Give scientific reasons: The human male never passes on the gene for haemophilia to his son.

(N/A) The gene for haemophilia is present on the $X$-chromosome. $A$ male has only one $X$-chromosome,which he receives from his mother,and a $Y$-chromosome,which he receives from his father. During reproduction,the human male passes his $X$-chromosome to his daughters and his $Y$-chromosome to his sons. Since the haemophilia gene is located on the $X$-chromosome,a father cannot pass this gene to his son because the son inherits the $Y$-chromosome from the father,not the $X$-chromosome.