Variation and Mutation
Biology (Year 12)
Chromosome mutations are alterations that affect whole chromosomes and whole genes, as opposed to a point mutation on individual nucleotides. There are three main categories of mutations in terms of variation;
There are five types of structural chromosome mutations:
1. Monoploidy is when some organisms are monoploid (1n) in terms of their chromosome number. A key example of monoploidy in nature is male bees. Male bees do not need to be diploid (2n) to function, whereas other organisms' gametes which are haploid (n) must meet and have their chromosomes pair to form a diploid (2n) organism. 2. Polyploidy occurs when meiosis fails, which results in half the gametes containing two copies of chromosomes (2n) and the rest having none. Fertilisation of these gametes may result in a triploid (3n) zygote or even a tetraploid (4n) zygote. Polyploidy is common in plants, ferns and algae but is fatal in most animals. 3. Aneuploidy refers to the addition or loss of one chromosome from a cell (2n + 1 or 2n - 1), which may involve a changing of the structure of a chromosome, or a loss/gain of part of a chromosome. This can occur in meiosis, when homologous pairs fail to segregate, leaving one daughter cell with all the chromosomes. Two main conditions which arise from this mutation are trisomy and monosomy. As mentioned earlier, there are five types of structural chromosome mutation. These 5 types can be remembered as 'DIDNT'.
Deletion: A chromosome may face double-strand breaks at two positions meaning that a section drops out of the chromosome.
Inversion: Occurs when a chromosome breaks in two places, and the broken segment rotates 180 degrees before rejoining the sequence. This reverses the normal sequence of genes.
Duplication: An extra copy is made of a section of a chromosome and inserted into the same chromosome or another chromosome.
Nondisjunction: The failure of chromosomes to separate during meiosis, causing gametes to have too many or too few chromosomes.
Translocation: A section of a chromosome can break off and reattach to another chromosome.
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