What does traits will only be expressed if both recessive allies are passed down from each parent otherwise i'll be hidden in dominant allele what does that mean

Recessive Inheritance

When a trait is recessive, an individual must have two copies of a recessive allele to express the trait. Recessive alleles are denoted by a lowercase letter (a versus A). Only individuals with an aa genotype will express a recessive trait; therefore, offspring must receive one recessive allele from each parent to exhibit a recessive trait. One example of a recessive inherited trait is a smooth chin, as opposed to a dominant cleft chin. Let (S) represent the dominant allele, and (s) represent the recessive allele. Only (ss) individuals will express a smooth chin. To determine the probability of inheritance of a smooth chin (or any other recessive trait), the genotypes of the parents must be considered. If one parent is heterozygous (Ss) and the other is homozygous recessive (ss), then half of their offspring will have a smooth chin.  In other words, any of the offspring will have a fifty percent chance of having a smooth chin. A Punnett square can be used to determine all possible genotypic combinations in the parents.

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the answer to your question is tooth decay.