Attraction, repulsion, and the red queen of genes

At the level of individual organisms, mutual attraction, mutual repulsion and the type of "one-sided" attraction - where A is attracted to B but B is repelled from A - are familiar phenomena.

This post is about these kinds of attraction and repulsion between pairs of genes at a genetic level.

• Mutual attraction: it has long been appreciated by geneticists that interdependent traits have some tendency to cluster together on chromosomes. The reason for this seems fairly obvious. If:
  • Two traits are functionally interdependent;
  • They are coded for by separate genes;
  • The genes are on the same chromosome;
  • The associated organism is sexual;
  • Neither gene is close to fixation;
  ...then mutations that reduce the distance between them - and so increase their linkage - will be selectively favoured. This reduces the chance of the genes being divided during meiosis.

• Mutual repulsion: this is likely to happen when two genes benefit from not being linked together. This might happen if the genes code for traits involved in disease resistance - for example. There are cases where you want your offspring to have a different genotype from you - in order to avoid parasites traveling from parent to child. If you have two genes that code for a blood-group trait, you probably want to give your offspring different blood group from yourself - and minimizing linkage does that. Repulsion is the opposite of attraction.

• Mutual indifference: this is a "dustbin" category, representing the absence of attraction and repulsion.

Attraction, repulsion and indifference need not be mutual. In parasite-host relationships the parasite is attracted to the host, but the host expressed repulsion towards the parasite. The same dynamics apply to genes - in the case of selfish genetic elements. These are like parasites that are part of their host's genome. They will be attracted to - and seek linkage with - useful host genes. However the useful host genes would prefer not to be linked to the selfish genes. This produces red-queen like dynamics at the genetic level - where parasitic selfish genes chase host genes around.

Motion histories can't be used to conclusively infer attraction. For example, if A migrates towards B, it may not be because A and B are attracted, but rather because both are attracted to C.

These kinds of effect also apply to cultural phenomena. For example, hammer memes tend to be attracted to nail memes - whereas Catholic and Islamic memes tend to repel each other.