Genetics and Health - Genetic Inheritance

For inheritance of simple genetic traits, the two inherited copies of a gene determine the phenotype (the observable characteristic) for that trait. When genes for a particular trait exist in two or more different forms that may differ between individuals and populations, they are called alleles. For example, brown and blue eye colors are due to different alleles for eye color. For every gene, the offspring receives two alleles, one from each parent. The combination of inherited alleles is the genotype of the organism, and its expression—the observable characteristic—is its phenotype.

For many traits the phenotype is a result of an interaction between the genotype and the environment. Some of the most readily apparent traits in humans, such as height, weight, and skin color, result from interactions between genetic and environmental factors. In addition, there are complex phenotypes that involve multiple gene-encoded proteins and the alleles of these particular genes are influenced by other factors, either genetic or environmental. So while the presence of certain genes indicates susceptibility or likelihood to develop a certain trait, it does not guarantee expression of the trait.

For a specific trait, some alleles may be dominant while others are recessive. The phenotype of a dominant allele is always expressed, while the phenotype of a recessive allele is expressed only when both alleles are recessive. Recessive genes continue to pass from generation to generation, however, they are only expressed in individuals that do not inherit a copy of the dominant gene for the specific trait.

There also are some instances, known as incomplete dominance, when one allele is not completely dominant over the other, and the resulting phenotype is a blend of both traits. Skin color in humans is an example of a trait often governed by incomplete dominance, with offspring appearing to be a blend of the skin tones of each parent. Further, some traits are determined by a combination of several genes (multigenic or polygenic), and the resulting phenotype is determined by the final combination of alleles of all the genes that govern the particular trait.

Some multigenic traits are governed by many genes, and each contributes equally to the expression of the trait. In such instances, a defect in a single gene pair may not have a significant impact on expression of the trait. Other multigenic traits are predominantly directed by one major gene pair and only mildly influenced by the effects of other gene pairs. For these traits, the impact of a defective gene pair depends on whether it is the major pair governing expression of the trait or one of the minor pairs influencing its expression.

A range of other factors enters into whether a trait will be evidenced and the extent to which it is expressed. For example, different individuals may express a trait with different levels of severity. This phenomenon is known as variable expressivity.

The Influence of Heredity on Health

It has long been known that heredity affects health. Genetics explains how and why certain traits such as hair color and blood types run in families. Genomics, a discipline that is only about two decades old, is the study of more than single genes; it considers the functions and interactions of all the genes in the genome. In terms of health and disease, genomics has a broader and more promising range than does genetics. The science of geno-mics relies on knowledge of and access to the entire genome and applies to common conditions, such as breast and colorectal cancer, Parkinson's disease, and Alzheimer's disease. It also has a role in infectious diseases once believed to be entirely environmentally caused, such as human immunodeficiency virus (HIV, which is the virus that causes acquired immune deficiency syndrome [AIDS]) infection and tuberculosis. Like most diseases, these frequently occurring disorders are due to the interactions of multiple genes and environmental factors. Genetic variations in these disorders may have a protective or a causative role in the expression of diseases.

It is commonly accepted that diseases fall into one of three broad categories: those few that are primarily genetic in origin, disorders that are largely attributable to environmental causes, and the majority of conditions in which genetics and environmental factors make important, though not necessarily equal, contributions. As understanding of genomics advances and scientists identify genes involved in more diseases, the distinction between these three classes of disorders is diminishing. This chapter considers genetic testing, some of the disorders believed to be predominantly genetic in origin, and some that are the result of genes acted on by environmental factors.