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Errors in the process of meiosis can cause damage to chromosomes or alter their overall number in the cell. Ideally, the process of meiosis distributes chromosomes evenly to daughter cells. However, it is possible for nondisjunction to occur, meaning members of a pair of chromosomes fail to separate. Chromosomes may not move apart properly during meiosis I, or the sister chromatids may not separate during meiosis II. Either of these conditions can result in a cell with two of the same type of chromosome and another cell with no copy at all (Figure 1).

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What kind of mutation is it when a part of one chromosome is removed and added to another chromosome?

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You can visualize types of structural changes by looking at a karyotype. The changes impact the genes housed in that section of DNA and can equate to health impacts on the individual. However, sometimes structural changes are present, but they don’t relate to a difference in phenotype.

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An accident during mitosis or meiosis, in which members of a pair of homologous chromosomes or sister chromatids fail to move apart properly.

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What kind of mutation is it when a segment of a chromosome is removed or lost?

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What kind of mutation is it when part of a chromosome is placed in the wrong orientation compared to the rest of the chromosomes?

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What kind of mutation is it when two chromosomes swap segments?

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Remember that before a cell undergoes mitosis or meiosis, its DNA is replicated. The two copies, called sister chromatids are temporarily held together at a specific location of the chromosome called the centromere. This location divides the chromosome into two regions called arms. (Figure 2). The location of this centromere is a distinguishing feature of each pair of chromosomes.

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When sperm and egg meet, the resulting cell should have a complete 46 chromosomes. However, if one of these gametes does contain more or fewer than the typical 23, the overall chromosome number in body cells will be altered.

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Breakage of chromosomes can occur. The change may only impact one part of a particular chromosome, or it may even involve swapping of segments between two chromosomes. These structural changes typically relate to segments of the chromosome that are altered, deleted, or added.

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What kind of mutation is it when a segment of the chromosome is replicated and inserted next to the original copy?

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Which do you think is easier to identify on a karyotype - changes in chromosome number or in chromosome structure? Explain your answer.

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Numerical changes are perhaps the easiest to spot on a karyotype. Humans typically have 23 pairs of chromosomes. The presence of only one chromosome in the pair is referred to as a monosomy (one missing chromosome). The presence of three of the same chromosome is referred to as a trisomy (one extra chromosome).