Kin selection sometimes only requires one niece or nephew gained for every offspring lost, with implications for the heritability of male homosexuality

Abstract

Male homosexuality, if genetically determined, presents a conundrum for natural selection. Male homosexuality is considered likely to reduce the number of children the male homosexual has. It has been suggested that genetically inherited male homosexuality passes on by kin selection, by creating a Gain in the number of Nieces and Nephews (GNN) to the male homosexual which offsets his Loss in number of Direct Offspring (LDO). However with the assumption of genetic inheritance of the trait and classical kin selection theory, the criterion for propagation of such an allele is that there are two nieces or nephews gained for every direct offspring lost, or GNN > 2LDO. This factor of 2 presents a high hurdle. However, I postulate the existence of what I shall call a Mother Offspring Modification allele or a MOM allele, defined as an allele which has the phenotypic effect that it makes a mother who is carrying it modify some of her children developmentally, in some specific way under certain conditions, but, importantly, independently of whether that child carries the allele. A powerful possible channel for such a modifying influence is in utero, by, for example, secretion of hormones which affect fetal development. A MOM allele has two distinct traits, one is the Offspring Trait (such as the trait of homosexuality) and the other is the Mother Trait which is the trait of making the offspring carry the Offspring Trait. When a MOM allele offspring trait expresses in a child, importantly, the child has only a 50% chance of carrying the allele. The offspring trait is therefore not genetically inherited in the usually understood sense that the carrier of the trait carries the allele. It may potentially be less confusing to say the trait is MOM-inherited rather than inherited. All of the mother’s grandchildren are equally likely, with a probability of one quarter, to carry a copy of the MOM allele, regardless of whether those grandchildren’s parents have the offspring trait. A successful propagation strategy for a MOM allele is therefore to merely indiscriminately increase the number of grandchildren overall to the mother who carries it. If a MOM allele in a mother makes her son homosexual, then there is only a 50% chance that the homosexual son carries the allele, and a probability of one quarter that his offspring carry it, and this figure is the same for his nephews and nieces, and the criterion for its propagation is only GNN > LDO, a much lower hurdle than the GNN > 2LDO of the regular kin selection theory. There is already considerable evidential support for the theory that male homosexuality is commonly caused in utero, providing evidence that male homosexuality is propagated by a MOM selection mechanism rather than classical inheritance and classical kin selection theory. It is speculated that the male child of a sister of a male homosexual may be somewhat more likely to be homosexual than will the male child of a brother of a male homosexual, or even than the male child of a male homosexual himself, because of MOM allele effects. The concept of a MOM allele can be generalized to both fathers and mothers and called a Parent Offspring Modification (POM) allele, in which an allele in a parent codes a trait called the parent trait in the parent which makes the parent developmentally modify a child towards having the offspring trait. In the case of a POM allele in which the parent trait is active in a father, the modification of the child by the father towards the offspring trait would have to be due to the father’s behavior as it affects the child and is somewhat more limited in its scope as a mechanism than a MOM mechanism. It is speculated that various MOM or POM alleles may produce effects other than homosexuality, with both parent trait and offspring trait being found in both sexes.