A recessive gene can assert itself only when an individual inherits two copies of that gene, one from each parent.

Here's why:

* Genes come in pairs: We inherit one copy of each gene from our mother and one copy from our father.

* Dominant vs. recessive: Genes can be dominant or recessive. A dominant gene will always express its trait, even if only one copy is present. A recessive gene will only express its trait if both copies of the gene are recessive.

Example:

Imagine a gene for eye color. Let's say:

* B: Dominant gene for brown eyes

* b: Recessive gene for blue eyes

Here are the possible combinations:

* BB: Brown eyes (dominant gene expressed)

* Bb: Brown eyes (dominant gene expressed)

* bb: Blue eyes (recessive gene expressed)

So, even if someone carries a recessive gene for blue eyes (b), they will still have brown eyes if they also have at least one dominant gene for brown eyes (B). They are called "carriers" of the recessive gene.

It's only when they inherit two copies of the recessive gene (bb) that the blue eye trait will be expressed.

In summary:

A recessive gene will assert itself when an individual is homozygous recessive for that trait, meaning they have two copies of the recessive gene.