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18.1) Variation

18.1) Variation

 

Variation: is the differences between individuals of the same species.

 

Genetic variation are variations that are determined by genes.

Phenotypic variations may be brought about by genes, but can also be caused by the environment, or a combination of both genes and the environment.

Acquired characteristic – eg. A tan, you cannot inherit a suntan.

Inherited characteristic – eg. Black skin.

 

Continuous variation results in a range of phenotypes between two extremes, eg. height in humans.

Discontinuous variation results in a limited number of phenotypes with no intermediates, eg. tongue rolling.

 

Discontinuous variation is under the control of a single pair of alleles or a small number of genes.

An example is human blood groups. A person is one of four blood groups: A, B, AB or O.

There are no groups in between.

 

When recording and presenting results of investigations, use line/histogram for continuous and bar chart for discontinuous variation.

 

Mutations:

Mutation is a spontaneous genetic change. Mutation is the way new alleles are formed.

Gene mutation: is a change in the base sequence in DNA.

 

Causes of mutation:

Ionising radiation and some chemicals increase the rate of mutation.

  • chemical mutagens – such as tar from cigarette smoke.
  • ionising radiation – gamma rays, X-rays and ultraviolet rays.
  • The greater the dose of radiation a cell gets, the greater the chance of a mutation.

 

Sickle-cell anaemia:

  • With sickle-cell anaemia, the haemoglobin molecule differs from normal haemoglobin by only one amino acid (represented by a sequence of three base).
  • valine replaces glutamic acid. This could be the result of faculty replication at meiosis.
  • When the relevant parental chromosome replicated at gamete formation, the DNA could have produced the triplet -CAT- (valine) instead of -CTT- (glutamic acid).
  • In this case, a change of just one base (from A to T) makes a significant difference to the characteristics of the protein (haemoglobin).
  • A person with sickle-cell disease has inherited both recessive alleles (Hb^SHb^S) for defective haemoglobin.
  • The heterozygous (Hb^AHb^S) have no symptoms of anaemia but are more resistant to malaria than the homozygotes Hb^AHb^A. It appears that the malaria parasite is unable to invade and reproduce in the sickle cells.
  • The selection pressure of malaria, therefore, favours the heterozygotes over the homozygotes and the potentially harmful Hb^S allele is kept in the population.
  • When Africans migrate to countries where malaria does not occur, the selective advantage of the Hb^S allele is lost and the frequency of this allele in the population diminishes.
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