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The fact that a trait such as height in humans varies over a wide range of values is due to multiple alleles and the environmental influences on the expression of these genes.
How frequently a gene is expressed is its penetrance. It is described as the proportion of individuals that carry the gene and manifest the associated phenotype. Even though the trait is dominant or recessive and the basis for determining for such trait is present on both chromosomes, a gene with incomplete penetrance might not be expressed.
The unaffected carrier of an abnormal gene can pass it on to their offspring, who may develop the clinical abnormalities, even if the defective allele is not expressed (nonpenetrance). The genealogy tends to skip a generation in these situations. Patients with little or no expression may be thought to have a form of the illness.
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