Update: I had a wrong assumption.

  1. After triple checking, I now see that how ordinary monozygotic twins arise is 1 sperm and 1 ovum and then later the zygote splits up.

    • (1.1. and that that phil and lil from rugrats are opposite sex and identical twins is actually an extremely underrated genetic miracle, possibly giving more credibility to the rugrats theory/hypothesis. There's also semi-identical twins in a song of ice and fire - though i guess not game of thrones -, Ben 10, legend of korra, KND, baka to test, and i think samurai jack, etc)
  2. Honest to God, what i weirdly remembered was that ordinary 'monozygotic' twins arise from 2 sperm 1 ovum and then this 'zygote' or whatever of 2 sperm and 1 ovum splits up into 2 zygotes i.e. the splitting up of the 'zygote' is a natural consequence from that 2 sperm have fertilised 1 ovum. From what I weirdly remembered, it would be natural to ask what if the 2 sperm were from different males.

    • (2.1. Additionally, the Phil and Lil thingy is not really a genetic miracle since all you need is a 22X sperm and a 22Y sperm to fertilise 1 ovum. Damn television. i really thought opposite sex identical twins were not that rare.)
  3. However, apparently it is possible for 2 more sperm to fertilise 1 ovum, in a case called semi-identical twins or something, but there are apparently only 2 reported cases.

New question: Okay in the unlikely event that what would be semi-identical twins would arise from sperm from different males, well, then what?

From wikipedia:

Homopaternal superfecundation refers to the fertilization of two separate ova from the same father, leading to fraternal twins;6 while heteropaternal superfecundation is referred to as a form of atypical twinning where, genetically, the twins are half siblings.

  1. There doesn't seem to be a distinction between heteropaternal superfecundation occurring with two ova and with one ovum. How will monozygotic heteropaternal superfecundation twins look like as opposed to dizygotic heteropaternal superfecundation twins and to identical twins?

  2. Are/Were there real life cases of monozygotic heteropaternal superfecundation?

The heteropaternal superfecundation selected cases given on wikipedia seem to be for dizygotic. I tried looking up on google, but I've found so far only dizygotic.

  • 2
    $\begingroup$ Have you thought about how many chromosomes your one zygote with heteropaternity would have? $\endgroup$
    – mgkrebbs
    Commented Jul 22, 2020 at 6:58
  • $\begingroup$ @mgkrebbs No. The extent of my familiarity with genetics is what I learned in a non-elective class in secondary school. Thanks for commenting. Please enlighten me. But I do remember the 22X, 22Y stuff I guess. What's the difference if the 2 sperm are from the same father vs from a different father? $\endgroup$
    – BCLC
    Commented Jul 23, 2020 at 1:42

1 Answer 1


A normal singleton child is a result of the fertilization of an ovum (with 23 chromosomes) by a sperm (with 23 chromosomes). After fertilization, the cell (with 46 chromosomes) is called a zygote. All normal human cells (other than ova and sperm) have 46 chromosomes.

Monozygotic twins (identical twins) occur when the set of cells resulting from the early divisions of a single zygote separate into two groups. Each of these two groups of cells proceed to form a normal embryo, with resulting (if carried to term) in twins. Since these twins came from the same zygote, they have the same 46 chromosomes and so are essentially genetically identical.

Now how could this monozygotic process occur with two fathers being involved? Only by having two separate sperm combine with a single ovum. This would result in the zygote having 69 (3x23) chromosomes. This could happen, but would be an extreme case of aneuploidy (abnormal number of chromosomes). Such a genome is massively defective due to the presence of too many gene copies, and the embryo (whether singleton or twin) will be spontaneously aborted. Only a few kinds of aneuploidy can result in a viable fetus, almost all having only a single extra chromosome (one of the smaller ones); the most common case is having 3 copies of chromosome #21, which results in Down Syndrome.

So, monozygotic heteropaternal superfecundation has not been observed because the resulting pregnancy is very unlikely to go to term. (It's just as unlikely when the two sperm are from the same man.)

  • $\begingroup$ thanks. i apparently remembered my biology/genetics from secondary school wrong. i updated question. follow-up 1: but your answer is still the same...right? $\endgroup$
    – BCLC
    Commented Jul 23, 2020 at 10:08
  • $\begingroup$ follow-up 2: do you really mean 'the resulting pregnancy would never go to term' rather than 'the resulting pregnancy has never in human history been recorded as having gone to term' ? $\endgroup$
    – BCLC
    Commented Jul 23, 2020 at 10:09
  • 1
    $\begingroup$ Never say "never" with biology - yes, there have been a very few reports of twins apparently resulting from two sperm entering an ovum. In the well documented 2019 report, the result was twins who were both mixtures (chimeras) of cells derived from both sperm, some XX and some XY. They hypothesize that an initial 3-way division separated the 2x69 chromosomes into 3 cells with 46 chromosomes each. I've softened the wording in my answer above. $\endgroup$
    – mgkrebbs
    Commented Jul 23, 2020 at 23:36
  • $\begingroup$ right i never said 'never happened' i said 'never...been recorded'....anyway thanks for answering and thanks in particular for putting up with my wrong assumption about monozygotic twins $\endgroup$
    – BCLC
    Commented Jul 25, 2020 at 7:05

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