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Predestination (2014) is a well-crafted time travel story, which I found quite remarkably consistent.
There was however one point in the plot that tickled the suspension of my disbelief.

The movie tells of the birth of a baby, whose parents are Jane, and

John.

Both parents respective genetic materials must be

the same, because Jane and John are the same individual, a few years (and 1 time travel) apart.

The movie suggests that the baby was created through classical human fertilization.

As this article about genes inheritance recalls, human gametes (ovum and sperm) contain half the genetic material of the parent ; and all gametes produced are genetically different from each other.
In other words gametes contain a random half of the parent's genetic material (which is why non-twin biological siblings are not genetically identical).

However the movie also reveals that the baby is

the same individual as Jane (and John).

For this to be consistent, it would mean that

the combination of the ovum provided by Jane, and the sperm provided by John, results in the exact same genome as that of Jane/John.

I estimate that this has a probability of happening of less than 0.00000000000142%, arguably smaller than the probability of time travel to ever become possible...

Were there hints or assumptions in the story explaining the occurrence of this improbable event?

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In order for the plot to be an 'infinitely reoccurring perfect loop', the baby fertilized has to an exact copy of Jane/John.

This is required because any change in the genome of the baby would essentially give rise to the possibility of the baby taking different decisions than what their mother would have in an identical environment with identical experiences (giving the baby 'free will').

I estimate that this has a probability of happening of less than 0.00000000000142%

Assuming your calculation to be true, however small the probability is, the infinite causal loop is consistent ONLY when this happens. And if you find this probability to be absurdly low then the probability that Jane behaves in a way that leads up to her 'creating' the causal loop in the first place is also absurdly low.

In essence, what we're watching on screen is possible only if the 0.00000000000142% chance event happens. So there weren't any actions explicitly made to allow it to happen, it's implied it happened. There was no other way

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Fiction aside, I don't see how the quote

contain half the genetic material of the parent ; and all gametes produced are genetically different from each other.

necessarily would contradict the scenario in the movie.

Take another quote (gamete being 50% of the respective chromosomes):

Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete.

The (obvious) assumption of this rule is that both parents are different thus have different genetic material thus taking a random 50% of one parent (e.g. legs + hair) and combining it with the inverse 50% of the other parents (e.g. hands + eyes) would result in one possible permutation of their shared attributes. (very simplified)

In the movies case however, both parents have the same genetic material, thus taking 50% from one and the remaining 50% from the other would result in exactly the same sequence as a result.

You are still correct in the sense that both gametes will contain a random number of chromosomes and there is a high probability that there would be a shared chromosome in both sets. I am assuming this could result in some kind of disability. How does this work in cloning of animals? Maybe someone with greater knowledge of this topic can shed some light onto this.

@Mithoron commented:

you don't consider alleles - for example child of two black haired identical clones could be blonde.

Based on this comment. We basically end up in an "Avengers: Endgame" scenario. Most viewers gladly accepted to see only the one (or two) out of 14 million possible outcomes to be brought to the silver screen. It is the same thing here. Out of a vast set of possible constructions, the most suitable for the narrative was picked. It is not likely, but possible.

And one could even argue that the time travel that happens in the movie has some side-effects when it comes to genetic information.

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  • " both gametes will contain a random number of chromosomes " no way! "taking 50% from one and the remaining 50% from the other would result in exactly the same sequence as a result" - no it wouldn't, you don't consider alleles - for example child of two black haired identical clones could be blonde.
    – Mithoron
    Jan 12 at 17:37
  • 1. well, maybe it is not completely random, but does it matter in this argument? Certainly not. Does it warrant an aggressive comment? Definitely not. A simple correction would have sufficed. 2. Point taken. I will update my answer.
    – BestGuess
    Jan 13 at 8:31

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