Can a man with haemophilia pass it onto a) his son or b) his grandson?
Haemophilia is a sex-linked recessive condition that causes serious bleeding problems. In the human genome there are 22 autosomal chromosome pairs and 1 sex chromosome pair. Males are always XY and females are XX. The mutated haemophilia allele can be considered as Xh, as it is found only on the X chromosome. Recessive means that females must be homozygous Xh Xh to show the haemophilia phenotype, whereas males only need one Xh allele. This is because if females are XH Xh the dominant healthy XH allele will be expressed, no haemophilia phenotype will be seen and the female can be considered a carrier. Whereas males only have one X chromosome, and no equivalent healthy allele is found on the Y chromosome, therefore if the disease-causing allele Xh is inherited it will always be expressed. The result of this is that males can only inherit haemophilia from their mothers, whereas females can inherit the disease-causing allele from both their mothers or fathers. Females are, however, less likely to show the haemophilia phenotype as they can be carriers and both their mother and father must have the disease allele for them to have the possibility of expressing the haemophilia phenotype. Males can inherit the condition even if only the mother carries the Xh haemophilia allele.In this example consider a haemophilic grandfather as XhY. If this grandfather reproduced with a healthy woman XHXH then the son must be a healthy XHY. This is because the son must inherit the Y allele from the grandfather, to be male, and an XH from the mother. Therefore the answer to a) is no a son cannot inherit haemophilia from his father but can from his mother had the mother been a carrier or had the disease. Looking at b) the answer is yes as the grandfather may pass on the disease-causing Xh to his daughter, or the mother of the grandson. The mother may be Xh Xh (expressing the haemophilic phenotype) or XH Xh (being a carrier of the condition), depending on whether she inherits Xh or XH from the grandmother. If you consider the haemophilic mother Xh Xh, the son has a 100% chance of being haemophilic as he would inherit a Y from his father and must inherit a Xh from his mother. If the mother was XH Xh, the son would have a 50% chance of having haemophilia as he may have inherited either XH or Xh from his mother. The best way to consider this problem is with punnet squares to visualise the genotypes of the possible offspring.
