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Single-Nucleotide Polymorphisms (SNPs) (view source)
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== Single-Nucleotide Polymorphisms (SNPs) == | == Single-Nucleotide Polymorphisms (SNPs) == | ||
Of the 3.2 billion base pairs we all have 90% are identical in all humans. Of the four nitrogenous bases (A, C, G, or T) each of those base pairs are homozygous meaning they can only have one of the four possible alleles. They simply make us human. The places where it is possible for a variance to occur are called SNPs which stands for Single-nucleotide polymorphisms. SNPs are the main force behind DNA and what gives it it's genealogical value. When two individuals have enough matching SNPs in a row, this becomes a matching segment. The more matching SNPs there are, the bigger the segment is. If a segment is big enough (morethan 15cm), then the segment must be identical by descend (IBD) which means the two individuals share that segment because they both descend from a common ancestor who passed on that segment of DNA to both of them. The more matching segments there and the bigger they are, the closer two test takers are probably related. By testing a sample of a person's | Of the 3.2 billion base pairs we all have 90% are identical in all humans. Of the four nitrogenous bases (A, C, G, or T) each of those base pairs are homozygous meaning they can only have one of the four possible alleles. They simply make us human. The places where it is possible for a variance to occur are called SNPs which stands for Single-nucleotide polymorphisms. SNPs are the main force behind DNA and what gives it it's genealogical value. When two individuals have enough matching SNPs in a row, this becomes a matching segment. The more matching SNPs there are, the bigger the segment is. If a segment is big enough (morethan 15cm), then the segment must be identical by descend (IBD) which means the two individuals share that segment because they both descend from a common ancestor who passed on that segment of DNA to both of them. The more matching segments there and the bigger they are, the closer two test takers are probably related. By testing a sample of a person's SNPs and then comparing them to everyone else in the database, it is possible to identify a person's genetic relatives. Most major companies will test 500-700k SNPs. | ||
In practice only two alleles are ever found at any one SNP the vast majority of the time. The allele that is most common is called the major allele the less common is called the minor allele. In autosomal DNA, each person will have two alleles at each spot, one inherited from their mother and one inherited from their father. This means that at each SNP tested a person can have one of three combinations: <br> <br> | In practice only two alleles are ever found at any one SNP the vast majority of the time. The allele that is most common is called the major allele the less common is called the minor allele. In autosomal DNA, each person will have two alleles at each spot, one inherited from their mother and one inherited from their father. This means that at each SNP tested a person can have one of three combinations: <br> <br> | ||
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3. One copy of the major allele and one copy of the minor allele (heterozygous) <br> <br> | 3. One copy of the major allele and one copy of the minor allele (heterozygous) <br> <br> | ||
When two people have at least once matching allele at a SNP, it is a half match, if both SNPs match it is a full match, and if neither SNP matches it is | When two people have at least once matching allele at a SNP, it is a half match, if both SNPs match it is a full match, and if neither SNP matches it is a non match. Since there are only three possible combinations at each spot, most people will be either a full or half match at any one SNP by coincidence even though they are not related. In fact, somebody who is heterozygous will match everybody on earth at that spot. This is why it is important that hundreds or thousands of SNPs in a row match to be confident that a matching segment is truly identical by descent. | ||
== Phasing == | == Phasing == | ||