arise when DNA in chromosomes is broken. The broken ends may remain unrepaired or may be joined with those of another break, to form new combinations of genes, such as translocations. A translocation between chromosomes 8 and 21 in humans causes acute myeloid leukemia by increasing the activity of c-myc, a gene involved in cell replication.
Translocations often cause human infertility, because they interfere with the normal distribution of chromosomes during meiosis. Chromosomes pair up before separating, as eggs or sperm are formed, and the correct pairing depends on matching sequences between them. Structural aberrations also include inversions and duplications of pieces of chromosomes.
Most chromosomal aberrations lead to the formation of chromosomal fragments without centromeres. Centromeres are crucial for proper chromosomal division, during both mitosis and meiosis. Therefore a chromosomal fragment is likely to be lost from one of the daughter cells formed after cell division.
Structural aberrations are nonetheless common in evolutionary history. As a result, although the chromosomes of mouse and man are quite different in appearance, most genes have the same neighbors in the two species, representing the ancestral mammalian arrangement, even if they have been moved to another chromosome as shown in Figure 3.
"Numerical chromosomal aberrations," the third category of mutations, are changes in the number of chromosomes. In some cases, the whole genome has been duplicated (called polyploidy) and the mutant has, for example, four of each chromosome (and is thus tetraploid) rather than the usual two (diploid, as in humans). These are much more common in the evolution of plants than animals. In other cases, only one or a few of the chromosomes are involved, which is referred to as aneuploidy. Down syndrome, in which a person has an extra chromosome 21, is an example of such a mutation. Aneuploidy may also involve the loss of a chromosome. The absence of one of the sex chromosomes, X or Y, is a mutation in humans that results in Turner's syndrome, in which there is only one X.
"Transposon-induced mutations" are the fourth category of mutations. Transposable genetic elements (transposons) are pieces of DNA that can copy themselves and insert into a new location in the genome. They were first discovered by Barbara McClintock, a U.S. geneticist and Nobel laureate in 1950. When transposons jump into a new position, the insertion may disrupt a gene and thus mutate it, usually inactivating it. Sometimes the transposon jumps again, and the activity of the gene it leaves is restored. Often, however, the transposon stays in the original position, permanently disrupting the gene. Some forms of hemophilia are due to transposon insertion. Transposon mutations have been extremely common in human evolution, and such mutations are still occurring.
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