Hets carry the allele of interest in a recessive fashion. Meaning their DNA contains the coding sequence for the desirable trait, but it is not expressed phenotypically by that individual, because they only retain one copy. When that allele is passed onto offspring, and the offspring receive another of the same allele from the second parent, they will express the phenotypic trait.
To expand on what was stated above, each animal has chromosomes that exist in homologous pairs, meaning one set of chromosomes from mom, one set from dad. During meiosis, when gametes are created the homologous chromosomes split apart, to form a haploid cell. So statistically you have a 50% chance of passing on the DNA from mom and a 50% chance of passing on the DNA from dad. If the individual is heterozygous for a trait, this is why if bred to a normal there is a 50% chance of the offspring receiving the gene. And when you combine two hets to breed, they each have a 50% chance of passing on the trait, so their offspring will have a 25% chance of getting BOTH alleles from the parents (50%x50%)
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