A study done at Newcastle University by Corry Gellatly studied 927 family trees containing 556,387 people from North America and Europe. The studied showed that men are more likely to have daughters if they have more sisters and more likely to have sons if they have more brothers. The study also suggests that there is an undiscovered gene that controls whether a mans sperm contains more X or Y chromosomes, which would affect the sex of his children.
Gellatly demonstrated that men carry 2 different types of alleles that result in three possible combinations in a gene that controls the ratio of X and Y in sperm. The three possibilities is mm, which produces more Y sperm so have produce sons, mf, which produces an equal number of X and Y sperm and produce an equal number of sons and daughters, and ff, which produces more x sperm so produce more daughters. This gene is passed on from both parents so, this explains why this is roughly an equal amount of men and women throughout the population. If there are to many men, females can find a mate easier so men with more daughters will pass on more of their genes, causing more females to be produced in later generations. This gene could also explain why there is an increase of boys born after wars and an increase of boys in the population in general because more males die in childhood.
This study is a good way of understating the ratio of males and females in our populations. It can also be used to understand how mass events such as war effects the ratios in our population as well, which could help populations overcome drops in either females or males. It could also be helpful in genetic consulting because if a couple if trying to have either a boy or a girl they can look at their family pedigrees and determine if they are more likely to have a boy or a girl.