What is Thalassemia?
How is it Inherited?
The thalassemias are disorders of the blood, so we have to describe blood and its functions before we can talk about thalassemia
What is blood? (Figure 1)
Blood is a part of your body. it is pumped round by your heart, and circulates in the blood vessels that spread it through your whole body. Blood vessels are arteries, capillaries and veins. When you heart pumps blood out, it flows first into the big arteries, and then passes through smaller arteries to the capillaries. Capillaries are so small that you can only see them with a microscope, but they are very important because while blood is flowing through them it gives out the air and food is carrying to the tissues of the body, and picks up wastes to take away. After this the blood flows into the veins, which finally carry it back to the heart.
What is blood made of?
Blood is made up of a light yellow liquid, called plasma, and of three types of "cells" (Figure 2). In fact, your whole body is made up of tiny building blocks called cells. far too small to see. In most tissues they are stuck together, but in the blood they float round freely in the plasma. There are 4 types of blood cells1: red cells2, white cells3 and platelets.
What does blood do?
Each part of the blood has its own special function.
The plasma carries water, salts and materials such as food, hormones and drugs to your tissues, and carries away wastes, to be got rid of through your lungs (in your breath) and through your kidneys, in your urine.
Your white blood cells defend your body against infections.
Your platelets main function is to stop you losing blood if you hurt yourself. They stick together and block up your blood vessels when they get damaged, so they stop any more blood escaping.
You have many more red blood cells than white blood cells. The red blood cells are full of hemoglobin, which is red, and this is what makes your blood look red. Hemoglobin picks up oxygen from the air in your lungs, and carries it round to your tissues, where it lets it go. To live, your tissues need to breath, so they need oxygen.
What is "anemia"?
If you have too few red blood cells, or there is too little hemoglobin in them, you have "anemia" - which simply means a shortage of blood. If the anemia is mild, it does no harm and you may not even notice it, but if it is severe, you are ill, because your tissues don't have enough oxygen.
1Blood Cells are often called blood corpuscles - the Latin for "tiny bodies"
2Red Cells are often called erythrocytes - the Greek for red cells.
3White blood cells are often called leukocytes - the Greek for white cells.
How do you measure anemia?
By measuring the amount of hemoglobin in your blood. This is quite easy. We describe the result as grams (g)4 of hemoglobin (Hb) per decilitre (dl)5 of blood.
So we write you hemoglobin level as, for instance, 10g/dl, = Ten Grams Per Decilitre. This is what we mean when we talk about your hemoglobing level. Of course, in thalassemia major it changes all time, because of the transfusions you receive.
The usual Hb level is about 13 - 16 g/dl for men
about 11 -14 g/dl for women
Mild anemia means an Hb level of about 9 - 11 g/dl
Severe anemia means an Hb level of less than 8 g/dl
Thalassemia is an inherited characteristic of the blood. It reduces the amount of hemoglobin your body can make, so it can lead to anemia.
How is thalassemia passed on from parents to their children?
Every characteristics of your body is controlled by "genes" which contain a detailed pattern that controls your growth from an embryo, and all your physical functions. Genes are present in every cell of your body. One gene for each characteristics comes from your mother, the other from your father. So, among many other genes, you have two genes that control how hemoglobin is made in each of your red blood cells.
"Normal" people are normal because they have two normal genes for hemoglobin.
Healthy carriers of b-thalassemia-trait have one normal gene for hemoglobin and one altered one, they are healthy because one gene is working well. Since one gene is inherited from each parent, at least one of their parents must be a carrier.
People with b-thalassemia-major have two altered genes for hemoglobin, one inherited from each parent, so both their parents must be carriers.
4One gram is one thousandth of a kilogram.
5One decilitre is one tenth of a litre.
Children are conceived when an egg from the mother meets a sperm from the father. Eggs and sperms are made so that they carry only one of each gene from the parent. When the sperm and egg meet and become one, the material the baby will be made from has two genes for every characteristic again, one from the mother and one from the father (Figure 3).
The eggs or sperms from normal people always carry normal genes for hemoglobin, and so cannot transmit thalassemia. When healthy carriers of thalassemia produce eggs or sperms, each egg or sperm carries either a normal gene or a thalassemia gene, but not both so half the eggs are thalassemic and half are normal; and half the sperms are thalassemic and half are normal.
Now let us consider three sorts of married couples:
- Both Parents are "normal"
They cannot possibly pass on thalassemia trait or thalassemia major to their children (Figure 4)
- One Parent has a thalassemia trait and one is "normal"
All the children must inhert a normal gene from the "normal" parent. However, they may inherit a normal of a thalassemia gene from the carrier parent. For each child there is a one in two (50 %) chance of inheriting the thalassemia gene from the carrier parent, If this happens the child will have thalassemia trait.
There is also a one in two (50 %) chance of inheriting the normal from the carrier parent, if this happens the child will be completely normal. None of this couple's children can have thalassemia major (Figure 5)
- Both the Parents are b-thalassemia carrier, i.e. they are a "couple at risk."
When the mother produces an egg (once a month) the egg is either completely normal or completely thalassemic. There is no way of telling in which order they will come. And when the father produces sperms, half are completely normal and half are thalassemic (Figure 6).
When the mother produces a normal egg, it does not matter what kind of sperm meets it. If the normal egg meets a normal sperm, the child will be completely normal. If the Normal egg meets a thalassemic sperm, the child will bbe a healthy carrier.
But if the mother produces a thalassemic egg, it matters very much what kind of sperm meets it. If the sperm is normal, the child will be a healthy carrier of thalassemia. But if the thalassemic egg meets a thalassemic sperm, the child will have thalassemia major.
This is why couples of carriers have a one in four (25 %) chance in each pregnancy of having a child with thalassemia major, one in two (50 %) chance of having a child with thalassemia trait, and one in four (25 %) chance that the child will inherit a normal gene from both parent, and so will be completely normal
These chances are the same in each prenancy.