12.3 Genetic Disorder

Genetic diseases can arise from mutations in the genes of sex chromosomes. These disorders are called sex-linked disorders.

• X Chromosome and Y Chromosome Differences:

• The Y chromosome is smaller and contains fewer genes than the X chromosome.

• Sex-linked disorders typically occur due to mutations on the X chromosome.

• Females and Sex-Linked Disorders:

• Females have two X chromosomes (XX). If one X chromosome carries a mutation, the other X chromosome usually compensates, preventing the disorder from showing.

• Because the two X chromosomes in females are unlikely to carry the same mutation, they are typically carriers of the disorder, not affected by it.

• Males and Sex-Linked Disorders:

• Males have one X and one Y chromosome (XY). Since they only have one X chromosome, they cannot be carriers. If their single X chromosome carries a mutation, they will exhibit the symptoms of the disorder.

(a) Colour Blindness

Colour blindness is the inability to distinguish between certain colours.

• Cause:

• Colour blindness occurs when the eye’s optical nerve cells lack the pigments required to identify specific colours.

• Red-green colour blindness: The most common type, where the patient lacks a pigment that allows them to differentiate between red and green.

• Blue-yellow colour blindness: Occurs when there is a deficiency in pigments responsible for distinguishing blue and yellow.

• Prevalence:

• 1 in 10 men are colour blind, but only a few women are affected. Women are less likely to show symptoms as they typically have two X chromosomes, one of which can compensate for the mutated one.

• External Factors:

• Certain medications like hydroxychloroquine (used to treat rheumatism) can trigger colour blindness by damaging the colour pigments in the eyes.

• Identification:

• Colour blindness can be identified using the Ishihara chart, which displays numbers embedded in coloured dots. A colour-blind person will see a different number than a healthy person.

• Genetic Inheritance:

• Carrier Female (XX’) and Male with Sex-linked Disorder (X’Y):

• Daughter may be a carrier (XX’) or affected (X’X’).

• Son may be normal (XY) or affected (X’Y).

• Inheritance ratio: 1:1:1:1 (sick girl, carrier girl, sick boy, normal boy).

(b) Thalassemia

Thalassemia is a genetic disorder affecting red blood cells, causing them to break down prematurely, leading to anemia.

• Prevalence:

• In places like Bangladesh, thalassemia is a common hereditary blood disorder, with about 7000 babies born with it each year.

• Genetics:

• Thalassemia is an autosomal recessive disorder. This means a person must inherit defective genes from both parents to express the disease.

• Thalassemia can be caused by mutations in either the alpha (α) or beta (β) globulin genes in red blood cells.

• Alpha thalassemia: Caused by defective or absent production of alpha globulin.

• Beta thalassemia: Caused by defects in the beta globulin gene and is also known as Kulir thalassemia.

• Types:

• Thalassemia Major: Occurs when both parents pass on defective genes, and the child inherits both defective alpha or beta globulin genes.

• Thalassemia Minor: Occurs when a child inherits only one defective gene (either from the father or mother), and the child becomes a carrier but does not show symptoms.

• Inheritance Pattern:

• If both parents are carriers (healthy but silent carriers), the chance of having a child with thalassemia is 1/4 (25%).

• Symptoms:

• In severe cases, thalassemia may cause the baby to die in the womb.

• Babies born with major thalassemia will show signs of anemia as early as birth, which worsens within the first year of life.

• Treatment:

• Thalassemia requires regular blood transfusions and specific medication to manage symptoms.

• Patients should avoid iron-enriched foods and medicines, as iron buildup can damage organs, particularly the liver, and cause complications like jaundice.

• Life expectancy for those with major thalassemia is typically limited to 20-30 years.

Key Takeaways

• Sex-linked disorders occur primarily due to mutations in the X chromosome, with females typically being carriers and males exhibiting symptoms directly.

• Colour blindness is caused by deficiencies in colour-identifying pigments and is more common in males than females.

• Thalassemia is a hereditary blood disorder caused by defective globulin genes, and it can be managed but requires lifelong treatment.