If a gene is located on the X chromosome, does that mean males are never heterozygous for these genes?

Question

As said in the title I am a little confused on the idea of traits located on the sex chromosomes. The example we worked on in class was haemophilia, a recessive trait located on the X chromosome. Does this mean males only have one allele to choose from while females have two? Does this mean that only females have the ability to be heterozygous in sex linked traits?

Answer 1

Heterozygosity is not used to describe heteromorphic chromosomes, such as XY in human males because the alleles are by default, heteromorphic (different). Instead, "Hemizygosity" is used (Hemi- meaning half). This comes up in other biological systems as well, i.e. the females of other organisms have heteromorphic sex chromosomes rather than the males (e.g., female birds are designated with ZW and the homomorphic males are ZZ), and there are odd creatures like the platypus that has five pairs of sex chromosomes (some of which function like XX/XY and others are more like ZW/ZZ).