From a historical and biological perspective, heredity is the transfer of traits from a parent organism to its offspring. Traditional conceptualizations of heredity have focused on genes and the expression of genetic code that is transferred during reproduction. More recently and in response to knowledge about the limits of genetics in the human phenotypes and behavior, the conceptualization of heredity has been expanded to include the transfer of characteristics of the parent organism to offspring via a range of mechanisms, to include social institutions.
First described as animalcules and ultimately the basis of the school of scientists known as "spermists," early thinkers such as Anton van Leeuwenhoek recognized that there were microscopic parts of the human existence. Others went further to suggest that sperm contained little men who were small representations of adults. As such, heredity was determined by the male of the species, and the role of women in reproduction was simply to carry the homunculus that had been deposited by the male. As a reflection of societal values and the diminished value of women, this theory of heredity prevailed throughout the seventeenth century.
In a controversial and radical move forward, Gregor Mendel was credited for determining the rules associated with genetic transfer in the 1800s in a series of experiments using garden peas. He established patterns of inheritance by observing frequency of traits such as seed color and based on assumptions that the frequency was a direct function of specified patterns of genetic transmission. It was he who observed patterns and coined subsequent rules of genetics, notably the basis of modern genomic theories.
During this era science recognized that both the male and female contributed to heredity and that the ovum and sperm fused toward the development of a synergistic being. The mechanism of this transmission was later determined to be deoxyribonucleic acid (DNA), which was carried on chromosomes. From these seminal discoveries emerged the notion of the "central dogma" that indicates that DNA codes for ribonucleic acid (RNA) in transcription and RNA codes for proteins through a trans-lational process. DNA was ultimately recognized as the blueprint in the direction of cellular activities, tissue and organ functions, and organismic activity and reproduction in both plants and animals.
The molecular structure of DNA was deduced in 1953 by James Watson and Francis Crick and has since served as the basis for understanding modern molecular and behavioral genetics. DNA is characterized as a polymeric double helix containing repeating nucleotide bases linked to phosphorylated sugars. These DNA molecules are arranged in linear or circular chromosomes, specific to species. In some organisms, chromosomes are circular and singular, but in most higher order organisms, chromosomes exist in linear and duplicate form. For example, humans have twenty-three pairs of chromosomes, where inheritance is derived from both mother and father. The sequence of repeating units that make up DNA determines the organism’s genotype and, ultimately, pheno-type. Genetic variation occurs when there is a change in the DNA sequence secondary to biological or environmental provocation.
We have grown to recognize that heredity significantly influences how we look and how we behave in, anticipate, and respond to our environmental context. This includes how and what we contract in terms of disease, how disease susceptibility is manifest in subsequent generations, and how wellness is defined. In a reciprocal fashion, our environmental context modifies the relationship between genotype and phenotype. Genotype influences phenotype but may produce several different phenotypes, depending on the environmental context. One example is phenylketonuria. This disease is caused by a genetic defect that results in a buildup of phenylalanine, which causes brain damage in children. However, if the affected child’s diet contains low levels of phenylalanine, mental retardation is prevented. This environmental change prevents disease presentation, even though the genotype would otherwise predict disease and mental retardation.
Inheritance in humans is often difficult to study. First, all study methods outside the laboratory are observational; scientific ethics prevent us from forcing or selectively controlling mating in humans. Secondly, it is very difficult to study human heredity prospectively because we have very long generation times. As a consequence, a number of techniques have been discovered that uniquely and creatively provide insight into the human inheritance. These include the use of pedigree, twin, and adoption studies. Pedigree studies give scientists a long-term picture of the inheritance of a given trait, or of several traits, by several generations of a given family. Specific rules regarding pedigrees allow researchers to determine the pattern of inheritance, such as whether a trait is autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, or Y-linked. Dizygotic and monozygotic twins offer insight into the influence of environment on diseases with a significant genetic etiology. Since monozygotic twins share the same genetic makeup, it is expected that the manifestation of a genetic trait would be the same for the pair if they are in the same environment. Low concordance in monozygotic twins signals to investigators that environmental factors play a large role in the characteristic. Similarly, but from an environmental perspective, adoption studies also assist with determining the influence of environment and genetics on human functioning. Persons who are adopted often have few genes in common with their parents. However, they share the same environment for a number of years and often have health characteristics similar to those of their adoptive parents. Comparisons are made between adoptees and their adoptive parents, as well as between adoptees and one or both natural parents.
Inheritance is also of interest to those who study human behavior. For example, because of advances in statistical procedures and the use of twin methodologies in recent years, several researchers have begun evaluating the genetic influence of common psychiatric and personality traits, coping, and other behaviors. For example, using behavioral and molecular genetic analyses, Whitfield et al. (2006) recently reported that up to 35 percent of the variance in coping may be genetically mediated. This study suggests that genetics may provide a baseline for coping that is ultimately malleable and susceptible to learning and environmental influences. Similarly, Whitfield et al (2007) reported that 60 percent of individual smoking behavior was genetically mediated, with very few meaningful differences in the genes that determine or influence smoking behavior between racially classified social groups.
Studies like those reported above have been used by some to promote biological explanations for social phenomena and suggest genetic predispositions for favorable or unfavorable social outcomes. Notably, the prevailing scientific evidence strongly challenges genetic explanations in the etiology of social outcomes and suggests that societal inequities are much more salient than are genetic factors.
The recent growth of genomics as a science has frequently exceeded our planning and thinking on topics such as ethics, morals, and the law. Our physical capacity to disentangle the building blocks of human existence has not always been equaled by our appreciation for the impact of such knowledge. For example, should predispositions identified by genetic testing be reported to insurance companies as an acceptable factor in the calculation of risk and an influence on the cost of insurance premiums? Should our ability to manipulate phenotypic characteristic in humans (blond hair, blue eyes, tall, etc.) be used in a fashion representative of a modern-day extension of the concept of eugenics put forth by Francis Galton in the mid-1800s? The eugenics movement advocated selective breeding for the purpose of producing desirable human phenotypes. Should parents be able to preselect the characteristics of their children prior to birth? We will likely continue to develop reactive ethics, morals, and laws as the result of advances in the genomic sciences. We view the future of the study of heredity to include a range of social, psychological, biological, and genetic influences. Genetics serves as a necessary but insufficient factor to understand the scope of intergenerational stability and variation.
