X-Linked Recessive Inheritance

Child Health

By Dr Shan Narayanan

The “sex chromosomes” are the X-chromosome and the Y-chromosome. Females have two X-chromosomes, and males have one X-chromosome and one Y-chromosome. Examples of X-linked recessive disorder are Haemophilia and Glucose 6 Phosphate Dehydrogenase deficiency (G6PD deficiency).

The abnormal gene is located on the X-chromosome, and the risk for children depends on the gender of the affected parent. If the mother has the abnormal gene, there is a 50% risk for passing that gene onto her children with each pregnancy. This is illustrated in diagram 1.

If the father has the abnormal gene, all of his daughters will get the abnormal genes but none of his sons will have the abnormal gene. This is illustrated in the diagram 2.

Daughters who inherit the abnormal gene from either parent do not have the illness, thus they become carriers of the illness. Sons who inherit the abnormal gene from their mother will have the illness.

Glucose 6 Phosphate Dehydrogenase (G6PD) Deficiency

Glucose 6 Phosphate Dehydrogenase is an enzyme in the red blood cells. Enzymes are special proteins that bring about chemical changes in the body. Enzymes are found in many organs and cells in the body including mouth, stomach, intestinal fluids and blood. G6PD protects the red cells from destruction by chemicals following the intake of fava beans (diagram 3) and various medication (including the antibiotics, Co-Trimoxazole).

This deficiency is inherited in a sex-linked recessive manner. Thus males have the illness and females are carriers. The affected male inherits the abnormal gene from his carrier mother.

This deficiency is common among Africans, Mediterraneans and South-East Asians. In Malaysia, it is common among the Chinese and Malay population.

All babies born in Malaysia are checked for G6PD deficiency. A simple blood test is performed on the baby’s cord blood. If the baby is G6PD deficient, the parents are counselled and given the list of medication and food stuff to avoid. Some babies with G6PD deficiency may develop jaundice needing phototherapy.

Individuals with the G6PD deficiency lead a near normal life by avoiding the offending medication and foodstuff. What if the individual consumes the offending agent?

The chemical in the medication or food stuff will cause the unprotected red blood cells to break. This gives rise to sudden onset of anaemia and jaundice. As large numbers of red cells are broken down, there will be blood in the urine. The kidney is overloaded with the red cell breakdown products and may not function efficiently (kidney failure). In this situation, the affected individual will be hospitalised and treated. The treatment includes blood transfusion.

In conclusion, G6PD deficiency is a minor illness, complications can be prevented by avoiding the offending medication and foodstuff.

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