Each major Y-DNA haplogroup has many, many subgroups within it, each defined by a different Y-DNA SNP (pronounced snip). In most cases, members of these major haplogroups tend to have similar sets of STR values, called STR signatures. This means that the STR signature for an individual can reliably predict their major haplogroup. However, this prediction is very general and corresponds reliably only to an ancient haplogroup, not a more refined subgroup. In order to confirm to which subgroup a person belongs, SNP testing is required.
In other words, STR testing can provide a predicted haplogroup, but only SNP testing can provide a confirmed haplogroup.
Haplotype vs. Haplogroup
You may have heard of the terms haplotype and haplogroup, and while these two terms sound very similar, they have different meanings. Understanding the distinctions between them is crucial to understanding Y-DNA as it relates to genetic genealogy.
You can read more about the distinction between the two here, but to explain it briefly, a haplogroup is a haplotype shared by multiple people. Haplotypes are unique sets of genetic markers that can vary in size from the entire sequence of DNA (genome) all the way down to a single nucleotide. At the genome level, each person has a unique haplotype unlike anyone else.
When a male patriarch migrates away from a group of people, they take their own unique haplotype with them as well. Y-DNA remains relatively unchanged from one generation to the next, so the Y-DNA haplotypes of the male descendants will be very similar (if not identical) to the Y-DNA haplotype of the patriarch. Haplotypes that are shared by more than one person are called haplogroups.
As generations go by, SNPs and STRs accumulate. When these mutations are passed down to subsequent generations, they begin to form their own unique subgroups. These subgroups all contain portions of the original haplogroup; however, they are all considered to be subgroups of the same ancestral haplogroup.
Predicted vs. Confirmed Haplogroup
Each haplogroup is defined by a specific SNP, so only SNP testing such as the Big Y-700 or a SNP pack can provide a confirmed haplogroup. However, FamilyTreeDNA has specified a 12 STR signature range for each haplogroup. Because of the wide variation of STR values found within a haplogroup, the STR range refers to an ancestral haplogroup, meaning one that was founded thousands of years ago in most cases. Based on their 12 marker signature, we can predict that they too will share the SNP that defines that ancestral haplogroup.
In some cases, we are unable to accurately predict a haplogroup if these STR marker values do not fall within a typical range. In cases such as these, FamilyTreeDNA runs a complementary test for defining SNPs called a backbone test. This collection of SNPs, called a panel, includes the defining SNP for each ancestral haplogroup in order to accurately provide an ancestral haplogroup for customers.