"Heterozygous" refers to a condition where an organism has two different copies of a specific gene or trait. So, instead of having identical copies, there is variation or diversity in the genetic information. Full definition
They identified a compound heterozygous mutation in the ARSA gene: c. 542T > G (exon 3) and the novel mutation c. 1013T > C (exon 6). (interactive-biosoftware.com)
In mice heterozygous for tau - lacZ targeted to the melanopsin gene locus, β - galactosidase — positive RGC axons projected to the SCN and other brain nuclei involved in circadian photoentrainment or the pupillary light reflex. (science.sciencemag.org)
Compound heterozygous mutations in UBA5 causing early - onset epileptic encephalopathy in two sisters. (decode.com)