Downs syndrome - same family

Hello,

I was reading the other day that Downs may be caused
from ...

brother and sister having sex, then have baby, this baby
could be born with downs, or

cousins, blood cousins marrying or having child together... baby
may be born with downs.

I then was speaking to a friend of mine who is a nurse and she said, yes, if the genes are too similar when having a child this could happen.

Has anyone heard this?

I have an aunt that was born with Downs.
I have never asked my father about this, the above, it
would not go over very well with him, so am asking on here.

thanks,

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