The type of allele that only affects phenotype in a homozygous chromosome is called a recessive allele.

Here's why:

* Recessive alleles are only expressed when two copies of the allele are present (homozygous).

* Dominant alleles, on the other hand, will be expressed even if only one copy is present (heterozygous).

Example:

Let's say "B" represents the dominant allele for brown eyes and "b" represents the recessive allele for blue eyes.

* BB: Brown eyes (homozygous dominant)

* Bb: Brown eyes (heterozygous)

* bb: Blue eyes (homozygous recessive)

In this case, a person with the genotype "bb" will have blue eyes because they have two copies of the recessive allele. However, a person with the genotype "Bb" will have brown eyes because the dominant "B" allele masks the effect of the recessive "b" allele.