Natural Selection in Action

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Natural Selection in Action

Higher Biology

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Natural Selection in Action

Most mutations produce inferior versions of original gene

Some mutations allow adaption to a changing environment

Mutant allele gives mutant form of organism a selective advantage

Change in environment

-abiotic factor (e.g. pollution)

-biotic factor (e.g. disease)

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Sickle cell anaemia

Genetically transmitted disease of the blood

Caused by presence of abnormal haemoglobin S

Abnormality occurs as result of mutation

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Haemoglobin S

H – allele for normal haemoglobin

S – allele for haemoglobin S

People homozygous for mutant allele (SS)

-sickle-shaped red blood cells

-inefficient at carrying oxygen

-cells clump together interfering with circulation

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Symptoms of Sickle Cell Anaemia

Fatigue

Breathlessness

rapid heart rate

delayed growth and puberty

susceptibility to infections

ulcers on the lower legs (in adolescents and adults)

jaundice

attacks of abdominal pain

weakness

joint pain

fever

Vomiting

bloody (hematuria) urination

excessive thirst

excessive penis pain

Priapism

chest pain

decreased fertility

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Incomplete dominance

Allele H incompletely dominant to allele S

Heterozygotes – HS

Allele S partially expressed

Sickle Cell Trait

-a third of the haemoglobin is S

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Sickle Cell Anemia in Africa

Allele S rare in populations as semi-lethal

Some parts of Africa up to 40% of population is HS

People with Sickle Cell Trait resistant to malaria

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Distribution of malaria

Distribution of sickle cell trait

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Sickle Cell Trait and Malaria

In malarial regions

-natural selection favours people with genotype HS

People that are HH will die during serious outbreaks of the disease

Hs loses selective advantage in non-malarial sites

Slide 11

Industrial Melanism in Peppered Moth