Disclaimer: The Open Encyclopedia Project attempts to provide precise and relevant information in the Health categories for educational purposes only. We do not guarantee the accuracy or currency of the information contained on these pages and cannot be responsible for any harm or adverse outcomes as a consequence of relying on or using this material. People who intend to rely or use this information as clinical advice or for treatment are advised not to but should consult an appropriate registered (licensed) medical professional.

Further Information

Related websites

Genetic Disorders

Genetic disorders include any condition that may arise from differences in the genetic code (mutations) of an individual. Genetic disorders may follow different inheritance patterns, or mutations may occur sporadically. Some genetic disorders may be present at the time of delivery and may be associated with birth defects and/or mental retardation depending on the severity of the condition. Others may not present until childhood or adulthood. There are also many conditions that involve multiple genetic differences as well as environmental influences. For example, adult-onset diabetes is thought to be due to differences in the HLA genotype as well as lifestyle factors, such as diet and exercise.

The mechanisms of genetic diseases vary as well. Down Syndrome is the result of having three copies of chromosome 21 (trisomy 21). Huntington's Chorea patients have an excess of trinucleotide repeats (repetitive sequences of three DNA bases). Color-blindness in males may be due to absence of genes encoding particular retinal cones.

Patterns of Inheritance

Genes are typically encoded as pairs of "alleles" on two separate chromosomes. An individual inherits one allele for a particular gene from each parent. When this individual mates, he or she transfers either of the inherited alleles to the offspring. The combination of genes and alleles leads to the diversity of gene expression.

Some alleles are considered "dominant", meaning that its encoded characteristics (phenotype) will be expressed regardless of the phenotype of its counterpart allele in the body. For example, the brown eye allele (B) is dominant over the blue eye allele (b). Blue eye traits are called "recessive". If a brown-eyed parent with a BB combination produces offspring with a blue-eyed parent (bb), the child will be born brown-eyed. The alleles of this child will be B and b, one from each parent. If this brown-eyed individual (Bb) mates with a blue-eyed parent (bb), the progeny will either be brown-eyed (Bb) or blue-eyed (bb). The brown-eyed parent can give either a B or a b allele. The other parent can only pass on a b allele. Thus, the combinations are Bb or bb. Although more complicated, the transfer of genetic diseases is similar in concept as the transfer of eye color or height.

Through classical Mendelian genetics, the selection of alleles to pass down is random and independent of other genes. In reality, there are many traits that are "linked" based on the distances of the genes on the same chromosome. This fact has permitted geneticists to discover trends in familial diseases and to discover markers that can potentially detect the presence of a genetic abnormality. This has influenced parenthood counseling, breast cancer screening, and early detection of a host of other diseases. It has also spurred great debate on the medical ethics of genetic screening by insurance companies, potential employers, and the government.