This matches the incidence of singleton one baby conception. Gender is not determined "by chance", as chance is defined as "a possibility of something happening,fortuitous, accidental". Since neither is older than the other, what is the probability of having any gender combination? The question you're asking is essentially a version of an old riddle: Search Search the site
If they are fraternal there is an equal chance of having a same sex pair as having an opposite sex pair, and if they are same sex there's an equal chance of having two girls or two boys.
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