Having only one copy of a gene instead of the normal two, usually because that gene is located on the X chromosome in males.
From Greek 'hemi-' meaning 'half' and 'zygous' from 'zygon' meaning 'yoked' or 'paired'; coined in early 20th-century genetics to describe organisms with unpaired genes.
Male humans are hemizygous for X-linked genes, which is why color blindness and hemophilia are much more common in men than women—they only have one copy to rely on!
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