How is it possible for a parent to carry a gene for a genetic disorder and not know they possess this gene?

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A carrier is an individual that carries and is capable of passing on a genetic disorder. However, a carrier may or may not display symptoms of the disorder. This may be because the disorder carried by the individual is a recessive trait.

Many human genes are composed of two alleles. Alleles are varieties of genes. Alleles can be dominant or recessive. A dominant allele is the “stronger” variety of a gene. A recessive allele is the “weaker” variety of the gene.

A genotype is the genetic combination of alleles that an individual has for a trait. Phenotypes are the physical representation of a genotype. Because dominant alleles are “stronger” than recessive alleles, only one dominant allele is needed to produce a dominant phenotype, while two recessive alleles are needed to produce a recessive phenotype.

Genotypes can be homozygous dominant, heterozygous, or homozygous recessive. Homozygous dominant genotypes have two dominant alleles and will show the dominant phenotype. A heterozygous genotype has one dominant and one recessive allele. Since a heterozygous genotype contains a dominant allele, it will show the dominant phenotype. Homozygous recessive genotypes contain two recessive alleles. Having a homozygous recessive genotype is the only way an individual can show the recessive phenotype. Many genetic disorders are from recessive alleles, so if the parent is heterozygous, they may not know they possess the gene that carries the disorder.


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