Giants AFTER the flood - how ?

Gen 6
4 There were giants in the earth in those days; ..
Except the ark, the FLOOD killed everything that breathed air.
17...., to destroy all flesh, wherein is the breath of life, ...
Noah had uncorrupted DNA
9.....perfect in his generations,
Logic dictates that Noah's wife & 3 sons were the same since Christ came from the line of Shem.
Shem's wife would also have had uncorrupted DNA.
Numbers 13:33
33And there we saw the giants, ....we were in our own sight as grasshoppers, and so we were in their sight.
So how did giants return after the flood ?
The wife of either Ham or Japeth had the Nephilim gene/mutation ???