The chart used to show the possible gene combinations in a cross between two organisms is called a Punnett square.

Here's how it works:

1. Genes: The Punnett square considers the alleles (different versions of a gene) that each parent contributes to their offspring.

2. Gametes: It represents the possible gametes (sperm and egg cells) that each parent can produce, with each gamete carrying one allele for each gene.

3. Grid: The Punnett square is a grid, with the possible gametes from one parent listed across the top and the possible gametes from the other parent listed down the side.

4. Combinations: By combining the alleles from each parent, the Punnett square shows all the possible genotype combinations for their offspring.

The Punnett square is a simple and visual tool for understanding how genes are inherited and the probability of different traits appearing in offspring.