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Large-scale mutations

Problems with cell division can produce cells with an abnormal number of chromosomes.

If this occurs during the formation of gametes, the fetus can have too few or too many chromosomes.

In most cases, having the wrong number of chromosomes is lethal to a human fetus, resulting in miscarriage. However there are exceptions.

Down's syndrome is a genetic disorder that causes severe intellectual disability. It is caused by an individual having three repeats (instead of two) of chromosome 21.

Having three copies of a chromosome is called trisomy.

This image shows the trisomy in chromosome 21 of a Downs syndrome patient.
This image shows the trisomy in chromosome 21 of a Downs syndrome patient.

A large-scale mutation is a mutation that results in changes to a large section of a DNA strand.

Large-scale mutations normally occur during chromosomal division and segregation.

Small scale mutations, on the other hand, normally occur during DNA replication.

Large-scale mutations often come about during homologous recombination in meiosis (the production of sperm and eggs). If homologous recombination is unequal, a large section of the chromosome may be lost or gained.

Large-scale mutations appear to have had a greater effect on evolution, when compared to small scale mutations.

Duplications of Hox genes is thought to have had a significant effect on the evolution of animals. These genes code for an organism's structure, such as the location of limbs.

Extra copies of entire genes allow the organism to evolve new functions without losing existing functions.

There are several common types of large-scale mutations:

  • Gene duplication: a whole gene, or block of genes, is repeated on the chromosome. Gene duplications often result from unequal crossover during meiosis.

    If the chromosomes do not line up correctly, the recombined sections will not be equivalent. This results in a loss of genes from one chromosome and a gain on the other.

  • Gene deletion: loss of a whole gene or a block of genes. Gene deletions are also caused by unequal recombination during meiosis.
  • Chromosomal translocation: the movement of genes from one chromosome to another non-homologous chromosome. Some cancers are a result of translocations.
  • Chromosomal inversion: a section of genetic material is reversed on the chromosome. This is often unproblematic but can lead to reproductive problems.
  • Non-disjunction: homologous chromosomes fail to separate during meiosis, resulting in either a gain or loss of a chromosome in the cell.
  • Aneuploidy: errors in cell division can result in changes in the number of chromosomes.