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If a pair of twins developed when their mother's egg split in half and they were fertilized by different sperm, could one of them develop situs inversus? The child with the condition would be male, the other one, without the condition, would be female. They both look very much alike, like, hard to tell apart alike, and do have mirrored traits, but they aren't identical twins. Is any of this possible?

I should point out that the whole 'egg splitting before it's fertilized' is a theory that some scientists developed, it has not been proven, I believe, but it's known as 'polar body twins' or 'half identical twins'.

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  • $\begingroup$ Would i be ok to use the vanishing twin phenomenon in your story? If the right fetus absorbs the other, you could have a situs invertus kid with no identical twin and no genetic defect. $\endgroup$
    – BabiBaba
    Commented Aug 13, 2015 at 11:38

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Eggs don't split.

Identical twins happen when one egg is fertilized then part way through gestation it splits into two.

Non-identical twins happen when the mother releases two eggs at once and both are fertilized by different sperm.

However the situation you are asking for is possible, however very unlikely. There are documented cases of it happening: https://www.ncbi.nlm.nih.gov/pubmed/6540028

The most likely cause is that something happens after the egg splits to cause one twin to mirror but not the other.

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Situs inversus is generally an autosomal recessive genetic condition,...

So sure. It happens by means other than "mirror twins" so a mirror can have a normal sibling.

I don't think eggs split. They are already haploid. DZ twins come from two different eggs released at once. They may indeed have different fathers.

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