chapter 14 question 52
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a gene for the mn blood group has codominant alleles m and n. if both children in a family are of blood type m, which of the following situations is possible?
both children are heterozygous for this gene
each parent is either m or mn
each parent must be type m
neither parent can have the n allele

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# Brief Explanations:
- **For the first option**: If both children are heterozygous ($MN$), their blood - type would be $MN$ (because of codominance), not $M$. So this option is wrong.
- **For the second option**:
    - If one parent is $M$ ($MM$) and the other is $MN$:
        - The parent with $MM$ can only give $M$ allele. The parent with $MN$ can give either $M$ or $N$ allele.
        - The possible genotypes of children are $MM$ (from $M$ (from $MM$ parent) and $M$ (from $MN$ parent)) or $MN$. But if by chance, both children get $M$ from the $MN$ parent (in addition to $M$ from the $MM$ parent),  both children can be $MM$ (blood - type $M$).
    - If both parents are $M$ ($MM\times MM$), all children will be $M$.
    - If one parent is $M$ ($MM$) and the other is $M$ ($MM$), all children are $M$. So it is possible that each parent is either $M$ or $MN$ and have two children of blood - type $M$.
- **For the third option**: It is not necessary that each parent must be type $M$. As shown above,  a parent can be $MN$ and still have children of blood - type $M$ (if the other parent is $M$ and the $MN$ parent donates $M$ allele to both children).
- **For the fourth option**: If one parent is $MN$ (has $N$ allele) and the other is $M$ ($MM$), as long as the $MN$ parent donates $M$ allele to the children (and the $MM$ parent donates $M$ allele), children can be $M$. So parents can have $N$ allele (in one of them).

# Answer:
each parent is either $M$ or $MN$

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