Children born from the same pregnancy. Twins can result from either a single zygote (monozygotic, or identical twins) or from two, separately-fertilized egg cells (dizygotic, or fraternal twins).

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Limb polarity such as writing handedness, golf handedness and natural football foot

Define the term "limb polarity" to mean things such as writing handedness, golf handedness, throwing ball arm preference and natural boxing hand. I avoid using the term "handedness" because of its ...
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Is a double-yolk egg capable of producing two viable chicks?

As per title: If fertilized, is a double-yolk egg capable of producing two viable chicks (that could survive to adulthood)? Have there been any reported cases of such (presumably) fraternal twins?
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Identical twins' genders

Can identical twins have different genders (as in biological sex)? I thought they should have absolutely the same genes. If they can, what caused it? I'd assume it MAY be because of errors in ...
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Which disorders are fully concordant?

I work in neuroscience, mostly Alzheimer's disease (AD), with some work in autism spectrum disorder (ASD). The work is in gene regulation and epigenomics. I'm familiar with monozygotic twin (MZ) ...
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Is there a limit to twinning?

Can developing twins further divide and subdivide and become identical quadruplets or octuplets or...? Can twinning be induced, either in utero or in vitro? Might it be possible to just keep ...
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When does identical twinning happen?

What is the window during which division into twins is possible (for instance can it still happen after implantation in the uterus)? Is one twin the "original" and the other its clone?
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Are identical twins exactly the same?

http://www.scientificamerican.com/article.cfm?id=identical-twins-genes-are-not-identical According to this article some identical twins show differences with respect to their copy number variants. ...