In mammals, which have an XX/XY system of sex determination, the X chromosome has many more genes than the Y chromosome. This explains the inheritance of such conditions as colour blindness and haemophilia. They are caused by recessive alleles, and usually do not show the condition in females. This is because those females carrying the mutant are most likely to be heterozygous for the allele. However, if the mutant allele is carried by a male, the condition will develop, because his X chromosome has the mutant allele, but his Y chromosome has no allele at that locus.