"semi-identical" twins (sesquizygotic)

[jim mcnamara]

A set of twins was born in Queensland Australia that is almost identical. This was not supposed to happen.
Identical twins arise from a single fertilized egg, derived from fertilization by a single sperm cell, that divides into two separate but genetically identical fertilized eggs. After this the two cells go their merry way on toward creating two genetically identical babies. In the Queensland case, the children share 100% of maternal DNA (as expected), but only 78% of paternal DNA. This means that the fertilized egg most likely got its DNA from two separate sperm cells.

https://medicalxpress.com/news/2019-02-semi-identical-twins-pregnancy.html

This may also be thought of as a kind of chimera: one egg, two sets of male DNA:
https://en.wikipedia.org/wiki/Chimera_(genetics)

 

[Ryan_m_b]

This was not supposed to happen.

In my experience with biology, and particularly genetics and development, human inclinations towards clear categories are constantly smashed apart.

 

[jim mcnamara]

I think it is more of a matter of sample size and detection. If you observe something 100K times with the same result, using constant methods, people tend to ossify the concept.

Then new technology comes along expanding the available sample size as well as providing previously unavailable methods, voila! something new shows up. Blood types are good example. At the other end of the spectrum might be exosomes. Not detected and reported until ~2012.