A   M91, P97
•     A*   -
•     A1   P108
�    �     A1*   -
�    �     A1a   M31, P82
�    �     A1b   P114, P305
•     A2   M6, M14, M23, M29/P3/PN3, M49/Page41, M71, M135, M141, M196, M206, M212, M276/P247,
               M277/P248, MEH1, P4, P5, P36.1, Page52, Page71, Page87
�    �     A2*   -
�    �     A2a   M114
�    �     A2b   P28
�    �     A2c   P262
•     A3   M32
�    �     A3*   -
�    �     A3a   M28, M59
�    �     A3b   M144, M190/Page35, M220, P289, Page50
�    �     �     A3b*   -
�    �     �     A3b1   M51/Page42, P100, P291
�    �     �    �     A3b1*   -
�    �     �    �     A3b1a   P71, P102
�    �     �     A3b2   M13, M127, M202, M219, M305/Page17, Page53, Page77/V10
�    �     �    �     A3b2*   -
�    �     �    �     A3b2a   M171
�    �     �    �     A3b2b   M118

NOTES:

• Identical SNPs that were discovered separately are listed in alphabetical order, not necessarily in the order of discovery, and separated by "/". Examples: M29/P3/PN3, M276/P247 and M277/P248.
• PK1 (formerly A2) and M63 (formerly A3b2) were removed from the tree due to the uncertainty of their placement.

Y-DNA haplogroup A represents the oldest branching of the human Y chromosome tree, thought to have begun about 60,000 years ago. Like Y-DNA haplogroup B, the A lineage is seen only in Africa and is scattered widely, but thinly across the continent. These haplogroups have higher frequencies among hunter-gather groups in Ethiopia and Sudan, and are also seen among click language-speaking populations. Their patchy, widespread distribution may mean that these haplogroups are remnants of ancient lineages that once had a much wider range but have been largely displaced by more recent population events.

The most commonly seen sub-groups of haplogroup A are A2 (A-M6), A3b1 (A-M51), and A3b2 (A-M13). Sub-groups A2 and A3b1 are seen in South Africa, with A3b1 seen exclusively among the Khoisan. The range of A3b2 is restricted to Eastern Africa and at lower frequencies among Cameroonians. About 1.1% of African-Americans belong to the sub-group A3b2.

References:

Cinnioglu et al, Excavating Y-chromosome Haplotype Strata in Anatolia. (pdf) Human Genetics. 114:127-148, 2004.
Cruciani et al, A Back Migration from Asia to Sub-Saharan Africa Is Supported by High-Resolution Analysis of Human Y-Chromosome Haplotypes. American Journal of Human Genetics, 70:1197-1214, 2002.
Cruciani et al, A Revised Root for the Human Y Chromosomal Phylogenetic Tree--The Origin of Patrilineal Diversity in Africa. The American Journal of Human Genetics, doi:10.1016/j.ajhg.2011.05.002, 2011.
Deng et al, Evolution and Migration History of the Chinese Population Inferred from the Chinese Y-chromosome Evidence. (pdf) Journal of Human Genetics, 49:339-348, 2004.
Karafet et al, New Binary Polymorphisms Reshape and Increase Resolution of the Human Y-Chromosomal Haplogroup Tree. Abstract. Genome Research, published online April 2, 2008. Supplementary Material.
Mohyuddin et al, Detection of Novel Y SNPs Provides Further Insights into Y Chromosomal Variation in Pakistan. Journal of Human Genetics, 2006.
Rozen et al, Remarkably Little Variation in Proteins Encoded by the Y Chromosome's Single-Copy Genes, Implying Effective Purifying Selection. American Journal of Human Genetics. 2009 December 11; 85(6): 923-928.
Semino et al, Ethiopians and Khoisan Share the Deepest Clades of the Human Y-Chromosome Phylogeny. (pdf) American Journal of Human Genetics, 70:265-268, 2002.
Shen et al, Reconstruction of Patrilineages and Matrilineages of Samaritans and other Israeli Populations from Y-Chromosome and Mitochondrial DNA Sequence Variation. (pdf) Human Mutation, 24:248-260, 2004.
Valone et al, Y SNP Typing of African-American and Caucasian Samples Using Allele-Specific Hybridization and Primer Extension. (pdf) Journal of Forensic Science, 49:4, July 2004.

Additional Resources:
ISOGG Wiki - What you need to know about Genetic Genealogy.
Y Haplogroup A, Bonnie Schrack.
The African DNA Project (A), Dr. Ana Oquendo Pabon.

Corrections/Additions made since 1 January 2011:

• Added P305, Page50, Page52, Page53, Page 71, Page77, Page87 on 11 March 2011.
• Added haplogroup projects to Additional Resources on 22 March 2011.
• Added Rozen et al (2009) on 24 March 2011.
• Added Page17, Page35, Page41, Page42 on 30 March 2011.
• Changed in Additional Resources, admin for Y Haplogroup A from Vince Vizacherro to Bonnie Schrack on 27 May 2011.
• Added Cruciani et al (2011) on 27 May 2011.
• Added V10 on 7 June 2011.
• Added Bonnie Schrack as a Contact Person on 23 October 2011.

Contact People for Haplogroup A: Ana Oquendo Pabon and Bonnie Schrack.

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