Final answer:
Recessive genes persist in a population because they can be carried by individuals who have one recessive and one dominant allele for a trait. These genes can be passed to offspring and remain in the gene pool even if they're not phenotypically expressed, which is why they never totally disappear.
Step-by-step explanation:
Recessive genes never disappear from a population because they can be carried by heterozygous individuals without being expressed phenotypically. This means that individuals who are carriers have one dominant and one recessive allele for a particular trait. Due to the principles of Mendelian genetics, particularly the law of segregation, the recessive allele is passed down to offspring just as the dominant allele is passed. The presence of recessive diseases like cystic fibrosis—expressed only when two recessive alleles are inherited—remains in the population because even if both parents are carriers (heterozygous), there is only a
chance for any given child to have the disease, while there is a
chance for the child to be a carrier as well.
According to the Hardy-Weinberg equilibrium, which is
allele frequencies in a population remain constant from generation to generation unless specific factors cause them to change. Since both dominant and recessive alleles are transmitted through carrier states, the recessive genes persist over generations even if they don't manifest in the phenotype. Furthermore in the case of X-linked recessive traits, which are more common in males than females, these genes can also be transmitted through carrier females. Collectively these factors act to sustain the presence of recessive genes within a population over time.