The correct explanation is that In a recessive disorder, only a single functioning allele is necessary to determine a normal phenotype.
A carrier of a disease is known to be any person that is said to have the disease trait, and they do not show any kind of physical symptoms of the said ailment.
Note that the carrier do have present in them a gene mutation in the recessive gene. The dominant gene is said to overpower the recessive gene, making the carrier to have the disease and can be able to pass on the said gene to their child.
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