A child exhibiting a heritable trait that neither parent exhibits could be explained by several factors, the most likely being the following:
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Recessive Alleles: The trait may be a recessive trait, meaning that both parents may carry one copy of the recessive allele (which does not manifest in them) and one copy of the dominant allele. If the child inherits the recessive allele from both parents, the trait can manifest in the child.
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New Mutations: It is also possible that a new mutation occurred in the child that was not present in the parents. Although this is less common, it can lead to the expression of traits that are not observed in either parent.
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Incomplete Penetrance or Expressivity: Traits can sometimes have incomplete penetrance, where not all individuals with a particular genotype express the associated phenotype, or varying expressivity, where the trait may present differently among individuals.
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Polygenic Inheritance or Environmental Influences: The trait could arise from the interaction of multiple genes and environmental factors, which might not be obvious or exhibited in the parents.
In most cases, recessive alleles are the most common explanation for such a situation.