Complex sentence: A SNP in which both alleles produce the same polypeptide sequence is called a "synonymous polymorphism" (sometimes called a silent mutation)--if a different polypeptide sequence is produced the polymorphism is a "replacement polymorphism".
Split into simpler sentences: A SNP in which both alleles produce the same polypeptide sequence is called a "synonymous polymorphism" (sometimes called a silent mutation). If a different polypeptide sequence is produced the polymorphism is a "replacement polymorphism".