A gene that produces an effect only in the homozygous state is called a recessive gene.

Here's why:

* Homozygous: Means having two identical copies of a gene.

* Recessive: Means that the gene's effect is only visible when two copies of it are present.

Example:

Consider a gene for eye color, where the dominant allele (B) codes for brown eyes and the recessive allele (b) codes for blue eyes.

* BB: Homozygous dominant - Brown eyes

* Bb: Heterozygous - Brown eyes (dominant allele masks the recessive one)

* bb: Homozygous recessive - Blue eyes

In this case, the blue eye color (b) only appears in the homozygous recessive state (bb).

Important Note: Some genes can have incomplete dominance where the heterozygous state shows a blended phenotype. For example, a flower with one red allele and one white allele might have pink petals.