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Non-disjunction

When two chromatids do not separate during a meiotic division, resulting in one gamete with two copies of the chromosomes and another with no copies.

Daughter cell

One of the two identical cells produced as a result of cell division.

Phenotype

The physical or biochemical characteristics of an organism resulting from the genotype and the environment.

Polysomy

When an individual has more than the normal number of copies of a particular chromosome.

Monosomy

When an individual has only one copy of a chromosome.

Mutations

Mutations can occur during mitosis and can cause serious problems for cells, as described on page 5.

Mutations can also take place during meiosis. These may be produced in the same way as mitotic mutations, when the DNA replicates in interphase between cell divisions.

But in meiosis there are other ways in which mutations can arise – and in some ways, meiotic mutations more significant than mitotic mutations. Regardless of what happens to the cells of the body as they grow and divide mutations will not be passed on to the next generation. But if a mutation occurs during the formation of the gametes, then that mutation (and its good or bad effects) can be passed on to the offspring and become a permanent feature of their genetic material.

Mutations are the key to evolution. Without the mutations that take place as the gametes are formed, organisms would not develop differences in their phenotype. These differences can give them an advantage if conditions change – or mean that they can no longer compete successfully. As a result of natural selection, mutations that give an organism an advantage will become common in a population. Organisms containing a mutation that causes a disadvantage will quickly die out.

mutations

A series of mutations in each of these organisms along with natural selection means they are each well adapted to survive in their particular habitat

How do mutations occur?

Any of the types of mutation that you saw on page 5 can take place during interphase of meiosis as well.

Non-disjunction

As the chromosomes and then chromatids separate in the two meiotic divisions, sometimes things do not work properly. The chromatids do not always separate. This can result in one daughter cell with two copies of a chromosome and another with none. This is known as non-disjunction of the chromosomes.

If either of these cells is fertilised by another, normal gamete, the individual formed will have either three chromosomes, or only one. If you have more copies than normal of a chromosome it is known as polysomy. If you have only one copy of a chromosome it is known as monosomy. For human beings, in most cases monosomy means a fetus will not develop and survive. Examples of non-disjunction include:

  • The sex chromosomes: Non-disjunction of the sex chromosomes is not uncommon. It results in individuals who are polysomic or monosomic for the sex chromosomes. XXY gives Kleinfelter’s syndrome which affects one in every 600 live male births and XO, when the cells are monosomic for the sex chromosomes, gives Turner’s syndrome. In most cases non-disjunction of the sex chromosomes causes fertility problems and sometimes other problems with growth and development as well. sex chromosomes Some of the possible results of non-disjunction of the sex chromosomes
  • Chromosome 21: Non-disjunction of chromosome 21 can give an individual three copies of the chromosome. This results in Down’s syndrome, causing various degrees of learning difficulties, heart abnormalities, lack of muscle tone, visual problems and more. Any zygote that forms with only one copy of chromosome 21 has a very severe version of the problems of Down’s syndrome. Few affected babies survive.

Translocation

The process of recombination creates opportunities for other types of mutation to take place as well. One of the most common is translocation. When this happens, a piece from one chromosome breaks off and rejoins, not to one of the matching chromatids but to another chromosome entirely. If the translocation is balanced – genes simply swap places – then the individual may suffer few effects. If the translocation is unbalanced – one chromosome loses a section that attaches to another chromosome, leaving one chromosome short and the other long – this can have serious consequences. Examples include Burkitt’s lymphoma (chromosomes 8 and 14), core binding factor acute myeloid lymphoma (chromosomes 8 and 21) or Ewing’s sarcoma (chromosomes 11 and 22).