When two haploid gametes contain different alleles of a given gene, the resulting offspring is called heterozygous for that gene.

Here's why:

* Haploid gametes: Each gamete (sperm or egg) carries only one copy of each chromosome, and thus only one allele for each gene.

* Different alleles: If the two gametes contribute different versions (alleles) of a gene, the offspring will inherit one allele from each parent.

* Heterozygous: The offspring is considered heterozygous because it carries two different alleles for that specific gene.

For example, if one parent contributes the allele for brown eyes (B) and the other parent contributes the allele for blue eyes (b), the offspring will be heterozygous (Bb) for eye color.