Lesson 3: DNA's Role in Determining Your Traits
When a particular gene occurs in slightly different forms (sequences) in some people, the gene is said to be polymorphic. Most gene polymorphisms do not produce new traits, although some of them do. For example, the gene that encodes the ABO blood type trait is polymorphic; it has several SNPs and over 180 sequence variants. But all variants can be classified into the 3 groups, A, B, and O. In this example, there are 3 blood group traits. The trait is determined by a single gene that has SNPs. The SNP variants are classified into 3 different classes of alleles. An allele is a variant form of a gene that produces variation in a trait. Most human genes have a single known allele; the traits they produce appear the same in everyone.
SNPs = Polymorphisms
Genes have many polymorphisms but few alleles.