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We all have two copies of each gene, one inherited from our mother and the other from our father.

Inherited traits are determined by the combination of genes that we receive from our parents. This combination is called a genotype. Phenotype is the term used to describe the physical characteristics and traits that result from our genotype and interactions with our environment.

When a person has two identical copies of a gene, this is called homozygosity. The phenotype will reflect the variant, or allele, of the gene that is present. If both copies are normal, the person will have a normal phenotype. If both copies are mutated and cause a disease, the person will express the characteristic symptoms of that disease.

When a person has two different copies of a gene, this is called heterozygosity: one copy is normal, while the other copy is mutated. The phenotype will depend on dominance. If an allele is dominant, it will be reflected in the phenotype, hiding the other allele, which is called a recessive allele. However, the normal allele is not always dominant, just like the mutated allele is not always recessive. It depends on each particular disease.

People who have one mutated allele for a recessive disease are carriers of the disease allele but do not develop the disease themselves. However, they can pass this mutated copy of the gene on to their children. If their children also receive a mutated allele from the other parent, they will be homozygous and develop the disease.