[Janet]

Janet asked the Naked Scientists:

My question is this

I have blood type O, my Father is AB. my Mother is O. All of their children are O.

They said at the hospital that was rare. I have two children and my husband is O, our daughter is A- we haven't got our son's done yet.  Is it possible to carry on the blood types from generation to generation?  

My husband's grandmother is a rare type, AB- I believe, can't remember.  I was always under the assumption that the child received either the Father's or the Mother's blood type - or so I thought - I have been reading and learning this isn't so apparently.  Could put this in more easier terms?

Thanks.

What do you think?

[SquarishTriangle]

It seems you might have inherited a genetically recessive condition known as the Bombay phenotype.

Firstly, the ABO gene locus is responsible for determining which of the A and B antigens can be expressed on the surface of a person’s red blood cells based on the interaction between the 2 alleles (versions of the gene) at that site. One of the 2 alleles is inherited from each parent. In turn, each parent will have contributed to half the alleles at that gene.

The alleles that can occur at the ABO gene locus are generally denoted by: I^A, I^B and i. Only one copy of the A or B antigen producing gene is required to produce the respective antigen. Consequently, A type individuals can be either I^AI^A or I^Ai since the I^A allele (dominant) is able to mask the effect of the i allele (recessive). The expression of the B antigen can be similarly explained in B type individuals (I^BI^B or I^Bi). AB type individuals must have one of each the I^A and I^B alleles in order to have to genetic encoding to express both antigens A and B (I^AI^B), whereas both the I^A and I^B alleles are absent in O types individuals who express neither A or B antigens (ii).

In addition to the ABO gene, there is a 2nd gene (the H gene) that can influence the effect of the ABO gene and thus a person's blood type. The H gene is also inherited in 2 allelic forms (dominant H and recessive h), and is responsible for producing the precursor molecule to the A and B antigens, the H antigen (or substance H). The H antigen needs to be produced by the cell before it can be converted into A and B antigens. I^A and I^B actually encode the enzymes that enable this conversion to take place in the biochemical pathway.

In order to produce the H antigen, the person needs to have at least one copy of the H allele (HH or Hh). Individuals with the Bombay phenotype instead possess 2 recessive h alleles at the H gene locus (hh) and so the H antigen is not produced. As a result, the A and B antigens also cannot be produced even if I^A or I^B alleles are present at the ABO locus (there is nothing for the A and B enzymes to convert). In a blood type test, these individuals come up as O regardless of whether they are genetically A, B, AB, or O.

In your case, I would guess you probably have inherited 2 recessive h genes at the H gene locus in addition to at least one I^A (I’ll explain that in the next paragraph). Your father has 1 H allele since he expresses A and B type antigens, but also has 1 h allele (which you inherited). Your mother must have at least 1 h allele (in order for you to have 2). If she is Hh, she must also be ii (‘true' O type). Alternatively she could be hh and genetically, but inapparently, any of the ABO blood types. In the 1st situation,  75% of your parent’s offspring would be either A or B. Since you said all of their children were of O type (I’m assuming they had quite a few children), the 2nd situation would be more likely. You could therefore be genetically I^AI^A, I^AI^B or I^Ai (ie. you would be either A or AB if the H antigen was expressed.)

Because the Bombay phenotype is relatively uncommon, I would guess your husband is probably a true O type (ii) rather than another Bombay phenotype. If that is true then your daughter must have inherited her I^A from you and be an I^Ai A type.

I hope that makes some sort of sense to you!

[chris]

Brilliant answer!

[iko]

Cis-AB is another possibility:

1chromosome carries the O gene, the other A&B together.
Quite rare indeed, like type O Bombay...

http://en.wikipedia.org/wiki/Cis_AB

[Nizzle]

Hmm,

additional question after reading Squarish Triangle's post.

So Bombay phenotype is rare, Cis-AB is rare as well, and true-O is the result of a double recessive genotype.
Then how do you explain these numbers of blood type prevalence in humans:

Code: [Select]

O+ 37% O- 6%
A+ 34% A- 6%
B+ 10% B- 2%
AB+ 4% AB- 1%


They show that O+ is the most common blood type, which you don't expect of a double recessive genotype....

[rosy]

Rh+ is dominant, isn't it? So O+ not double recessive? (Or am I wrong, haven't looked it up..)

More to the point, a dominant gene can only dominate in those individuals who inherit it. And there's no reason to expect a particular gene to be more or less common in the population just because it's dominant, unless it confers a reproductive advantage on its carriers* (infact.. if, as I seem to recall from somewhere, O mothers are more likely to miscarry non-O babies than O babies there's a selection pressure in favour of O blood type, I think).

*Where a reproductive advantage is one which promotes any or all of reaching breeding age, or finding a partner, or successfully raising offspring to maturity, or even living long enough to support the grandkids.

[Nizzle]

So basically, i is much more abundant than I^A and I^B..

[rosy]

I imagine so... I don't have any evidence other than the stats you cite, but that's the implication.