Thalassaemia
Of every condition in this chapter, thalassaemia is the one a Sri Lankan reader is most likely to meet. It is inherited in exactly the same pattern as albinism, and that pattern is the reason it can be prevented.
Thalassaemia
An inherited condition in which the body cannot make normal haemoglobin, the protein in red blood cells that carries oxygen. It is caused by a recessive allele on an autosome.
What it means for someone who has it
Without enough working haemoglobin the blood carries too little oxygen, causing severe anaemia. A person with the major form needs regular blood transfusions throughout life, together with treatment to remove the iron that builds up from those transfusions.
The carrier cross, one more time
Write the normal allele T and the thalassaemia allele t. A carrier is Tt — healthy, usually with no idea they carry it. When two carriers have children:
| Tt × Tt | T | t |
| T | TT | Tt |
| t | Tt | tt |
- TT — one in four: neither affected nor a carrier.
- Tt — two in four: a healthy carrier, like both parents.
- tt — one in four: a child with thalassaemia.
Two healthy parents, an affected child
Neither parent is ill and neither has any sign of the condition, yet one child in four may have it. That is not bad luck and it is nobody's fault — it is what a recessive allele does, and it is exactly the F2 ratio Mendel counted in his pea plants.
This is why carrier testing exists
A simple blood test identifies carriers, who otherwise have no way of knowing. Sri Lanka runs a national thalassaemia programme built on exactly the reasoning on this page: if two carriers know they are both carriers, they know the one-in-four risk in advance and can get proper counselling. The genetics on this page is not an abstraction; it is the basis of a public health programme.