# [SPM Biology Blood] Why the answer is like that?😄

### Question

(b) An O-blooded man marries an AB-blooded woman. What is the possibilities that their child blood O? Explain why?
Seorang lelaki berdarah O berkahwin dengan perempuan berdarah AB. Apakah kemungkinan anak mereka berdarah O? Jelaskan mengapa.

Hi Farhaini,
I believe that the second sentence of the answer is incorrect.

We know that I^A and I^B are dominant alleles and I^O is the recessive allele,
so if the person’s genotype is I^AI^O, the blood group would be A
if the person’s genotype is I^BI^O, the blood group would be B
if the person’s genotype is I^AI^B, the blood group would be AB because neither allele is more dominant than the other, I^A and I^B are said to be codominant in this case.

Here’s a table showing the possible genotypes for each blood group. (Note: Some books or some resources on the internet will use i instead of I^O to represent the O allele)

Now, back to the question. An O-blooded man marries an AB-blooded woman. We know that for blood group O, the only possible genotype is I^OI^O. (So by saying that the man does not have the O allele is totally incorrect.) The same goes for the woman with the blood group AB where there is also only one possible genotype, I^AI^B.

Here is a schematic diagram to show you why I believe that the reasoning of the answer is incorrect even if the answer meant “The child does not have the O allele”.

As you can see, the child cannot be blood O, thus 0% chance. The first part of the given answer is correct. But you can observe that if the offspring (child) is blood A or blood B, the O allele, I^O is still present in both cases.

I think the better answer would be because the child would have genotype I^AI^O or I^BI^O, and the O allele is recessive, hence the child cannot have blood O, or you can say that the child will not have a pair of O alleles as the woman does not have the O allele. (But sometimes mark schemes want certain keywords included in your answer be sure to know what keywords to write!)

Hope this helps! I haven’t read bio stuff for 2 years so I apologize if I didn’t explain it well enough ^^

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I totally get it ! Thank you very much!

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