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How can a second normal copy of a gene reduce the effect of a harmful mutation?

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Four kittens with different fur patterns-three tabby and one black-and-white-lying together in a box, illustrating genetic variation among offspring. — heredity Each offspring is a combination of its two parents, receiving some dominant traits from its mother and others from its father.

In genetics, a second normal copy of a gene can reduce or mask the effect of a harmful mutation through the mechanism of dominance. When an individual inherits one normal allele and one mutant allele, they become a heterozygote.1

In cases of autosomal recessive conditions, the normal allele is typically capable of directing the production of sufficient functional protein to maintain a normal phenotype, effectively masking the presence of the harmful, recessive mutation. Because the normal allele is dominant, the deleterious effects of the mutation are not expressed in the individual, who remains phenotypically unaffected, though they may act as a carrier.12

This relationship is fundamental to understanding why certain genetic diseases only manifest when an individual is homozygous for the mutant allele—meaning they lack a functional copy of the gene to compensate for the mutation.13