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Genetic variation and genetic disorders


Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms.

Different forms of the same gene are called alleles (pronounced 'al-eels'). The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.

Alleles are dominantdominant: An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself or recessiverecessive: Describes the variant of a gene for a particular characteristic which is masked or suppressed in the presence of the dominant variant. A recessive gene will remain dormant unless it is paired with another recessive gene:

  • The characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair
  • The characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair

For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only have blue eyes if they inherit two copies of the allele for blue eyes.

individual A is heterozygous and has one allele for blue eyes (recessive). B is homozygous and has two alleles for brown eyes (dominant). C is homozygous recessive and has two alleles for blue eyes (recessive)

Individuals A and B have brown eyes - only individual C has blue eyes

The cells of offspring produced by asexual reproduction are produced by mitosismitosis: A type of cell division which produces daughter cells identical to the parent. from the parental cells. They contain the same alleles as the parent. However, sexual reproduction causes variation because when two gametes join together one of each pair of alleles comes from each parent.

DNA fingerprinting

Each person (apart from identical twins) has unique DNA [DNA: The material inside the nucleus of cells, carrying genetic information. DNA stands for Deoxyribonucleic Acid. ]. This can be used to identify individuals by DNA fingerprinting. This technique uses some of the small differences between the DNA from different people to make a picture rather like a barcode.

If enough parts of the DNA are tested, it is very unlikely that two identical DNA fingerprints would belong to two different people. This makes the method very useful for matching samples found at the scene of a crime to people suspected of committing the crime.

Back to Cell division and inheritance index

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