Changes

*Using two or more antibiotics at once can help ensure that those bacteria that are resistant to one of the drugs is killed by the other.

*Using two or more antibiotics at once can help ensure that those bacteria that are resistant to one of the drugs is killed by the other.

==='''Industrial melanism'''===

==='''Industrial melanism'''===

In England during the [[industrial revolution]] pollution killed mosses and lichens on tree trunks, turning them from light colours to dark brown.  Before this occurred most specimens of the peppered moth ''Biston betularia'' had white wings with black spots - giving a peppered appearance.  During the period with dark tree trunks (1849 to circa 1970) the white version of ''betularia'' was easily predated on by birds, decreasing the frequency of the white-wing allele.  Individuals with dark wings caused by high concentrations of melanin became much more frequent and the allele frequency for melanic moths increased.  When pollution levels were reduced in the late 1960s the trees gained more moss and lichens, making the trunks lighter and predation selected against melanic forms of the peppered moth.  It is now thought that the proportions of white to melanic moths in England have now returned to pree-1849 levels.

In England during the [[industrial revolution]] pollution killed mosses and lichens on tree trunks, turning them from light colours to dark brown.  Before this occurred most specimens of the peppered moth ''Biston betularia'' had white wings with black spots - giving a peppered appearance.  During the period with dark tree trunks (1849 to circa 1970) the white version of ''betularia'' was easily predated on by birds, decreasing the frequency of the white-wing allele.  Individuals with dark wings caused by high concentrations of melanin became much more frequent and the allele frequency for melanic moths increased.  When pollution levels were reduced in the late 1960s the trees gained more moss and lichens, making the trunks lighter and predation selected against melanic forms of the peppered moth.  It is now thought that the proportions of white to melanic moths in England have now returned to pree-1849 levels.

==='''Sickle cell anaemia'''===

==='''Sickle cell anaemia'''===

This is a condition where red blood blood cells have a sickle shape caused by faulty haemoglobin genes.  Suffers with full sickle cell anaemia have two alleles for the condition and none of their red blood cells are formed correctly.  These cells get stuck in blood capillaries and cannot carry oxygen properly.  This is clearly not advantageous and many sufferers die early in life.  Even though death occurs in some parts of East Africa up to 50% of babies carry the genes for sickle cell anaemia and 14% have anaemia<ref>Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4</ref>.  This can be explained as an individual with one normal allele and one sickle cell allele will have half normal red blood cells and half sickle red blood cells.  While this condition is not desirable they will ''not'' suffer from full anaemia and can live a normal life.  However, the protoctist ''plasmodium'' that causes malaria cannot survive in sickle red blood cells and the sickle cell allele is selected for.  People with two normal alleles for heamoglobin do not have anaemia but are much more susceptible to malaria and in a study of 100 children who died from sickle cell anaemia all but one child had two normal alleles<ref>Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4</ref>.

This is a condition where red blood blood cells have a sickle shape caused by faulty haemoglobin genes.  Suffers with full sickle cell anaemia have two alleles for the condition and none of their red blood cells are formed correctly.  These cells get stuck in blood capillaries and cannot carry oxygen properly.  This is clearly not advantageous and many sufferers die early in life.  Even though death occurs in some parts of East Africa up to 50% of babies carry the genes for sickle cell anaemia and 14% have anaemia<ref>Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4</ref>.  This can be explained as an individual with one normal allele and one sickle cell allele will have half normal red blood cells and half sickle red blood cells.  While this condition is not desirable they will ''not'' suffer from full anaemia and can live a normal life.  However, the protoctist ''plasmodium'' that causes malaria cannot survive in sickle red blood cells and the sickle cell allele is selected for.  People with two normal alleles for heamoglobin do not have anaemia but are much more susceptible to malaria and in a study of 100 children who died from sickle cell anaemia all but one child had two normal alleles<ref>Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4</ref>.