I live in UK and I have heard that when a new child is born we do aqiqah. My question is that, I know Aqiqah is not obligatory but why do we sacrifice two goats for boy and one goat for girl (I think according to Hadith). If Aqiqah is a way of gratitude then why there is discrimination between boy and girl? For parents both are equal.
We believe that aqiqah is not a practice that forms part of the religious practices (sunan) instituted as part of the religion by the Prophet (pbuh). For detail: Please refer to: Aqiqah
As for your question about the number of the animals to be slaughtered for a boy I believe that is upon the believer to decide. There has been a difference of opinion on the issue from the earliest times. I believe that it was considering the cultural values that for the boy the people have been offering two animals for the boy. We see that it is an expression of thanks to God Almighty. When someone is blessed even today with a boy they are happier. This finds a reflection in their reaction to the blessing. In the past the value of the boy have always been more as the boy used to bear the financial burden of the family and share the responsibility with the father. Therefore, the people used to be happier when granted by God with a boy. This does not mean that you shun daughters. It only means that the level of happiness is more when you are blessed with a boy. Some people can be happier at the birth of a girl. They may offer more offerings on the birth of a baby girl.
From the earliest authorities Imam Malik considered the view that only one animal may be offered at the birth of a boy as well as a girl. He based his view on a hadith which goes like this:
حدثني يحيى عن مالك عن نافع أن عبد الله بن عمر لم يكن يسأله أحد من أهله عقيقة إلا أعطاه إياها وكان يعق عن ولده بشاة شاة عن الذكور والإناث
Whenever someone among his family asked him for aqiqah (ie whenever someone was born in his home) he would grant the request. He would offer a sheep for a boy and a sheep for a girl.