Haemophilia

Haemophilia is a sex-linked recessive trait. The gene is recessive and linked on the X chromosome. So, males having one abnormal gene will get the disease, but females need two. Normally, females, only the carriers, can grow to puberty and propagate the gene to the next generation, but haemophiliac females cannot. It is because haemophiliac females cannot survive through their menstrations.

The abnormal gene makes the patient fail to produce the cannot produce the factor VIII, or the anti-haemophiliac globulin (AHC), for making blood clot. So, the coagulation of blood is difficult.

The symbols used in the inheritance of haemophilia are as follow.:

Let	X be the normal gene on X chromosome;
	X be the haemophiliac gene on X chromosome. It is recessive.
	Y be the Y chromosome. There is no active related gene on it.

Then, the performance of various geneotype would be as follow :

XX	is the normal female;
XX	is the female carrying the haemophiliac gene;
XX	is the haemophiliac female;
XY	is the normal male;
XY	is the haemophiliac male.

Reference :

(1)	(book)	Advanced level Biology for Hong Kong, Bk 2, by Y.K. Ho, Manhattan Press, pp. 180
(2)	(book)	Understanding Biology for Advanced Level, 4th Edition, by Glenn and Susan Toole, Stanley Thornes Press, pp. 165-167.
(3)	(book)	Biology : Principles and Processes, by Roberts, Reiss and Monger, Nelson Press, pp. 695-697
(4)	(book)	A-level Biology,by W.D. Phillips and T.J.Chilton, Oxford University Press, pp. 339-341
(5)	(book)	Advanced Biology, Principles and applications, by C.J.Clegg with D.G.Mackean, John Murray Press, pp. 628-629..
(6)	(web)	Wikipedia
(7)	(web)	HKEMB

(03.08.2008)