Genetic diseases: sickle cell anaemia, haemophilia and Down syndrome

What you'll learn

This revision guide covers three major genetic diseases tested in the CXC CSEC Human and Social Biology examination: sickle cell anaemia, haemophilia and Down syndrome. You will understand the causes, inheritance patterns, symptoms and management of these conditions. These genetic diseases appear regularly in Section B questions and require precise knowledge of inheritance mechanisms and their effects on Caribbean populations.

Key terms and definitions

Gene mutation — a change in the DNA sequence of a single gene that may alter the protein produced, causing genetic disorders such as sickle cell anaemia

Chromosome mutation — a change in the number or structure of chromosomes, such as the presence of an extra chromosome 21 in Down syndrome

Recessive allele — an allele that only expresses its characteristic when two copies are present (homozygous recessive condition)

Sex-linked inheritance — inheritance of characteristics controlled by genes located on the X or Y sex chromosomes, such as haemophilia

Carrier — an individual who possesses one copy of a recessive allele for a genetic disorder but does not show symptoms of the disease

Homozygous — having two identical alleles for a particular gene (e.g., HbS HbS for sickle cell anaemia)

Heterozygous — having two different alleles for a particular gene (e.g., HbA HbS for sickle cell trait)

Non-disjunction — the failure of chromosomes to separate properly during cell division, resulting in abnormal chromosome numbers

Core concepts

Sickle cell anaemia: causes and inheritance

Sickle cell anaemia is caused by a gene mutation in the haemoglobin gene on chromosome 11. The mutation causes the production of abnormal haemoglobin S (HbS) instead of normal haemoglobin A (HbA). This single gene mutation involves the substitution of one amino acid in the haemoglobin protein.

The inheritance pattern follows autosomal recessive inheritance:

• HbA HbA — normal individual with healthy red blood cells
• HbA HbS — carrier with sickle cell trait (usually asymptomatic)
• HbS HbS — affected individual with sickle cell anaemia

When both parents are carriers (HbA HbS):

• 25% chance of unaffected child (HbA HbA)
• 50% chance of carrier child (HbA HbS)
• 25% chance of affected child (HbS HbS)

Sickle cell trait provides resistance to malaria, which explains its higher frequency in Caribbean populations descended from West African ancestry. Countries like Jamaica, Trinidad and Tobago, and Barbados have significant prevalence rates due to this selective advantage in malaria-endemic regions.

Sickle cell anaemia: symptoms and effects

The abnormal haemoglobin S causes red blood cells to become rigid and sickle-shaped (crescent-shaped) when oxygen concentration is low. These deformed cells cause multiple problems:

Blood flow complications:

• Sickled cells block small blood vessels, reducing blood flow to organs
• Painful episodes called "crises" occur when blood flow is restricted
• Tissue damage results from lack of oxygen delivery

Anaemia symptoms:

• Red blood cells are destroyed more rapidly (10-20 days instead of 120 days)
• Reduced oxygen-carrying capacity throughout the body
• Fatigue, weakness and shortness of breath
• Jaundice due to breakdown of haemoglobin

Organ damage:

• Spleen damage leading to increased infection risk
• Kidney problems from chronic lack of oxygen
• Delayed growth and development in children
• Stroke risk from blocked blood vessels in the brain

Management approaches:

• Adequate hydration to prevent sickling
• Pain medication during crises
• Folic acid supplements to support red blood cell production
• Blood transfusions in severe cases
• Avoiding triggers (cold, dehydration, high altitudes)
• Penicillin prophylaxis to prevent infections

Haemophilia: causes and sex-linked inheritance

Haemophilia is a sex-linked genetic disorder caused by a recessive allele located on the X chromosome. The disorder prevents normal blood clotting due to the absence or reduced production of clotting factors (Factor VIII in Haemophilia A, Factor IX in Haemophilia B).

The inheritance pattern shows distinct male and female outcomes:

Possible genotypes:

• XH XH — normal female
• XH Xh — carrier female (produces some clotting factors)
• Xh Xh — affected female (extremely rare)
• XH Y — normal male
• Xh Y — affected male (has haemophilia)

Males are much more likely to have haemophilia because they only need one recessive allele (on their single X chromosome) to express the condition. Females need two recessive alleles, which is rare because it requires both parents to carry or have the disorder.

Inheritance from carrier mother (XH Xh) and normal father (XH Y):

• 25% normal female (XH XH)
• 25% carrier female (XH Xh)
• 25% normal male (XH Y)
• 25% affected male (Xh Y)

This means 50% of male offspring from a carrier mother will have haemophilia, while 50% of female offspring will be carriers.

Haemophilia: symptoms and management

Haemophilia symptoms arise from the inability to form stable blood clots:

Bleeding complications:

• Prolonged bleeding from minor cuts and injuries
• Spontaneous internal bleeding, especially in joints (knees, elbows, ankles)
• Severe bleeding from dental procedures or surgery
• Bruising easily from minor trauma
• Blood in urine or stool from internal bleeding

Joint damage:

• Repeated bleeding into joints (haemarthrosis) causes pain and swelling
• Chronic joint damage and arthritis develop over time
• Restricted movement and disability in affected joints

Management strategies:

• Regular injections of clotting factor concentrates (Factor VIII or IX)
• Immediate treatment of bleeding episodes with clotting factors
• Avoiding activities with high injury risk
• Dental care to prevent gum bleeding
• Pain management (avoiding aspirin which increases bleeding risk)
• Physiotherapy to maintain joint function
• Genetic counselling for family planning

Caribbean health systems provide Factor VIII concentrate through regional blood banks, though access can be limited in some islands due to cost and storage requirements.

Down syndrome: chromosomal basis

Down syndrome results from a chromosome mutation rather than a gene mutation. The condition involves having three copies of chromosome 21 instead of the normal two copies (trisomy 21). This occurs in approximately 1 in 700-800 live births globally, with similar rates observed across Caribbean populations.

Cause — non-disjunction: The extra chromosome results from non-disjunction during meiosis (sex cell formation). Chromosomes fail to separate properly during cell division, creating a gamete with 24 chromosomes instead of 23. When this gamete fuses with a normal gamete (23 chromosomes), the resulting zygote has 47 chromosomes instead of 46.

Non-disjunction can occur during:

• Formation of egg cells (most common)
• Formation of sperm cells (less common)

The risk of non-disjunction increases significantly with maternal age, particularly after age 35. This is why older mothers have higher risk of having children with Down syndrome.

Chromosomal pattern:

• Normal individual: 46 chromosomes (23 pairs)
• Down syndrome individual: 47 chromosomes (trisomy 21)

Unlike sickle cell anaemia and haemophilia, Down syndrome is not typically inherited from parents. It occurs as a random error during cell division, though the extra chromosome can be passed to offspring if the affected individual reproduces.

Down syndrome: characteristics and support

Down syndrome affects physical development, intellectual ability and health:

Physical characteristics:

• Distinctive facial features (flattened face, upward-slanting eyes)
• Short stature and reduced muscle tone
• Single crease across the palm
• Small ears and protruding tongue
• Short neck with excess skin

Developmental effects:

• Intellectual disability ranging from mild to moderate
• Delayed speech and language development
• Learning difficulties requiring specialized education
• Delayed motor skill development

Health complications:

• Heart defects present in approximately 50% of cases
• Increased susceptibility to infections
• Hearing and vision problems
• Higher risk of leukaemia
• Thyroid problems
• Early onset of Alzheimer's disease

Support and management:

• Early intervention programs for development
• Special education support in Caribbean schools
• Speech and occupational therapy
• Regular medical monitoring for associated health conditions
• Surgical correction of heart defects when necessary
• Social support networks for families
• Vocational training programs for independence

Many individuals with Down syndrome in Caribbean countries participate in community programs and can live semi-independent lives with appropriate support.

Worked examples

Example 1: A woman who is a carrier for haemophilia (XH Xh) marries a man with normal blood clotting (XH Y).

(a) Draw a genetic diagram to show the possible genotypes of their children. (4 marks)

(b) State the probability that their male children will have haemophilia. (1 mark)

Solution:

(a)

Parents:        XH Xh    ×    XH Y
                (carrier)     (normal)

Gametes:        XH  Xh        XH  Y

Offspring:      XH XH    XH Xh    XH Y    Xh Y
                (normal  (carrier (normal (affected
                female)  female)  male)   male)

Award marks for:

• Correct parental genotypes (1 mark)
• Correct gametes shown (1 mark)
• All four offspring genotypes shown (1 mark)
• Phenotypes identified or ratio shown (1 mark)

(b) 50% or 1 in 2 or 1:1 ratio (1 mark)

Example 2: Explain why sickle cell anaemia is more common in Caribbean populations than in European populations. (4 marks)

Solution:

• Sickle cell trait (HbA HbS) provides resistance/protection against malaria (1 mark)
• Malaria was/is endemic in West Africa (1 mark)
• Caribbean populations have significant West African ancestry due to the transatlantic slave trade (1 mark)
• Individuals with sickle cell trait had survival advantage in malaria regions, so the allele frequency remained high in these populations (1 mark)

Example 3: A couple has a child with Down syndrome. Explain how this condition arises and state whether their next child is likely to have the same condition. (4 marks)

Solution:

• Down syndrome is caused by an extra copy of chromosome 21/trisomy 21 (1 mark)
• Results from non-disjunction during meiosis/sex cell formation (1 mark)
• Chromosomes fail to separate properly, creating a gamete with 24 chromosomes (1 mark)
• The next child is unlikely to have Down syndrome because non-disjunction is usually a random event/not inherited (1 mark) (Note: Accept that risk slightly increases with maternal age)

Common mistakes and how to avoid them

• Confusing carrier status with affected status: Remember that carriers (HbA HbS or XH Xh) have one normal allele and typically show no or mild symptoms. Only homozygous recessive (HbS HbS) or hemizygous males (Xh Y) are affected with full symptoms.

• Mixing up the causes: Sickle cell anaemia and haemophilia result from gene mutations (changes in DNA sequence), while Down syndrome results from chromosome mutation (extra whole chromosome). These are fundamentally different types of mutations.

• Incorrect haemophilia inheritance patterns: Female carriers (XH Xh) do not usually show symptoms because they have one functional allele. Never write that carrier females have haemophilia — they carry the allele but are not affected.

• Stating that Down syndrome is inherited from parents: In most cases, Down syndrome results from random non-disjunction during gamete formation, not from inheriting genes from parents. The error occurs during the formation of the egg or sperm cell.

• Forgetting to show working in genetic diagrams: Always show parental genotypes, gametes, and offspring genotypes clearly. Use a Punnett square or branching diagram to demonstrate all possible combinations.

• Misunderstanding the link between sickle cell and malaria: The sickle cell trait (heterozygous condition HbA HbS) provides malaria resistance — not sickle cell anaemia (HbS HbS), which is debilitating. This selective advantage explains the persistence of the HbS allele in populations.

Exam technique for "Genetic diseases: sickle cell anaemia, haemophilia and Down syndrome"

• Command words matter: "Explain" requires you to give reasons or mechanisms (worth 3-4 marks), while "state" or "name" needs brief answers (1 mark each). "Describe" requires characteristics or features without needing to explain why.

• Genetic diagram questions: Always set up your diagram systematically — parental phenotypes and genotypes, gametes produced, offspring genotypes using a Punnett square, then offspring phenotypes and ratios. Show all steps even if the question doesn't explicitly ask for them to maximize partial credit.

• Use precise scientific terminology: Write "non-disjunction during meiosis" not "chromosomes don't split properly." Write "red blood cells become sickle-shaped due to polymerization of haemoglobin S" rather than "blood cells change shape." Examiners award marks for accurate biological language.

• Link structure to function: When explaining symptoms, always connect the genetic cause to the biological effect. For example: mutation → abnormal haemoglobin → sickled red blood cells → blocked capillaries → reduced oxygen delivery → tissue damage and pain. This chain of reasoning demonstrates understanding and earns multiple marks.

Quick revision summary

Genetic diseases arise from gene or chromosome mutations. Sickle cell anaemia (gene mutation, autosomal recessive) causes abnormal haemoglobin and sickled red blood cells, leading to pain crises and anaemia. Haemophilia (gene mutation, X-linked recessive) prevents blood clotting, predominantly affecting males. Down syndrome (chromosome mutation, trisomy 21) results from non-disjunction, causing intellectual disability and characteristic physical features. Know inheritance patterns, symptoms and the distinction between gene and chromosome mutations. Practice genetic diagrams showing all working steps for maximum examination marks.