Understanding Sickle-Cell Anaemia Inheritance

Sickle-cell anaemia is a genetic disorder that affects red blood cells. Normally, red blood cells are round and flexible, easily passing through blood vessels. In sickle-cell anaemia, the red blood cells become sickle-shaped or crescent-shaped due to an abnormal type of haemoglobin. These sickle-shaped cells are rigid and can block blood flow, leading to pain, organ damage, and other serious health problems.

Genetic disorders are passed down through families in different patterns. These patterns depend on which chromosome carries the gene (autosome or sex chromosome) and whether the trait is dominant or recessive.

• Autosomal inheritance: The gene is located on one of the autosomes (chromosomes 1-22), which are the same in males and females.
• Sex-linked inheritance: The gene is located on a sex chromosome (X or Y). Most sex-linked traits are X-linked.
• Dominant trait: Only one copy of the altered gene is needed for a person to have the trait or disorder.
• Recessive trait: Two copies of the altered gene (one from each parent) are needed for a person to have the trait or disorder. Someone with only one copy is typically a carrier and usually does not show symptoms, but can pass the gene to their children.

Sickle-Cell Anaemia Inheritance Pattern

Sickle-cell anaemia is caused by a mutation in the HBB gene, which provides instructions for making a part of haemoglobin. This gene is located on chromosome 11, which is an autosome. Therefore, sickle-cell anaemia is an autosomal disorder.

The sickle-cell trait is inherited in a recessive manner. This means:

• A person with two copies of the normal haemoglobin gene (let's say HbA HbA) does not have sickle-cell anaemia or sickle-cell trait.
• A person with one copy of the normal gene and one copy of the sickle-cell gene (HbA HbS) has sickle-cell trait. They are carriers and usually do not have severe symptoms of sickle-cell anaemia, although they might experience some issues in certain conditions like low oxygen.
• A person with two copies of the sickle-cell gene (HbS HbS) has sickle-cell anaemia.

For a child to be born with sickle-cell anaemia (HbS HbS), they must inherit one HbS gene from each parent. If both parents have sickle-cell trait (HbA HbS), there is a 25% chance with each pregnancy that the child will have sickle-cell anaemia (HbS HbS), a 50% chance they will have sickle-cell trait (HbA HbS), and a 25% chance they will be unaffected (HbA HbA).

Analyzing the Options

Let's look at the provided options based on our understanding of sickle-cell anaemia inheritance:

1. Autosome linked dominant trait: Incorrect, as sickle-cell anaemia requires two copies of the altered gene (HbS HbS) to manifest fully, indicating recessive inheritance. It is autosome linked, but not dominant.
2. Sex-linked dominant trait: Incorrect, as the gene is located on chromosome 11, an autosome, not a sex chromosome.
3. Sex-linked recessive trait: Incorrect, as the gene is located on chromosome 11, an autosome, not a sex chromosome.
4. Autosome linked recessive trait: Correct, as the gene is located on an autosome (chromosome 11) and the condition requires two copies of the altered gene to be present for full expression (recessive inheritance).

Therefore, sickle-cell anaemia is an autosome linked recessive trait.

Revision Table: Genetic Inheritance Patterns

Additional Information on Sickle-Cell Trait

It's important to differentiate between sickle-cell anaemia and sickle-cell trait. Having sickle-cell trait (HbA HbS) means a person carries one copy of the sickle-cell gene. People with sickle-cell trait usually do not have the severe symptoms of sickle-cell anaemia. However, they are carriers and can pass the gene on to their children. If two people with sickle-cell trait have children, there is a risk with each pregnancy that their child will inherit two copies of the sickle-cell gene and have sickle-cell anaemia. Sickle-cell trait also offers some protection against malaria, which is why the gene is more common in areas where malaria is widespread.