How many homologous pairs do humans have and what is the exception?
Humans has 22 homologous pairs besides the 23 pairs which can be different.  The 23rd paird codes for gender and does not need to match.  The 21st pair is the pair that sometimes has a third which causes down syndrome.
Instruction: How many homologous pairs do humans have and what is the exception?
Context: 
Response: Humans has 22 homologous pairs besides the 23 pairs which can be different.  The 23rd paird codes for gender and does not need to match.  The 21st pair is the pair that sometimes has a third which causes down syndrome.