親族間のSTRとSNPの一致率

Relation	STR consistency rate (%)	SNP concordance rate (%)
parent and child	50%	50%
siblings	25-50%	50%
cousin (male)	12.5%	12.5%

STR is effective in identifying parent-child and sibling relationships, with match rates of 50% for parents and children, 25-50% for siblings and around 12.5% for cousins.

SNPs are suitable for extensive kinship and ancestry research, with 50% matches between parents and children, 50% between siblings and 12.5% between cousins.

Both methods are effective in confirming blood relations, but the appropriate method is chosen according to the purpose.

The reason why the Short Tandem Repeat (STR) match between siblings is not necessarily 50%, but falls in the range of 25% to 50%, is because the genes are passed on randomly from both parents. This is explained in more detail below.

On the inheritance of STRs.

As STR patterns are randomly selected from the DNA of the parents, the proportion of DNA shared between siblings is variable. Specifically, match rates may range from 25% to 50% for the following reasons

Randomness of inheritance from parents:

Each parent has two sets of genes (inherited from the mother and father) and it is random which genes the child will inherit. There is no certainty that siblings will inherit the same genes.
Siblings may both inherit the same version of their father’s genes, or they may inherit different versions.

Variant combinations:

If siblings inherit the same STR markers from the same parent, the match rate is 50%.However, they may inherit different versions of the STR from one parent, resulting in some markers being different, which may reduce the match rate to around 25%.

Differences between identical and fraternal twins:

Identical twins have exactly the same DNA, so the STR concordance rate is 100%.Dizygotic twins and normal siblings, on the other hand, share 50% of their DNA, but the match rate fluctuates between 25% and 50% because the gene combinations are chosen at random.

In the case of SNPs (Single Nucleotide Polymorphism), the reason why the match rate between parents and children and between siblings is roughly 50% is that SNPs deal with variations in a single base (a component of DNA) and the overall genetic pattern is more linearly This is because the overall pattern of genes is transmitted more linearly from parent to child.

Based on the differences between SNPs and STRs, the following explains why SNPs have a 50% match rate between siblings.

1. Characteristics of SNPs

SNPs are a method of testing for points in DNA where one base (A, T, C or G) is replaced by another (polymorphism). These SNPs are a huge number in the overall genetic information, but are inherited in a relatively simple form as part of the genetic information passed from parent to child.

For example, one of the father’s SNPs and one of the mother’s SNPs is passed on from parent to child .

2. How SNPs are transmitted between siblings

SNPs passed from parents to their children are randomly inherited as part of the gene 50% of the time.

Between siblings, 50% of SNPs match because they inherit 50% of their DNA from each of their parents. Siblings could theoretically have 50% of the total in common because they share common parents.

However, due to the random nature of SNPs, certain SNPs are more likely to be shared between siblings, resulting in an almost 50% match. Genes are passed on extensively through SNPs, which are fine units, with an almost 50% match between parents and children and between siblings.

3. Why SNPs are 50% (vs. STRs)

Inheritance of SNPs is simple because each SNP is an independent mutation point. This means that individual SNPs are randomly inherited from either parent, which is often a 50% match between siblings.

STRs, on the other hand, are based on the length of repeated sequences and are therefore more susceptible to random combinations, which can lead to variations in match rates between siblings.

Conclusion

SNPs deal with genetic variation at the level of a single base, and the reason for the 50% match between parents and children and between siblings is that 50% of genes are inherited from each parent to their children; SNPs are inherited in a simple pattern, which is why the match between siblings stabilizes at around 50%.

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