1. Y-chromosome:
The Y chromosome, found only in males, is composed of 58 million base pairs and contains 83 genes which code for only 23 proteins. Most Y chromosome tests examine between 12 and 67 STR markers. Each STR has a numerical value. DYS392, for example, has a value of 11, 12, 13, 14, 15, of 16, with 13 being the most common. The numerical values for all the STRs tested can then be compared to the thousands of other numerical sets available in DNA databases. If two people have identical numerical values, it is more likely that they have a recent common ancestor.

2. X-chromosome:
The X chromosome, found in both males and females, is more than 153 million base pairs and contains roughly 1000 genes. The use of X chromosomes to study genealogical relationships is still relatively new. The X chromosome, just like the Y, contains STRs, called X-STRs. The problem with studying X-STRs is that the entire X chromosome undergoes recombination during meiosis. In other words, in females the two X chromosomes randomly swap information and genes. Some tests use "haplotype blocks", or regions of X-STRs that are inherited intact over several generations.

3. mtDNA:
Mitochondrial DNA (mtDNA) is a small circle of DNA located inside a small organelle found inside our cells, the mitochondria. mtDNA is only 16,569 base pairs long and contains just 37 genes. Unlike nuclear DNA, mtDNA is only passed on from mother to child; although males inherit mtDNA from their mothers, they do not pass it on to their children. This unique feature of mtDNA allows it to be used for tracing matrilineage, the inheritance of mtDNA from mother to child. When mtDNA is tested for genealogical purposes, a region of the DNA is sequenced for SNP mutations. The results of the mtDNA test are most often used to determine a person's mtDNA haplogroup and haplotype.

4. Autosomal:
Autosomal DNA is the 22 pairs of non-sex chromosomes found within the nucleus of every cell. Autosomal DNA tests examine SNPs, or alleles, located throughout all of the DNA. One of the most popular uses of autosomal DNA testing is to determine an individual's ethnic heritage. According to recent research, ethnic groups can contain distinctive alleles that are different from all other ethnic groups. The presence of that allele in an individual's DNA suggests that they are descended from that ethnic group.

mtDNA Databases
Mitomap
Mitosearch
The mtDNA Population Database
mtDNA Test Results Log


Y-Chromosome Databases
SMGF
Ybase
YHRD
Ysearch
Y-STR Database