Revision Notes

18.2 Mutations

Causes & Effects of Mutations

  • Mutations are random genetic changes
  • Most mutations have no effect on the phenotype as the protein that a mutated gene produces may work just as well as the protein from the non – mutated gene
  • Rarely, mutations lead to the development of new alleles and so new phenotypes and if they do, most have a small effect on the organism
  • Occasionally, the new allele gives the individual a survival advantage over other members of the species
  • For example:
    • A bird develops a mutation leading to a change in feather colours
    • This makes it more attractive to birds of the opposite sex
    • Which causes the bird to breed more frequently and have more chances of passing on the mutated phenotype to the next generation
  • Mutations can also lead to harmful changes that can have dramatic effects on the body – for example, sickle cell anaemia in humans
  • Mutations happen spontaneously and continuously but their frequency can be increased by exposure to the following:
    • Gamma rays, x – rays and ultraviolet rays – all types of ionising radiation which can damage bonds and cause changes in base sequences
    • Certain types of chemicals – for example chemicals such as tar in tobacco
  • Increased rates of mutation can cause cells to become cancerous, which is why the above are linked to increased incidence of different types of cancer
Extended Only

Sickle Cell Anaemia


  • Sickle cell anaemia was the first genetic disease to be described in terms of a gene mutation
  • A gene mutation is a change in the base sequence of DNA
  • The mutation changes the molecule haemoglobin, causing the red blood cells (RBC’s) to become stiff and sometimes sickle-shaped when they release oxygen to the body tissues
  • The sickled cells tend to get stuck in narrow blood vessels, blocking the flow of blood
  • As a result, those with sickle cell disease suffer painful “crises” in their joints and bones
  • They may suffer strokes, blindness, or damage to the lungs, kidneys, or heart. They must often be hospitalized for blood transfusions and are at risk for a life-threatening complication called acute chest syndrome
  • Although many sufferers of sickle cell disease die before the age of 20, modern medical treatments can sometimes prolong these individuals’ lives into their 40s and 50s


Sickle-cell anaemia, IGCSE & GCSE Biology revision notesSickle cell anaemia is caused by abnormal haemoglobin which changes the shape of red blood cells



  • There are two versions or alleles of the gene important for the inheritance of sickle cell anaemia : A and S
  • The two alleles are codominant, meaning there is no ‘dominant’ or ‘recessive’ version of the gene
  • Individuals with two A alleles (HbAHbA) have normal haemoglobin, and therefore normal RBCs
  • Those with two S alleles (HbSHbS) develop sickle cell anaemia
  • Those who are heterozygous for sickle cell (HbAHbS) produce both normal and abnormal haemoglobin (as the alleles are codominant)
  • Heterozygous individuals are usually healthy, but they may suffer some symptoms of sickle cell anaemia under conditions of low blood oxygen, such as high altitudes or during exercise
  • Heterozygous individuals are said to be ‘carriers’ of the sickle cell gene and are said to have ‘sickle cell trait
  • Inheritance of sickle cell trait:


Sickle cell genetic cross 1, IGCSE & GCSE Biology revision notesIf one parent is a carrier of the sickle cell trait, there is a ½ chance their offspring will inherit the trait


  • Inheritance of sickle cell disease:


Sickle cell genetic cross 2, IGCSE & GCSE Biology revision notesIf both parents are carriers of the sickle cell trait, there is a ¼ chance they will have a child that suffers from sickle cell disease


Sickle cell anaemia & natural selection

  • In the United States, about 1 in 500 African-Americans develops sickle cell anaemia
  • In Africa, about 1 in 100 individuals develops the disease
  • Why is the frequency of such a serious disease so much higher in Africa? The answer is to do with malaria
  • Malaria is a disease spread by mosquitoes that are endemic in many areas of Africa and causes over 1 million deaths per year
  • In the late 1940s, studies of diseases in populations suggested a connection between African populations, malaria and sickle cell disease
  • A theory was suggested; if the heterozygous individuals (HbAHbS) are protected from malaria, and the negative effects (of sickle cell) are only present in the small proportion of people who are homozygous for the affected allele (HbSHbS), then the affected allele could become more common
  • Later studies supported this theory, showing that African children who are heterozygous for the sickle cell allele have a ten-fold reduction in their risk of getting malaria
  • This means that there is a strong correlation between the prevalence of sickle cell anaemia in areas of the world where malaria is common


Sickle-cell and malaria, IGCSE & GCSE Biology revision notesIn areas of Africa where malaria is common, there is a corresponding higher rate of sickle cell disease

Exam Tip

You should be able to explain how these maps support the idea that having a sickle cell allele gives resistance to malaria.

You should also be able to use numerical data and graphs given in exam questions to explain this.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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