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Discussion Starter · #1 ·
A 20 year old Asian discovered accidentally to have microcytic anemia with no Heinz bodies. His uncle had Hemoglobin H disease. His mother had a history of still birth of hydrops fetalis.
If he marries a genotypically normal girl what are the chances that they'll have an affected baby?

a- 30%
b- 25%
c- 50%
d- 100%
e- Can't be predicted
 

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This is alpha thalassemia
I think it follows AR inheritance just like beta thalassemia
So is the correct answer is B ?
 

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Discussion Starter · #6 ·
The correct answer is 50%
because the parents of our patients had a stillborn with hydrops fetalis it means that they must both have the alpha genes in cis-configuration --,--/a,a.
So our patient must have got --,--/a,a as he has no Heinz bodies (so he's alpha thalassemia minor which means he got one normal allele from one parent and another cis-deleted allele from another parent)
So our patient has a 50% chance of giving the abnormal alleles and 50% chance of giving the normal alleles.
 

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The correct answer is 50%
because the parents of our patients had a stillborn with hydrops fetalis it means that they must both have the alpha genes in cis-configuration --,--/a,a.
So our patient must have got --,--/a,a as he has no Heinz bodies (so he's alpha thalassemia minor which means he got one normal allele from one parent and another cis-deleted allele from another parent)
So our patient has a 50% chance of giving the abnormal alleles and 50% chance of giving the normal alleles.
can someone show me the math here pls? I dont get the 50% unless it was an AD disorder, but as thalassemia is AR I come up with the same 25%
 

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Discussion Starter · #8 ·
Already explained

can someone show me the math here pls? I dont get the 50% unless it was an AD disorder, but as thalassemia is AR I come up with the same 25%
I've already explained that!
You need to review the alpha thalassemia genetics. It's AR but it's with 4 alleles and cis or trans configuration.
Our patient is having cis- configuration deletion in one of his chromosomes. Which means whenever he gives that configuration his babies are going to be thalassemia minor also.
The chance of producing a thalassemia major baby is zero because his wife is genotypically normal.
 

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That's true 473... but don't you have to have to combine the probabilities as these are both independent events?? We are dealing with two different set of genes as there are two alpha globulin genes. He has a mutated allele in both of his alpha globin genes (-a/-a). He has a 50% of giving one of the defective genes to his offspring but since we need him to give the defective gene in both loci... likelihood of both events occurring together = 1/4.
 

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473 is right. There are two alpha globulin genes on chromsome 16. Each alpha globulin gene contains two alleles giving a total of four alleles. Alpha thal trait is defined by loss of two alleles. If both alleles are lost from the same gene, this is a cis-configuration. If one allele is lost from each gene, this is a trans-configuration. The patient in question with alpha thal trait has inherited 2 mutated alleles that were lost from the same gene(cis configuration). This is NOT a case in which there was a mutated allele lost from each gene(trans-configuration). Since the patient is Asian, you can assume that the cis-configuration mutation is likely present as it is most commonly associated with this group; this can also be suggested by the fact that the patient in question has a +ve family history of hydrops fetalis. So this is a case where the patient has two mutated alleles in one alpha globulin gene and the other alpha globulin gene is totally normal (aa/--). If this patient has a child with a genotypically normal female(aa/aa), the chance of the offspring having alpha thal trait (aa/--) is 50% while the chance of being genotypically normal (aa/aa) is 50% as well.
 
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