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In theory each SNP can be one of the four nitrogenous bases (A, C, G, or T), but in practice only two are ever found at each specific spot the vast majority of the time. There is usually a major alle and a minor alle that is present in at least 5% of test takers. Each person will have two nitrogenous bases 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: two copies of the major alle (called a homozygous SNP), one copy of the major alle and one copy of the minor alle (called a heterozygous SNP), and two copies of the minor alle (called a homozygous SNP). When two people have at least once matching SNP at each spot, it is a half match, if both SNPs match it is a full match, and if neither SNP matches it is no match. Since there are only three possible combinations at each spot, many people will be either a full or half match at any given 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-thousands of SNPs in a row match to be confident that a matching segment is identical by descent and not just a coincidence. | In theory each SNP can be one of the four nitrogenous bases (A, C, G, or T), but in practice only two are ever found at each specific spot the vast majority of the time. There is usually a major alle and a minor alle that is present in at least 5% of test takers. Each person will have two nitrogenous bases 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: two copies of the major alle (called a homozygous SNP), one copy of the major alle and one copy of the minor alle (called a heterozygous SNP), and two copies of the minor alle (called a homozygous SNP). When two people have at least once matching SNP at each spot, it is a half match, if both SNPs match it is a full match, and if neither SNP matches it is no match. Since there are only three possible combinations at each spot, many people will be either a full or half match at any given 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-thousands of SNPs in a row match to be confident that a matching segment is identical by descent and not just a coincidence. | ||
If you download your DNA raw data file and open it, you can see exactly what genes you have at each of the tested SNPs. The order they are listed is arbitrary and it is impossible to know which gene came from each parent. If both genes are the same (A A for example) then one A came from mom and one from dad. If your DNA is heterozygous at a certain SNP (C G for example), the only way to know which parent gave you the C and which the G is by comparing against other relatives. Sorting out the paternal and maternal SNPS is called phasing. In this situation, if you compared your DNA against your mom's and at the spot where you have C G, your mom has G G then you must have inherited the C from your dad and the G from your mom. If you are C G and your mom is also C G, then it is still unclear which gene came from which parent and comparing against your dad or another relative would be necessary to figure it out. Programs such as GedMatch.com offer the ability to phase your DNA by comparing it against one or both parents. Using phased kits reduces the amount of false segments identified between you and a match and is a valuable tool for people interested in small DNA segments. | If you download your DNA raw data file and open it, you can see exactly what genes you have at each of the tested SNPs. The order they are listed is arbitrary and it is impossible to know which gene came from each parent. If both genes are the same (A A for example) then one A came from mom and one from dad. If your DNA is heterozygous at a certain SNP (C G for example), the only way to know which parent gave you the C and which the G is by comparing against other relatives. Sorting out the paternal and maternal SNPS is called phasing. In this situation, if you compared your DNA against your mom's and at the spot where you have C G, your mom has G G then you must have inherited the C from your dad and the G from your mom. If you are C G and your mom is also C G, then it is still unclear which gene came from which parent and comparing against your dad or another relative would be necessary to figure it out. Programs such as GedMatch.com offer the ability to phase your DNA by comparing it against one or both parents. Using phased kits reduces the amount of false segments identified between you and a match and is a valuable tool for people interested in small DNA segments. However, in cases where you and the parent being compared against are both heterozygous (like C G) the value becomes a no call and is discarded from the comparison. For this reason, comparing your DNA against both parents creates better results than just comparing against one. Perhaps in the spot where you and your mom are both C G, your father is C C, now it can be concluded you inherited the C from your father and the G from your mother. | ||
== Autosomal DNA == | == Autosomal DNA == | ||