The human genome--the whole of a person's DNA--contains thousands of genes. Each person has two copies of most genes, called alleles, with one allele inherited from the mother and one from the father. During development and throughout life, the contents of each allele allow the cell to make a protein required for development and life. However, some alleles code for proteins involved in detrimental traits and disease. Three major classifications of inherited detrimental traits determine how a detrimental trait is passed onto offspring.
Dominant Traits
Dominant detrimental traits require only one allele for a gene to be mutated in an individual for them to express the trait. The detrimental allele may be passed on from the mother, father or from both parents. Generally, if one parent expresses the detrimental trait, their child has a 50 percent chance of inheriting the trait, and if both parents express the trait, the child has a 75 percent chance of inheriting the trait.
An example of a dominent detrimental trait is the development of Huntington's disease, a neurodegenerative disorder. The University of Kansas medical center explains that Huntington's is caused by a mutant allele to the HD gene, and that transmission of one mutant allele is sufficient to cause Huntington's in offspring. Patients with a family history of Huntington's can undergo genetic testing to determine whether they carry the mutant allele, and whether they will develop the disease.
Recessive Traits
Recessive traits require that a person contain two mutant alleles to express the detrimental trait--both copies of the gene must be mutated. Parents that carry one mutant allele will not develop the detrimental trait, but they are considered carriers, because they can pass on the mutant allele to their offspring. If both parents carry a mutant allele associated with a detrimental trait, their child has a 25 percent chance of developing the trait and a 50 percent chance of being a carrier.
A relatively common recessive detrimental trait is cystic fibrosis, a disease characterized by abnormal mucous production that can interfere with breathing. Mutations to the gene CFTR can cause cystic fibrosis, according to the Genetics Home Reference, and patients containing two copies of the mutant gene develop the disease. Though incurable, patients with cystic fibrosis can control their disease with drugs and other therapies.
X-Linked Recessive Traits
In some cases, the inheritance of a detrimental trait can differ between males and females if the recessive mutant allele is found on the X chromosome. Males contain only one X chromosome, and females contain two. Therefore, a male may inherit one mutant allele and express a detrimental trait and a female may require two mutant alleles to develop the trait. As a result, X-linked detrimental traits are generally more common in men than in women.
An example of an X-linked detrimental trait is red-green color-blindness, an inability to see red and green. Though not a serious disorder, the inability to distinguish color can become a workplace hazard in some occupations if the ability to see color is necessary. The Lucile Packard Children's Hospital at Stanford explains that, because color blindness is an X-linked recessive trait, it occurs in males 16 times more often than in females.
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