Genealogy does not exist in a vaccum or isolation. There are cultural sociological factors as well as family expectations (maybe these are what are called genes) competing to bring about trends and direction on how people individually or collectively (family or bigger groups) adapt to to an ever changing landscape of social and political change. This is the classic nurture versus nature argument

So for those interested in the slightly bigger picture here are a couple of books to read up on the cultural sociological (i.e. nurture) influences on people in Ceylon/Sri Lanka. Knox's "An historical relation of the Island Ceylon in the East Indies" written in 16th century is a must to get a historical perspective. It has been the translated into Japanese, and inspired some of Daniel Defoes, "Gullivers Travels"

More modern studies include "Pfaffenberger, Bryan, Caste in Tamil culture" and Yalman, Nur Under the bo tree; studies" and are a must to compare and contrast sinhalese and tamil culture (see here for info on Leach and Yalman. Nur Yalman is somewhat heavy reading but "Bryce, Ryan ; L. D. Jayasena and D. C. R. Wickremasinghe. 1955, Sinhalese Village" is easier to read but hard to lay hands on. For a good overall book "Sri Lanka : history and the roots of conflict", edited by Jonathan Spencer is essential. Some of the conditions may no longer be directly applicable, specially to middle class urbanised individuals, but their thinking has been indirectly influenced to some extent by these type of social conditions that prevail or have prevailed in villages. I found Hooles "Exile Returned" a fascinating book on current soical conditions, with lots of footnotes on the historical antecedents of social customs (it seems its absolutely unavailable). It did become somewhat lopsided toward Christian Tamil mentality specially toward the end, but in all, still a good book. It is possible that Daya de Silvas "Memories of Rural Sri Lanka" might be a good complimentary book to compare Sinhalese tradition during the same time (I have not read it).

The more I have thought of the cultural differences between the Sinhalese and the Tamils, the most glaring difference is that of the traditional (and by law, in case of inestate) of inheritance. In the south among the Sinhalese its the eldest son who has the claim to inheritance by custom and by law in the case of intestate (not positive on this). In the North among the Tamils the custom is for the daughters to inherit. The implications are that if a man from Jaffna ever wishes to marry he has to succeed in some form or manner, regardless of his inherited position. 		 
 
Coomaraswamy, Ananda Kentish, 1877-1947, Mediaeval Sinhalese art, 		 
    A monograph on mediaeval Sinhalese arts and crafts, mainly as surviving in the eighteenth century, with an account of the structure of society and the status of the craftsmen. New York Pantheon Books,1956, N7305 .C6 1956

Given this is a genealogy site, info on Sir Muthu Coomaraswamy's grand son Rev. Dr. Rama Poonambalaam Coomaraswamy, M.D., (1929 - 2006), was a cardiac surgeon, then a psychiatrist and later a Traditionalist Catholic priest and exorcist, besides being a prolific writer on Traditionalist Catholicism and Perennialist topics. Rama Coomaraswamy, of mixed Tamil, English and Jewish ancestry, was the son of the famous Indologist Ananda Kentish Coomaraswamy, and of his fourth wife Luisa Runstein, an Argentine-born woman of Jewish descent. He is the grandson of the Tamil-Ceylonese lawyer and social pioneer Sir Muthu Coomaraswamy and his English wife Elizabeth Beeby. As such, Rama Coomaraswamy hailed from notable Tamil and English families. (more info from boston mahesh) 		 
 
Yalman, Nur. Under the Bo tree; Studies in Caste, Kinship, and Marriage in the interior of Ceylon. 		 
    Berkeley, University of California Press, 1967.
I cant emphasize enough the breadth and depth of this work. Do you want to figure out anthropology etc and want to figure it out in a SL context, read this book its an all time classic. Maybe some day this and Pfaffenberger work will be translated into Sinhala and Tamil. Even better would be if the populace can read the originals. 		 
 
de Silva, Daya. Memories of Rural Sri Lanka, 		 
    Published: Sarvodaya Vishvalekha, Ratmalana, Sri Lanka, 1997  
 
Gunasekera, Tamara, Hierarchy and egalitarianism : caste, class, and power in Sinhalese peasant society 		 
    Published: London ; Atlantic Highlands, NJ : Athlone Press, 1994 DS489.25.S5 G86d  
 
Hoole, S. Ratnajeevan H, The Exile Returned: A self-portrait of the Tamil Vellahlahs of Jaffna, Sri lanka. 		 
    Published Aruvi Publishers, Dehiwala, Sri Lanka. 1997  
 
Jayawardena, Kumari, (1995). The white woman's other burden : Western women and South Asia during British colonial rule. N 		 
    New York : Routledge, 1995 HQ1593.5 .J39  
 
Jiggins, Janice. Caste and family in the politics of the Sinhalese, 		 
    Cambridge ; New York : Cambridge University Press, 1978.  
 
Kemper, Steven, The presence of the past : chronicles, politics, and culture in Sinhala life 		 
    Ithaca : Cornell University Press, 1991 Wilder House series in politics, history, and culture DS489.15 .K46 1991  
 
Bryce, Ryan, L. D. Jayasena and D. C. R. Wickremasinghe. 1955, Sinhalese Village, 		 
    University of Miami Press, Coral Gables, Florida  
 
Leach, Edmund Ronald. Pul Eliya, a village in Ceylon; a study of land tenure and kinship. 		 
    Cambridge, University Press, 1961. GN489.1 .L4 1961  
 
Pfaffenberger, Bryan, Caste in Tamil culture : The religious foundations of Sudra domination in Tamil Sri Lanka 		 
    Syracuse, N.Y. : Maxwell School of Citizenship and Public Affairs, Syracuse University, 1982.  
 
Risseeuw, Carla. The fish don't talk about the water : gender transformation, power, and resistance among women in Sri Lanka, 		 
    New York : E.J. Brill, 1988. HQ1735.8 .R57 1988  
 
Ryan, Bryce. Caste in modern Ceylon; the Sinhalese system in transition. 		 
    New Brunswick, Rutgers University Press, 1953. DS489.15 .R9  
 
Singer, Marshall R. The emerging elite; a study of political leadership in Ceylon 		 
    Marshall R. Singer. Cambridge, Mass., M.I.T. Press, c1964 HN690.C4 S5  
 
Skjonsberg, Else. A special caste? : Tamil women of Sri Lanka 		 
    Else Skjonsberg. London : Zed Press, 1982  
 
Spencer, Jonathan, A Sinhala village in a time of trouble : politics and change in rural Sri Lanka 		 
    New Delhi : Oxford University Press, 1990. Oxford University South Asian studies series. DS489.8 .S64 1990  
 
Sri Lanka : history and the roots of conflict, 		 
    edited by Jonathan Spencer. London ; New York : Routledge, 1990.  
 
Gananath Obeyesekere (with Richard Gombrich): Buddhism Transformed: Religious Change in Sri Lanka