  Mathematicians have always been attracted to the field of genetics. I am
especially interested in the mathematical aspects of research on homosexuality.
Certain studies show that male homosexuality may have a genetic component that
is correlated with female fertility. Other studies show the existence of the
fraternal birth order effect, that is the correlation of homosexuality with the
number of older brothers.
  This paper is devoted to the mathematical aspects of how these two phenomena
are interconnected. In particular, I show that the fraternal birth order effect
produces a correlation between homosexuality and maternal fecundity. Vice
versa, I show that the correlation between homosexuality and female fecundity
implies the increase of the probability of the younger brothers being
homosexual.
