Heterozygous

having different allelic genes at one or more paired loci in homologous chromosomes

Description
Humans are diploid, meaning that they possess a duplicate set of chromosomes and hence two copies (a paternal and a maternal one) of most genes. Being heterozygous (heterozygosity) for a given gene means possessing two different copies of that gene (i.e. two copies encoding a different trait). If one of the traits is dominant, it will determine the phenotype (e.g. the health status) of the individual. Therefore, for dominant traits even heterozygotes show disease signs. For recessive disease traits, on the other hand, the healthy gene will predominate in heterozygotes, who are then symptom-free and referred to as disease carriers. The separation between dominant and recessive traits is not clear-cut, and for many conditions heterozygous individuals will show intermediate phenotypes, i.e. a comparably mild expression of disease signs or symptoms (see e.g. thalassaemia and sickle cell disease). See homozygous