Dal sito de "Il Sole 24ore" - 12/12/2008
Maschio o femmina? Quando a decidere sono i geni di papà.
Che i bambini possano nascere maschi o femmine già si sa da cosa dipende: la differenza è nei cromosomi sessuali, che presentano una doppia X nel caso delle femminucce e una X e una Y nel caso dei maschietti. E che a fare la differenza sia quindi l`uomo, che è l`unico a poter trasmettere la Y e quindi a poter determinare la nascita di un maschio, anche questo già è noto. Da una recente ricerca dell`University of Newcastle, guidata da Corry Gellatly e pubblicata sulla rivista Evolutionary Biology, è però emerso anche che alcuni uomini erediterebbero più di altri la tendenza a generare prole più in "azzurro" che in "rosa", e viceversa. In particolare, dalla ricerca è emerso che un uomo con tanti fratelli è portato geneticamente ad avere figli maschi, mentre un uomo con molte sorelle ha più possibilità di avere figlie femmine.
La ricerca ha coinvolto lo studio di 927 alberi genealogici, dal 1600 a oggi, contenente informazioni su 556.387 persone provenienti dal Nord America e Europa. "Lo studio degli alberi genealogici ha dimostrato che la possibilità di avere figli maschi o femmine è ereditaria. Ora sappiamo che gli uomini hanno maggiori probabilità di avere figli maschi se hanno fratelli maschi, ed è più probabile che abbiano delle femmine se hanno più sorelle. Per le donne, però, questa stessa predizione non può essere fatta", spiega Gellatly.
Sono gli uomini a determinare il sesso di un bambino, spiega il ricercatore, a seconda che tramandino la loro X (che combinandosi con la X materna dà vita a una bimba) o la Y (che con la X trasmessa dalla mamma dà vita a un bimbo). Lo studio dell`University of Newcastle, però, suggerisce che un gene, il cui funzionamento è in parte ancora da scoprire, determina la predisposizione genetica di ogni uomo ad avere figli maschi o femmine. Gellatly ha dimostrato che ogni uomo può avere tre differenti combinazioni nel gene che controlla la trasmissione, da parte dell`uomo nei confronti della prole, del cromosoma sessuale X o di quello Y. Dalla ricerca è emerso che quindi alcuni uomini sono più portati ad avere figlie femmine, altri più portati ad avere figli maschi ed altri ancora, indifferentemente, potrebbero geneticamente dar vita a prole al maschile o al femminile.
Nella ricerca, Gellatly dimostra che esistono quindi uomini con la prima combinazione, nota come mm, portati geneticamente a trasmettere la Y alla loro prole e quindi più portati ad avere figli maschi; uomini mf, cioè che possono trasmettere ai loro concepiti in uguale proporzione sia il cromosoma X che quello Y, e che quindi sono portati geneticamente ad avere uno stesso numero di figli e figlie; poi c`è la terza tipologia di uomini, quelli ff, portati quindi geneticamente a trasmettere la X, e quindi ad avere una prole in rosa. Fiocco rosa o azzurro? A fare la differenza, quindi, è sempre il papà. Adesso più di prima.

Da: "http://www.ncl.ac.uk/press.office/press.release/item/boy-or-girl-it-s-in-the-father-s-genes" (11th December 2008)
Boy or girl? It’s in the father's genes
A Newcastle University study involving thousands of families is helping prospective parents work out whether they are likely to have sons or daughters.
The work by Corry Gellatly, a research scientist at the university, has shown that men inherit a tendency to have more sons or more daughters from their parents. This means that a man with many brothers is more likely to have sons, while a man with many sisters is more likely to have daughters.
The research, published online today by the journal Evolutionary Biology, involved a study of 927 family trees containing information on 556,387 people from North America and Europe going back to 1600.
"The family tree study showed that whether you’re likely to have a boy or a girl is inherited. We now know that men are more likely to have sons if they have more brothers but are more likely to have daughters if they have more sisters. However, in women, you just can’t predict it," Mr Gellatly explains.
Men determine the sex of a baby depending on whether their sperm is carrying an X or Y chromosome. An X chromosome combines with the mother’s X chromosome to make a baby girl (XX) and a Y chromosome will combine with the mother’s to make a boy (XY).
The study suggests that an as-yet undiscovered gene controls whether a man’s sperm contains more X or more Y chromosomes, which affects the sex of his children. On a larger scale, the number of men with more X sperm compared to the number of men with more Y sperm affects the sex ratio of children born each year.
Sons or daughters?
A gene consists of two parts, known as alleles, one inherited from each parent. In his paper, Mr Gellatly demonstrates that it is likely men carry two different types of allele, which results in three possible combinations in a gene that controls the ratio of X and Y sperm;
• Men with the first combination, known as mm, produce more Y sperm and have more sons.
• The second, known as mf, produce a roughly equal number of X and Y sperm and have an approximately equal number of sons and daughters.
• The third, known as ff produce more X sperm and have more daughters.
“The gene that is passed on from both parents, which causes some men to have more sons and some to have more daughters, may explain why we see the number of men and women roughly balanced in a population. If there are too many males in the population, for example, females will more easily find a mate, so men who have more daughters will pass on more of their genes, causing more females to be born in later generations,” says Mr Gellatly.
More boys born after the wars
In many of the countries that fought in the World Wars, there was a sudden increase in the number of boys born afterwards. The year after World War I ended, an extra two boys were born for every 100 girls in the UK, compared to the year before the war started. The gene, which Mr Gellatly has described in his research, could explain why this happened.
As the odds were in favour of men with more sons seeing a son return from the war, those sons were more likely to father boys themselves because they inherited that tendency from their fathers. In contrast, men with more daughters may have lost their only sons in the war and those sons would have been more likely to father girls. This would explain why the men that survived the war were more likely to have male children, which resulted in the boy-baby boom.
In most countries, for as long as records have been kept, more boys than girls have been born. In the UK and US, for example, there are currently about 105 males born for every 100 females.
It is well-documented that more males die in childhood and before they are old enough to have children. So in the same way that the gene may cause more boys to be born after wars, it may also cause more boys to be born each year.
How does the gene work?
The trees, below, illustrate how the gene works. It is a simplified example, in which men either have only sons, only daughters, or equal numbers of each, though in reality it is less clear cut. It shows that although the gene has no effect in females, they also carry the gene and pass it to their children.
In the first family tree (A) the grandfather is mm, so all his children are male. He only passes on the m allele, so his children are more likely to have the mm combination of alleles themselves. As a result, those sons may also have only sons (as shown). The grandsons have the mf combination of alleles, because they inherited an m from their father and an f from their mother. As a result, they have an equal number of sons and daughters (the great grandchildren).
In the second tree (B) the grandfather is ff, so all his children are female, they have the ff combination of alleles because their father and mother were both ff. One of the female children has her own children with a male who has the mm combination of alleles. That male determines the sex of the children, so the grandchildren are all male. The grandsons have the mf combination of alleles, because they inherited an m from their father and f from their mother. As a result, they have an equal number of sons and daughters (the great-grandchildren).

Academic paper: Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio Gene. Corry Gellatly.
Published in: Evolutionary Biology, DOI 10.1007/s11692-008-9046-3