The following information is from the Wikipedia article on Haplogroup G2a3b1 (P303):
Haplogroup G2a3b1 (P303) is a Y-chromosome haplogroup. It is a branch of haplogroup G (Y-DNA) (M201). In descending order, G2a3b1 is additionally a branch of G2 (P287), G2a (P15), G2a3 (L30 or S126) and finally G2a3b (L141). This haplogroup represents the majority of haplogroup G men in most areas of Europe west of Russia and the Black Sea. To the east, G2a3b1-except in the Caucasus Mountains area-is just a large or small minority among G persons in such locales as Turkey, the Middle East, Iran, the southern Caucasus area, China and India.
All G2a3b1 men carry the P303 or S135 SNP Y-DNA mutation. There are also some short tandem repeat (STR) findings among G2a3b1 men which help in subgrouping them. Many of the men have an unusual value of 13 for marker DYS388, and some have 9 at DYS568. STR marker oddities are often different in each G2a3b1 subgroup, and characteristic marker values can vary by subgroup. Often the values of STR markers DYS391, DYS392 and DYS393, however, are respectively 10, 11 and 14 or some slight variation on these for all G2a3b1 men.
P303 first became available for public testing in fall 2008, and the P designation indicates it was identified at the University of Arizona. The mutation is found on the Y chromosome at position 20104736. The forward primer is TTCTTATTTGCTTTGAAACTCAG. The reverse primer is ATTGGCTTATCAGATTGACG. The mutation involves replacement of T by C. This mutation was actually first identified as S135 at Ethnoancestry in London, England, but it took some time to realize that P303, which was independently identified, and S135 were the same.
Research studies have not yet dated the origin of G2a3b1. Based on the number of mutations seen in 67-marker STR values, this P303 mutation perhaps occurred about 5,000-10,000 years ago, and the major subgroups developed their mutations several millennia later. The spread to Europe in some subgroups seems to have occurred primarily in the period of 2,500 to 1,500 years ago based on comparisons of samples from Europe west of the Black Sea to samples from more easterly locales. Several subgroups may have originated within Europe.
The distribution information here is based on the large collection of very likely or proven G2a3b1 samples in the Haplogroup G Project derived from multiple sources.
G2a3b1 definable subgroups are heavily concentrated in Europe west of Russia and the Black Sea, but small numbers are also found in a geographical belt extending from northern Africa to northeastern Asia.
G2a3b1 seems to represent the same percentage of the population in both central and southern Europe and usually represents half or more of the G seen in the population in these areas.
Isolated G2a3b1 samples are found also in North Africa (Tunisia, Libya and Egypt), in the Middle East (Saudi Arabia and Dubai), the Caucasus Mountains area (Armenia, Georgia, Azerbaijan, N. Ossetia in Russia, among Russian Kabardinians, Abazinia in Georgia), in Iran, in Uzbekistan and among the ethnic groups of northwesternChina and Russian Siberia. A distinctive Indian type of G2a3b1 exists, but its prevalence is unclear. And an isolated G2a3b1 sample from Malaysia exists.