A punnett square is a diagram that geneticists use to predict the genotypic outcome of a particular cross. It is named after the British geneticist Reginald Punnett, who developed it in 1905.

To find the gene combination on a punnett square, you first need to know the genotypes of the parents. The genotypes of the parents are listed on the top and side of the punnett square.

For example, if you are crossing a homozygous dominant parent (AA) with a homozygous recessive parent (aa), the punnett square would look like this:

```

| | A | a

|-----|-------|------|

| A | AA | Aa

| a | Aa | aa

```

The first row of the punnett square represents the possible genotypes of the gametes (sex cells) produced by the homozygous dominant parent. The second row of the punnett square represents the possible genotypes of the gametes produced by the homozygous recessive parent.

To find the gene combination of the offspring, you simply multiply the genotypes of the parents. For example, if the homozygous dominant parent produces an A gamete and the homozygous recessive parent produces an a gamete, the offspring will have the genotype Aa.

This means that the offspring will be heterozygous for the trait. Heterozygous individuals have one copy of the dominant allele and one copy of the recessive allele. Heterozygous individuals can express both dominant and recessive traits, but the dominant trait will be expressed more often.

In the case of the cross between the homozygous dominant parent and the homozygous recessive parent, the offspring will all be heterozygous for the trait. This means that they will all express the dominant trait.

Punnett squares can be used to predict the genotypic outcome of any cross between two individuals. They are a valuable tool for geneticists and breeders.