In his experiments, Mendel bred together plants with similar traits until he had distinct populations that were unvarying for those traits. Offspring of crosses of individuals within a population that always show the same trait are described as:

Homozygous

Explanation:

Homozygous individuals are those who do not have genetical variation, it means that they will always look the same generation after generation, those individuals are vulnerable to not adapt to natural selection due to his lack of genes.

You can see homozygous every day, just looking at some pure breeds of dogs or other animals.