Hemizygosity is the chromosomal condition in which only one allele of a gene occurs in the otherwise duplicate (diploid) chromosome set (hemizygous inheritance).
Hemizygosity affects all genes on the sex chromosomes in male individuals. Their chromosome pair consists of one X chromosome and one (much smaller) Y chromosome. Male individuals are therefore hemizygous with respect to all genes occurring only on the X chromosome and all genes occurring only on the Y chromosome. This implies that recessive traits are also expressed if they are located on this single (haploid) chromosome. This explains why some hereditary diseases affect male offspring much more frequently than female offspring.
See also heterozygosity (genetic endowment with duplicate but different genes, so-called alleles. In heterozygosity, the gene is present twice, but each allele is present only once).