Heterozygous individuals carry two copies (alleles) of same gene, one of which is recessive and another one dominant. This recessive allele when present in homozygous condition can cause some kind of genetic disorder. But in heterozygous condition, the effect of recessive allele is suppressed by the present of dominant allele. Hence the heterozygous individual is not affected. Butsuch individual carry genes (recessive allele) and can passed on those genes to their progenies. Hence heterozygous individual are called carriers only if they carry any defective gene but are not affected.