From the point of view of a maternally derived gene in Haldane, the three half-brothers are all offspring of his mother, so his maternally derived genes have a probability of one-half being present in each half-brother. For the sacrifice of one copy of the gene in himself, Haldane would be rescuing one and a half copies, on average, of his maternally derived genes. Natural selection acting in that situation on genes of maternal origin would favor the sacrificial behavior.

However, things look very different from the point of view of Haldane's paternal genes. Those three half-brothers are the offspring of different fathers, making them complete non-relatives. If genetic accounting were all that was important, no sacrifice, no matter how small, would justify any benefit, no matter how great, to his paternal half-sibs. Therefore, in this case, selection on paternally derived genes would prevent Haldane performing this sacrificial action.

This illustrates that different selective forces can act on different genes within an individual, pulling him in different directions, resulting in internal genetic conflicts. I suspect that how these conflicts are resolved is a matter of history, genetic politics, and knowing the details of the system. To answer questions like these, a lot of insight is going to come from the social sciences. Political science in particular is all about dealing with conflicts of interest within society with the formations of parties and factions, and I believe that if there are conflicts within the individual, you'll have a similar sort of internal politics.

I'm particularly interested in looking at situations in the real world where the Haldane story I just gave would apply—where there are potential conflicting selective forces acting within the individual. So far I've talked about conflicts between genes of maternal and paternal origin, but there are also possible conflicts between genes sitting on the sex chromosomes and genes sitting on the other chromosomes, or between genes sitting in the nucleus and genes sitting in mitochondria, or between our genetic inheritance and cultural transmission. I'm trying to develop a set of theories and tools for dealing with such situations.

Genomic imprinting is a fascinating phenomenon, and raises an interesting question: If information about the sex of the parent in the previous generation can be transmitted by such mechanisms, is there other historical information input from the environment that can be transmitted to the current generation and influence genetic expression? Would it be possible that if my great-grandmother experienced a famine or lived in a time of war, that this has put an imprint on the genome which is influencing gene expression in my own body?

My interest in genetic imprinting began while I was completing my doctorate at Macquarie University in Sydney. I began studying plant ecology and, in particular, how regeneration after fire takes place. I wandered around the bush a bit looking at plants, but my heart really wasn't in that. Through good fortune I got an opportunity to do a theoretical study on the evolution of the life cycles of plants, applying kin selection theory—the theory of parent-offspring conflict developed by Robert Trivers—to plants. By thinking about what's happening within seeds, I essentially had a theory of genomic imprinting ready to go the moment I heard of the phenomenon.

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