Simple Genetics Question

jmurt

Just a simple biology question regarding blood types - would appreciate any help:

In humans blood groups are determined by antigens present on the surface of red blood cells. The ABO system is controlled by 3 alleles; IA,IB and IO.

IA and IB are co-dominant while IO is recessive to both of the others

A woman of blood type A presents with a child of blood type AB. the woman's partner with blood type O denies paternity.

The most likely explanation is:

1. the man's blood type does not rule him out - but the woman is definitely not the mother
2. neither is a parent of the child
3. this is the child's biological parents
4. the man is the father


I thought 4 would be the answer but a solution guide gives 1, I have to say i'm stumped!

J

IrishKnight

Answer three and four can be discounted at any rate. If the male in question has blood type O, then he can only pass on the O allele to his offspring. For the offspring to be AB, they must get allele A from one parent and allele B from another. If the above couple have offspring, the offspring will either O or A

This brings us to Answer one and two. Answer one can be discounted, as the male can only give the O allele, it would be impossible for his offspring to be AB. Process of elimination beings us to answer 2.

Not really sure they the answer guide is saying 1, perhaps I am missing something, unlikely, but not impossible...

Improbable

jmurt wrote: »

A woman of blood type A presents with a child of blood type AB. the woman's partner with blood type O denies paternity.

The most likely explanation is:

1. the man's blood type does not rule him out - but the woman is definitely not the mother
2. neither is a parent of the child
3. this is the child's biological parents
4. the man is the father

As the mother is type A, she can be either (O,A) or (A,A)
As the father is type O, he has to be (O,O)
As the child is type AB, he/she has to be (A,B)

Now for the options:

1. This is not possible because the "father's" bloodtype DOES rule him out. Since the mother is A, she is expressedly NOT ruled out.

2. This is entirely possible because blood typing does not prove who the parents of a child are, it can only determine whether someone is NOT a child's biological parent.

3 and 4 have the same answer. Neither the mother or the father have a B allele, therefore they could not have passed it onto the child. The child DOES have a B allele so one of the child's actual biological parents must also have a B allele. Since they don't, 3 and 4 cannot be correct.

Based on the above information, and given the fact that it's multiple choice and not essay style, option 2 can be the only answer.

Tree

Out of curiosity, OP where was this question posed?

As has been worked out by the previous posters, 2 is the most correct answer. I'd be more used to seeing the question phrased in a way that would also permit the answer to involve the mother being a parent but the father not (as we can see from the working out that she is in a position to contribute an A to the child, and maternity disputes aren't that common in the real world as a nine month incubation is pretty conclusive).

Improbable

Tree wrote: »

Out of curiosity, OP where was this question posed?

As has been worked out by the previous posters, 2 is the most correct answer. I'd be more used to seeing the question phrased in a way that would also permit the answer to involve the mother being a parent but the father not (as we can see from the working out that she is in a position to contribute an A to the child, and maternity disputes aren't that common in the real world as a nine month incubation is pretty conclusive).

Yeah, that's what I was thinking as well, specifically about the first option. Feels like they got the order the wrong way around. Feels like it should have said the mother's blood type does not rule her out, but the father is definitely not the father.

jmurt

Thanks All - appreciate the input

@Tree this was a (poorly) worded question in a GAMSAT practice paper by Ozimed

doctoremma

Just to throw a spanner in here...OP, this almost certainly isn't relevant for your test where, on the face of it, 2 is not simply 'most correct', it's the 'only correct' possibility.

However, genetic mosaicism may provide some answers that confound this. Dad's blood may test as O/O but his genotype may be B/O in other tissues: to test as O/O (or, really O/-), he must have had a null mutation event in his B allele in blood lineages. If he maintains a working B allele in his germ line, he could pass this to his son.

Nothing is impossible in genetics. And discrepancies between genotype and blood group aren't unheard of! Anyone putting this answer on a test is getting top marks, pronto.

Improbable

doctoremma wrote: »

Just to throw a spanner in here...OP, this almost certainly isn't relevant for your test where, on the face of it, 2 is not simply 'most correct', it's the 'only correct' possibility.

However, genetic mosaicism may provide some answers that confound this. Dad's blood may test as O/O but his genotype may be B/O in other tissues: to test as O/O (or, really O/-), he must have had a null mutation event in his B allele in blood lineages. If he maintains a working B allele in his germ line, he could pass this to his son.

Nothing is impossible in genetics. And discrepancies between genotype and blood group aren't unheard of! Anyone putting this answer on a test is getting top marks, pronto.

In an essay style, I'd agree that the more knowledge you show, the better. But even if this was an essay style question and you were asked to justify your choice, option 1 still could not be the right answer because it also states that the mother definitely isn't the mother. Which is another wrench in the works.

In the interest of thoroughness, I will also mention Bombay Phenotype where the individual cannot produce the H antigen which is the precursor to the A and B antigens. So the father could be genotypically AB or BB or BO at his ABO allele but be homozygous recessive for the H antigen. Since he cannot make the precursor for the A or B antigens, he cannot make the A or B antigens themselves even though he may have the corresponding allele. In regular blood tests, he would come up as being O.