Re: Is the chance of having a girl or boy baby always 50-50?

A mother has two X-chromosomes, so she can paas on only an X-chromosome to her children. A 
father has one X-chromosome and one Y-chromosome. There is an equal chance that he will pass 
on either the X-chromosome or the Y-chromosome to his child. A child that receives his 
X-chromosome will have two X-chromosomes and be female. A child that receives his 
Y-chromosome will be XY and be a male. So, at this level there is a 50:50 chance.

However, this is not the whole story because normally the sex ratio at birth is not 1:1. 
Instead slightly more males are born - a recent study of 11,800 births in Japan found 1.21 
males for every 1 female. Environment in the womb plays a part. The same study found that 
smoking lowered the number of boys; if both parents smoked more than 20 cigarettes per day, 
the sex ratio changed to 0.82 males for every 1 female.

There can also be alterations in the SRY gene on the Y-chromosome, the gene that sets male 
development in motion. There can be individuals with XY who are female because the SRY gene 
on the Y-chromosome is damaged. And there can be XX individuals who are male because the 
SRY gene has somehow been transferred to an X-chromosome. But these are very rare.