simply refers to hemoglobin gene constituents. Genes are always in pairs and overall expression depends whether dominant, recessive or X-linked. So there is no problem when one of the genes is abnormal e.g. AS, AC. This is called a carrier state (sickle cell trait). Only when the two are abnormal then there is a great problem.
The genotypes in humans are AA, AS, AC, SS.
I hadn't heard of the genotype AC until I was researching for this article and found Dr. Ademola Peter's article on the subject of genotypes and blood groups. He's also the one I quoted above and throughout this article. Unfortunately is website is currently not accessible so I can't provide link to the actual article.
Anyway, I guess AC is not common in genotypes or I've simply not heard about it. But AS and AC are both abnormal take note.
And the great problem referred to above that we're trying to avoid with genotype matchmaking for intending couples is the Sickle cell disease.
Sickle cell disease (a recessive disorder) is a very serious medical condition with high prevalence rate in sub Saharan Africa. Intending couples must make sure to know their genotypes and be sure it's compatible for marriage before going ahead to get married.
So who can you marry and who shouldn't you marry based on Genotype compatibility?
Someone with the genotype AA can marry across. That is there's no risk of having a sicklier for a child even when he or she marries an SS. But AA marrying an SS can only result in AS children. There's no hope of having a child with the genotype AA. But there's no danger either.
Someone with the genotype AS should only marry someone with the genotype AA because if AS marries AS there's one out of 4 chances (AS, AS, AA and SS) that they will have a child with the sickle cell disease. Same applies when AC is combined. AC and AS combine will produce AA, AS, AC, SC, the sickle cell disease will be SC. Remember what the quote above from Dr. Peter said, it's only when two abnormal marry that there is a great problem.
Of course there are many cases where two abnormal was combined that is an AS and AS or AC and AC or AC and AS couples and had up to five or six children without a single sicklier among them. But why risk it? What if you're not so lucky? Can you forgive yourself when you end up having a child with the sickle cell disease and put the child through the agony the disease brings when you could have easily avoided it?
Compatible genotypes for marriage are:
AA marries an AA. That's the best compatible. That way you save your future children from having to worry about genotype compatibility in future.
AA marries an AS. You'll end up with kids with AA and AS which is good. But sometimes if you're not lucky all the kids will be AS. Like my elder sister who is AA and married an AS and was expecting to have kids with genotype mostly AA. But the last I heard two out of her 3 kids are AS; I'm not sure what the genotype of her third child is. Like I've said there's no problem having kids with AS except if eventually the kids marry people with the genotype AS.
AS and AS should not marry.
AS and SS shouldn't think of marrying.
And definitely SS and SS must not marry since there's absolutely no chance of escaping having a child with the sickle cell disease.
It is advisable that intending couples should know their blood groups before marriage. The blood groups are designated by the letters O, A, B, AB. Blood groups have a hereditary basis and depend upon a series of alternative genes, a fact sometimes utilized in solving the problems of disputed parentage.
Equally important and linked to the blood group is the Rhesus antigen, which is positive in 83% of the British population but about 95% among the black population.
Thus one can be O Rhesus positive (O+ve) or O Rhesus negative (O-ve). When an Rh +ve blood is given to an Rh -ve person, then Rhesus antibodies are stimulated; for instance, immunization of a woman can result from blood transfusion or injection of Rh positive blood. In women, also, immunization due to pregnancy can result from the passage of the Rh +ve antigen from a Rh positive fetus across the placenta into the circulation of a Rh negative woman.
It must be emphasized that when an Rh-ve woman is married to an Rh positive man, the chance for her becoming sensitized to the Rh antigen and thus having children affected with haemolytic (red cell breakdown) disease of the new-born is relatively small. The risk of developing antibodies increases with succeeding pregnancies, thus if a Rh-ve woman marries a Rh +ve man, there is 1 in 143 chance of Rhesus iso-immunization during the first pregnancy, 1 in 14 during the 2nd and 3rd; 1 in 12 during the 4th, and 1 in 8 during the 5th pregnancy.
The overall incidence of haemolytic disease of the newborn due to this problem is about 1 in 200 of all pregnancies. Usually, sensitization due to pregnancy practically never results in haemolytic disease in the first child; on the other hand, sensitization due to previous blood transfusion may cause the first child to be affected. Thus the importance of avoiding the transfusion of Rh +ve blood to a Rh -ve person must be emphasized.
In conclusion, in view of the low risk of Rhesus iso-immunization, Rhesus factor does not constitute an absolute contraindication why a Rh -ve woman should not marry a Rh +ve man. Even in the few cases where sensitization occurs, the affected child can still be effectively treated. There is a drug that can be used to prevent sensitization if the Rhesus status of the couple is known initially. There is no risk of Rhesus iso-immunization if both couple are Rhesus +ve, or are Rhesus -ve, or if a Rh -ve man marries a Rh +ve woman.
I'll conclude this article with Dr. Peter's final advice on the matter...
Know your blood group and genotype.
Help in preventing genetic diseases by marrying a partner of compatible genotype.
Determine the Rhesus status of every newly-born child.
Consult your obstetrician during pregnancy.
Genotype compatibility section of this article was put together with help from Dr. Peter's article and the book "Major Problems of Marriage, their Prevention and Management" written by Dr. T.O.K Osuafor.
Paragraphs of Blood group compatibility was taken from pages 27 & 28 of the book "Major Problems of Marriage, their Prevention and Management" written by Dr. T.O.K Osuafor.
Genetic compatibility is just one area of marriage compatibility tests between intending couples. For more see the article on marriage compatibility test.
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