Blood group and Genotype -Why you should know your blood group and genotype before getting married


Blood group and Genotype -Why you should know your blood group and genotype before getting married

As the world continues to evolve and change, many people are becoming conversant with certain steps to take before making a decision as important as marriage.

In many organizations, some medical tests have been made compulsory for intending couples to undergo in order to prevent the emergence of certain diseases in the offspring. Two of these required tests include blood genotype and blood group.

What is a genotype?

Genotype can simply be described as “gene type”; this defines the types of a gene present in an individual by means of inheritance. Genotypes are grouped into three based on dominance and prevalence:

  • Homozygous dominant
  • Heterozygous dominant
  • Homozygous recessive

Blood genotype

The blood is made up of haemoglobin which is a protein. This haemoglobin is classified as either Haemoglobin A or Haemoglobin S; with both forms having 2 alpha and 2 beta chains each.

The alpha chains of haemoglobin A (HbA) and Haemoglobin S (HbS) both have the same amino acid sequence, but differ in their beta chains. In HbS, both beta chains are mutated making Haemoglobin S the abnormal haemoglobin.

There are five blood genotypes:

  • AA
  • AS
  • SS
  • AC
  • SC

AC and SC are, however, very uncommon.

Individuals with genotype AA have two Haemoglobin A, while people with AS genotype have one Haemoglobin A and one Haemoglobin S. Those whose blood genotypes are SS have two Haemoglobin S.

The homozygous dominant genotype is AA, heterozygous dominant – AS, and the SS genotype is homozygous recessive.

How are blood genotypes determined?

Genes are transferred from parents to offspring and are therefore said to be genetic. This is why it is important to know your genotype and that of your partner before getting married and having children. The pattern of transfer is shown in the table below

Father’s genotype Mother’s genotype Genotypes of first four children Marriage option
AA AA AA, AA, AA, AA Can marry
AA AS AA, AS, AA, AS Can marry
AA SS AS, AS, AS, AS Can marry
AS AA AA, AS, AA, AS Can marry
SS AA AS, AS, AS, AS Can marry
AS AS AA, AS, AS, SS Cannot marry
SS SS SS, SS, SS, SS Cannot marry

Defects of blood genotype

The SS genotype is associated with sickle cell anaemia which leads to several disorders of blood and further crisis.

Blood group and effects

Just like blood genotype is important, it is also essential to know one’s blood group, especially the Rhesus factor.
One major disease associated with the Rhesus factor is referred to as the haemolytic disease of the newborn.

How is Rhesus antigen inherited?

The rhesus antigen is inherited as a dominant trait. The table below indicates what Rhesus factor a person may have based on inheritance

Genotype Rhesus factor
DD Positive
Dd Positive
dd negative

From the table above, one can infer that the rhesus is only negative when both alleles inherited are recessive. The rhesus factor of the parents and offspring is shown in the table below

Father Mother First four offspring
DD Dd DD, DD, DD, Dd
Dd DD DD, DD, DD, Dd
 Dd Dd DD, Dd, Dd, dd
DD dd Dd, Dd, Dd, Dd
dd DD Dd, Dd, Dd, Dd
dd dd dd, dd, dd, dd

Haemolytic disease of the newborn

If the father is rhesus positive (DD or Dd), and the mother is rhesus negative (dd), based on the table shown above, some of the children will be heterozygous dominant, therefore will be rhesus positive.

When having their first child who is Rhesus positive, no complications will ensue because blood from the foetus and the mother cannot mix in the womb.

However, during childbirth, some rhesus positive blood from the foetus may get into the mother’s blood causing an immune response/ reaction. This causes the mother’s body to produce antibodies against the rhesus positive blood.

When the mother conceives a rhesus positive (Rh+) child again, due to the Rh+ antibodies already present in the mother’s body as a result of the first pregnancy, the blood cells of the child in the womb is seen as “foreign” because the rhesus antibody can cross the placental barrier even when the antigen cannot.

The result is that the fetal red blood cells begin to agglutinate or clump causing haemolysis (destruction of red blood cells) which in turn leads to jaundice.

Other manifestations include hydrops fetalis; which is characterized by enlargement of the liver and spleen and may eventually lead to death.

This disease may also lead to Kernicterus characterized by brain damage because of a poorly developed blood-brain barrier in infants and resultant jaundice from haemolysis of red cells.

If detected on time, drugs can be given within 72 hours after delivery of the first child to prevent the Rhesus positive antibodies from forming.


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