Complex sentence: The R1a1 is the most common subclade within the family of the Y-DNA Haplogroup R1a, which share in common the M420 SNP mutation, and before the discovery of M420, R1a1 was itself referred to as R1a.
Split into simpler sentences: The R1a1 is the most common subclade within the family of the Y-DNA Haplogroup R1a. R1a shares in common the M420 SNP mutation, and before the discovery of M420, R1a1 was itself referred to as R1a.