Marry your Sister: Outbreeding Depression in Penna Ageing Model

If in the sexual Penna ageing model conditions are applied leading to complementary bit-strings, then marriages between brothers and sisters, or between close cousins, may lead to more offspring than for unrelated couples. Inbreeding is usually a bad thing for humans and many animals; the Egyp-tian pharaos liked marriages between brothers and sisters which led to the city named Philadelphia but did not prevent conquest by the Romans two millennia ago. Also in the Penna ageing model [1], an often used computer simulation model for biological ageing, such inbreeding depression has been found [2]: Too small populations die out because their genomes become too similar, activating many deleterious mutations which in a larger population with more genomic diversity would remain recessive and would not affect the health of the individual. However, outbreeding depression is also possible [3]: If the genomes are too different, survival becomes difficult. For example, donkey and horses cannot have grand children, and the senior author is the only known homo troglodytes and has no known children [4]. This effect may be connected with the possibility that the two haploid genomes in a species with sexual reproduction are complementary to each other [5], where with few exceptions for each gene one haplotype has one allele (" wild type ") and the other has the opposite allele (" mutant ") and where most mutations are recessive. Thus, even though about half of the genes are mutated, the phenotype is barely affected. For real humans instead of computer simulations, Helgason et al. [6] checked for all known marriages in Iceland 1800-1965 whether they were cousins and how many offspring they had. They found the greatest reproductive success, measured in the number of grand children, for third and fourth cousins. We now check for similar effects by simulating the Penna ageing model. Each individual in the Penna model [1] has a genome of two bit-strings of length L each; at age a only the first a bit positions are active. If the two alleles on one position are 00, 01, or 10, they do not affect the health. If one locus at position a is 11 instead, then the individual dies at age a. For ages above or at a minimum reproduction age R = 5L/8, each surviving female at each iteration randomly tries to find a male of reproductive age and then has B children with him; the two …