HEREDITY

CONCEPT

Heredity is the transmission of genetic characteristics from ancestor to descendant through the genes. As a subject, it is tied closely to genetics, the area of biological study concerned with hereditary traits. The study of heritable traits helps scientists discern which are dominant and therefore are likely to be passed on from one parent to the next generation. On the other hand, a recessive trait will be passed on only if both parents possess it. Among the possible heritable traits are genetic disorders, but study in this area is ongoing, and may yield many surprises.

HOW IT WORKS

Heredity and Genetics

As discussed at the beginning of the essay on genetics, the subjects of genetics and heredity are inseparable from each other, but there are so many details that it is extremely difficult to wrap one's mind around the entire concept. It is advisable, then, to break up the overall topic into more digestible bits. One way to do this is to study the biochemical foundations of genetics as a subject in itself, as is done in Genetics, and then to investigate the impact of genetic characteristics on inheritance in a separate context, as we do here.

Also included in the present essay is a brief history of genetic study, which reveals something about the way in which these many highly complex ideas fit together. Many brilliant minds have contributed to the modern understanding of genetics and heredity; unfortunately, within the present context, space permits the opportunity to discuss only a few key figures. The first—a man whose importance in the study of genetics is comparable to that of Charles Darwin (1809-1882) in the realm of evolutionary studies—was the Austrian monk and botanist Gregor Mendel (1822-1884).

GENES.

For thousands of years, people have had a general understanding of genetic inheritance—that certain traits can be, and sometimes are, passed along from one generation to the next—but this knowledge was primarily anecdotal and derived from casual observation rather than from scientific study. The first major scientific breakthrough in this area came in 1866, when Mendel published the results of a study on the hybridization of plants in which he crossed pea plants of the same species that differed in only one trait.

Mendel bred these plants over the course of several successive generations and observed the characteristics of each individual. He found that certain traits appeared in regular patterns, and from these observations he deduced that the plants inherited specific biological units from each parent. These units, which he called factors, today are known as genes, or units of information about a particular heritable trait. From his findings, Mendel formed a distinction between genotype and phenotype that is still applied by scientists studying genetics. Genotype may be defined as the sum of all genetic input to a particular individual or group, while phenotype is the actual observable properties of that organism. We return to the subjects of genotype and phenotype later in this essay.

MUTATION AND DNA.

Although Mendel's theories were revolutionary, the scientific establishment of his time treated these new ideas with disinterest, and Mendel died in obscurity. Then, in 1900, the Dutch botanist Hugo De Vries (1848-1935) discovered Mendel's writings, became convinced that his predecessor had made an important discovery, and proceeded to take Mendel's theories much further. Unlike the Austrian monk, De Vries believed that genetic changes occur in big jumps rather than arising from gradual or transitional steps. In 1901 he gave a name to these big jumps: mutations. Today a mutation is defined as an alteration of a gene, which contains something neither De Vries nor Mendel understood: deoxyribonucleic acid, or DNA.

Actually, DNA, a molecule that contains genetic codes for inheritance, had been discovered just four years after Mendel presented his theory of factors. In 1869 the Swiss biochemist Johann Friedrich Miescher (1844-1895) isolated a substance from the remnants of cells in pus. The substance, which contained both nitrogen and phosphorus, separated into a protein and an acid molecule and came to be known as nucleic acid. A year later he discovered DNA itself in the nucleic acid, but more than 70 years would pass before a scientist discerned its purpose.

THE DISCOVERY OF CHROMOSOMES.

In the meantime, another major step in the history of genetics was taken just two years after De Vries outlined his mutation theory. In 1903 the American surgeon and geneticist Walter S. Sutton (1877-1916) discovered chromosomes, threadlike structures that split and then pair off as a cell divides in sexual reproduction. Today we know that chromosomes contain DNA and hold most of the genes in an organism, but that knowledge still lay in the future at the time of Sutton's discovery.

In 1910 the American geneticist Thomas Hunt Morgan (1866-1945) confirmed the relationship between chromosomes and heredity through experiments with fruit flies. He also discovered a unique pair of chromosomes called the sex chromosomes, which determine the sex of offspring. From his observation that a sex-specific chromosome was always present in flies that had white eyes, Morgan deduced that specific genes reside on chromosomes. A later discovery showed that chromosomes could mutate, or change structurally, resulting in a change of characteristics that could be passed on to the next generation.

DNA MAKES ITS APPEARANCE.

All this time, scientists knew about the existence of DNA without guessing its function. Then, in the 1940s, a research team consisting of the Canadian-born American bacteriologist Oswald Avery (1877-1955), the American bacteriologist Maclyn McCarty (1911-), and the Canadian-born American microbiologist Colin Munro MacLeod (1909-1972) discovered the blueprint function of DNA. By taking DNA from one type of bacteria and inserting it into another, they found that the second form of bacteria took on certain traits of the first.

The final proof that DNA was the specific molecule that carries genetic information came in 1952, when the American microbiologists Alfred Hershey (1908-1997) and Martha Chase (1927-) showed that transferring DNA from a virus to an animal organ resulted in an infection, just as if an entire virus had been inserted. But perhaps the most famous DNA discovery occurred a year later, when the American biochemist James D. Watson (1928-) and the English biochemist Francis Crick (1916-) solved the mystery of the exact structure of DNA. Their goal was to develop a DNA model that would explain the blueprint, or language, by which the molecule provides necessary instructions at critical moments in the course of cell division and growth. To this end, Watson and Crick focused on the relationships between the known chemical groups that compose DNA. This led them to propose a double helix, or spiral staircase, model, which linked the chemical bases in definite pairs. Using this twisted-ladder model, they were able to explain how the DNA molecule could duplicate itself, since each side of the ladder contains a compound that fits with a compound on the opposite side. If separated, each would serve as the template for the formation of its mirror image.

Autosomes and Sex Chromosomes

Genetic information is organized into chromosomes in the nucleus, or control center, of the cell. Human cells have 46 chromosomes each, except for germ, or reproductive, cells (i.e., sperm cells in males and egg cells in females), which each have 23 chromosomes. Each person receives 23 chromosomes from the mother's egg and 23 chromosomes from the father's sperm. Of these 23 chromosomes, 22 are called autosomes, or non-sex chromosomes, meaning that they do not determine gender. The remaining chromosome, the sex chromosome, is either an X or a Y. Females have two Xs (XX), and males have one of each (XY), meaning that females can pass only an X to their offspring, whereas males can pass either an X or a Y. (This, in turn, means that the sperm of the father determines the gender of the offspring.)

Alleles

The 44 autosomes have parallel coded information on each of the two sets of 22 autosomes, and this coding is organized into genes, which provide instructions for the synthesis (manufacture) of specific proteins. Each gene has a set locus, or position, on a particular chromosome, and for each locus, there are two slightly different forms of a gene. These differing forms, known as alleles, each represent slightly different codes for the same trait. One allele, for instance, might say "attached earlobe," meaning that the bottom of the lobe is fully attached to the side of the head and cannot be flapped. Another allele, however, might say "unattached earlobe," indicating a lobe that is not fully attached and therefore can be flapped.

DOMINANT AND RECESSIVE ALLELES.

Each person has two alleles of the same gene—the genotype for a single locus. These can be written as uppercase or lowercase letters of the alphabet, with capital letters defining dominant traits and lowercase letters indicating recessive traits. A dominant trait is one that can manifest in the offspring when inherited from only one parent, whereas a recessive trait must be inherited from both parents in order to manifest. For instance, brown eyes are dominant and thus would be represented in shorthand with a capital B, whereas blue eyes, which are recessive, would be represented with a lowercase b. Genotypes are either homozygous (having two identical alleles, such as BB or bb) or heterozygous (having different alleles, such as Bb). The phenotype, however—that is, the actual eye color—must be one or the other, because both sets of genes cannot be expressed together.

Unless there is some highly unusual mutation, a child will not have one brown eye and one blue eye; instead, the dominant trait will overpower the recessive one and determine the eye color of the child. If an individual's genotype is BB or Bb, that person definitely will have brown eyes; the only way for the individual to have blue eyes is if the genotype is bb—meaning that both parents have blue eyes. Oddly, two parents with brown eyes could produce a child with blue eyes. How is that possible? Suppose both the mother and the father had the heterozygous alleles Bb—a dominant brown and a recessive blue. There is then a 25% chance that the child could inherit both parents' recessive genes, for a bb genotype—and a blue-eyed phenotype.

LEARNING FROM HEREDITARY LAW.

What we have just described is called genetic dominance, or the ability of a single allele to control phenotype. This principle of classical Mendelian genetics does not explain everything. For example, where height is concerned, there is not necessarily a dominant or recessive trait; rather, offspring typically have a height between that of the parents, because height also is determined by such factors as diet. (Also, more than one pair of genes is involved.) Hereditary law does, however, help us predict everything from hair and eye color to genetic disorders. As with the blue-eyed child of brown-eyed parents, it is possible that neither parent will show signs of a genetic disorder and yet pass on a double-recessive combination to their children. Again, however, other factors—including genetic ones—may come into play. For example, Down syndrome (discussed in Mutation) is caused by abnormalities in the number of chromosomes, with the offspring possessing 47 chromosomes instead of the normal 46.

REAL-LIFE APPLICATIONS

Population Genetics

Studies in heredity and genetics can be applied not only to an individual or family but also to a whole population. By studying the gene pool (the sum of all the genes shared by a population) for a given group, scientists working in the field of population genetics seek to explain and understand specific characteristics of that group. Among the phenomena of interest to population geneticists is genetic drift, a natural mechanism for genetic change in which specific traits coded in alleles change by chance over time, especially in small populations, as when organisms are isolated on an island. If two groups of the same species are separated for a long time, genetic drift may lead even to the formation of distinct species from what once was a single life-form. When the Colorado River cut open the Grand Canyon, it separated groups of squirrels that lived in the high-altitude pine forest. Over time, populations ceased to interbreed, and today the Kaibab squirrel of the north rim and the Abert squirrel of the south are different species, no more capable of interbreeding than humans and apes.

Where humans are concerned, population genetics can aid, for instance, in the study of genetic disorders. As discussed in Mutation, certain groups are susceptible to particular conditions: thus, cystic fibrosis is most common among people of northern European descent, sickle cell anemia among those of African and Mediterranean ancestry, and Tay-Sachs disease among Ashkenazim, or Jews whose ancestors lived in eastern Europe. Studies in population genetics also can supply information about prehistoric events. As a result of studying the DNA in fossil records, for example, some scientists have reached the conclusion that the migration of peoples from Siberia to North America in about 11,000 b.c. took place in two distinct waves.

Genetic Disorders

There are several thousand genetic disorders, which can be classified into one of several groups: autosomal dominant disorders, which are transmitted by genes inherited from only one parent; autosomal recessive disorders, which are transmitted by genes inherited from both parents; sex-linked disorders, or ones associated with the X (female) and Y (male) chromosome; and multifactorial genetic disorders. If one parent has an autosomal dominant disorder, the off-spring have a 50% chance of inheriting that disease. Approximately 2,000 autosomal dominant disorders have been identified, among them Huntington disease, achondroplasia (a type of dwarfism), Marfan syndrome (extra-long limbs), polydactyly (extra toes or fingers), some forms of glaucoma (a vision disorder), and hypercholesterolemia (high levels of cholesterol in theblood).

The first two are discussed in Mutation. Marfan syndrome, or arachnodactyly ("spiderarms"), is historically significant because it isbelieved that Abraham Lincoln suffered fromthat condition. Some scientists even maintain that his case of Marfan, a disease sometimes accompanied by eye and heart problems, was so severe that he probably would have died six months or a year after the time of his actual death by assassination at age 56 in April 1865.

RECESSIVE GENE DISORDERS.

Just as a person has a 25% chance of inheriting two recessive alleles, so two parents who each have a recessive gene for a genetic disorder stand a 25% chance of conceiving a child with that disorder. Among the approximately 1,000 known recessive genetic disorders are cystic fibrosis, sickle cell anemia, Tay-Sachs disease, galactosemia, phenylketonuria, adenosine deaminase deficiency, growth hormone deficiency, Werner syndrome (juvenile muscular dystrophy), albinism (lack of skin pigment), and autism. Several of these conditions are discussed briefly elsewhere, and albinism is treated at length in Mutation. Note that all of the disorders mentioned earlier, in the context of population genetics, are recessive gene disorders. Phenylketonuria (see Metabolism) and galactosemia are examples of metabolic recessive gene disorders, in which a person's body is unable to carry out essential chemical reactions. For example, people with galactosemia lack an enzyme needed to metabolize galactose, a simple sugar that is found in lactose, or milk sugar. If they are given milk and other foods containing galactose early in life, they eventually will suffer mental retardation.

SEX-LINKED GENETIC DISORDERS.

Dominant sex-linked genetic disorders affect females, are usually fatal, and—fortunately—are rather rare. An example is Albright hereditary osteodystrophy, which brings with it seizures, mental retardation, and stunted growth. On the other hand, several recessive sex-linked genetic disorders are well known, though at least one of them, color blindness, is relatively harmless. Among the more dangerous varieties of these disorders, which are passed on to sons through their mothers, the best known is hemophilia, discussed in Noninfectious Diseases. Many recessive sex-linked genetic disorders affect the immune, muscular, and nervous systems and are typically fatal. An example is severe combined immune deficiency syndrome (SCID), which is characterized by a very poor ability to combat infection. The only known cure for SCID is bone marrow transplantation from a close relative. Short of a cure, patients may be forced to live enclosed in a large plastic bubble that protects them from germs in the air. From this sad fact derives the title of an early John Travolta movie, The Boy in the Plastic Bubble (1976), based on the true story of the SCID victim Tod Lubitch. (The ending, in which Travolta, as Tod, leaves his bubble and literally rides off into the sunset with his beautiful neighbor Gina, is more Hollywood fiction than fact. Lubitch actually died in his early teens, shortly after receiving a bone marrow transplant.)

MULTIFACTORIAL GENETIC DISORDERS.

Scientists often find it difficult to determine the relative roles of heredity and environment in certain medical disorders, and one way to answer this question is with statistical and twin studies. Identical and fraternal twins who have been raised in different and identical homes are evaluated for multifactorial genetic disorders. Multifactorial genetic disorders include medical conditions associated with diet and metabolism, among them obesity, diabetes, alcoholism, rickets, and high blood pressure. Other such multifactorial conditions are a tendency toward certain infectious diseases, such as measles, scarlet fever, and tuberculosis; schizophrenia and some other psychological illnesses; clubfoot and cleft lip; and various forms of cancer. The tendency of a particular person to be susceptible to any one of these disorders is a function of that person's genetic makeup, as well as environmental factors.

Breeding within the Family

If there is one thing that most people know about heredity and breeding, it is that a person should never marry or conceive offspring with close relatives. Aside from moral restrictions, there is the fear of the genetic defects that would result from close interbreeding. How close is too close? Certainly, first cousins are off-limits as potential mates, though second or third cousins (people who share the same great-grandparents and the same great-great-grandparents, respectively) are probably far enough apart. Hence, the phrase "kissin' cousins," meaning a relative who is a distant enough to be considered a potential partner.

What kind of defects? Hemophilia, mentioned earlier, is popularly associated with royalty because several members of European ruling houses around the turn of the nineteenth century had it. Common wisdom maintains that the tendency toward the disease resulted from the fact that royalty were apt to marry close relatives. In fact, hemophilia has nothing to do with royalty per se and certainly bears no relation to marriages between close relatives. Research findings gathered over the course of more than three decades, beginning in 1965, indicate that many views about first cousins marrying may be more a matter of tradition than of scientific fact. According to information published in the Journal of Genetic Counseling and reported in the New York Times in April 2002, first cousins who have children together face only a slightly higher risk than parents who are completely unrelated. For example, within the population as a whole, the risk that a child will be born with a serious defect, such as cystic fibrosis, is 3-4%, while first cousins who conceive a child typically add another 1.7-2.8 percentage points of risk. Although this represents nearly double the risk, it is still a very small factor.

Researchers were quick to point out that mating should not take place between persons more closely related than first cousins. According to Denise Grady in the New York Times, "The report made a point of saying that the term 'incest' should not be applied to cousins, but only to sexual relations between siblings or between parents and children." First cousins, on the other hand, are a quite different matter, a fact borne out by the long history of people who married their first cousins. One example was Charles Darwin, who fathered many healthy children with his cousin, Emma Wedgwood.

WHERE TO LEARN MORE

Ackerman, Jennifer. Chance in the House of Fate: A Natural History of Heredity. Boston: Houghton Mifflin, 2001.

Center for the Study of Multiple Birth (Web site). <http://www.multiplebirth.com/>.

Clark, William R., and Michael Grunstein. Are We Hardwired?: The Role of Genes in Human Behavior. New York: Oxford University Press, 2000.

The Gene School (Web site). <http://library.thinkquest.org/19037/heredity.html>.

Genetic Disorders (Web site). <http://dir.yahoo.com/Health/Diseases_and_Conditions/Genetic_Disorders/>.

Grady, Denise. " Few Risks Seen to the Children of First Cousins ." New York Times, April 4, 2002.

Hawley, R. Scott, and Catherine A. Mori. The Human Genome: A User's Guide. San Diego: Academic Press, 1999.

Heredity and Genetics. The Biology Project at the University of Arizona (Web site). <http://student.biology.arizona.edu/sciconn/heredity/worksheet_heredity.html>.

Reproduction and Heredity (Web site). <http://www.usoe.k12.ut.us/curr/science/sciber00/7th/genetics/sciber/intro.htm>.

Ridley, Matt. Genome: The Autobiography of a Species in 23 Chapters. New York: HarperCollins, 1999.

Wynbrandt, James, and Mark D. Ludman. The Encyclopedia of Genetic Disorders and Birth Defects. New York: Facts on File, 2000.

KEY TERMS

ALLELE:

For any locus, one of two (or more) slightly different forms of a gene. These differing forms mean that alleles code for different versions of the same trait.

AUTOSOMES:

The 22 non-sex chromosomes.

CHROMOSOME:

A DNA-containing body, located in the cells of most living things, that holds most of the organism's genes.

DNA:

Deoxyribonucleic acid, a molecule in all cells, and many viruses, that contains genetic codes for inheritance.

DOMINANT:

In genetics, a term for a trait that can manifest in the offspring when inherited from only one parent. Its opposite is recessive.

GENE:

A unit of information about a particular heritable trait. Usually stored on chromosomes, genes contain specifications for the structure of a particular polypeptide or protein.

GENE POOL:

The sum of all the genesshared by a population, such as that of aspecies.

GENETIC DISORDER:

A condition, such as a hereditary disease, that can be traced to an individual's genetic makeup.

GENETIC DOMINANCE:

The ability of a single allele to control phenotype.

GENOTYPE:

The sum of all genetic input to a particular individual or group.

GERM CELL:

One of two basic types of cells in a multicellular organism. In contrast to somatic, or body, cells, germ cells are involved in reproduction.

HEREDITY:

The transmission of genetic characteristics from ancestor to descendant through the genes.

HETEROZYGOUS:

Having two different alleles—for example, Bb.

HOMOZYGOUS:

Having two identicalalleles, such as BB or bb.

LOCUS:

The position of a particular gene on a specific chromosome.

MUTATION:

Alteration in the physical structure of an organism's DNA, resulting in a genetic change that can be inherited.

NUCLEUS:

The control center of a cell, where DNA is stored.

PHENOTYPE:

The actual observable properties of an organism, as opposed to its genotype.

RECESSIVE:

In genetics, a term for a trait that can manifest in the offspring only if it is inherited from both parents. Its opposite is dominant.

SEX CHROMOSOMES:

Chromosomes that determine gender. Human females have two X chromosomes (XX), and males have an X and a Y (XY).

SYNTHESIZE:

To manufacture chemically, as in the body.

Heredity

BIBLIOGRAPHY

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 translational 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, phenotype. 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.

SEE ALSO Darwin, Charles; Determinism, Biological; Determinism, Genetic; Disease; Evolutionary Psychology; Genomics; IQ Controversy; Psychosomatics; Twin Studies

BIBLIOGRAPHY

Edwards, C. L., K. Whitfield, S. Sudhakar, et al. 2006. Parental Substance Abuse, Reports of Chronic Pain, and Coping in Adult Patients with Sickle Cell Disease (SCD). Journal of the National Medical Association 98 (3): 420–428.

Gillham, Nicholas Wright. 2001. A Life of Sir Francis Galton: From African Exploration to the Birth of Eugenics. New York: Oxford University Press.

Whitfield, K. E., D. T. Brandon, E. Robinson, et al. 2006. Sources of Variability in John Henryism. Journal of the National Medical Association 98 (4): 641–647.

Whitfield, K. E., G. King, S. Moller, et al. 2007. Concordance Rates for Smoking among African-American Twins. Journal of the National Medical Association 99 (3): 213–217.

Christopher L. Edwards

Goldie Byrd

heredity In popular parlance, the word ‘heredity’ is used to explain the observation that every living organism gives rise, through reproduction, to a look-alike organism. In biology and medicine, it is a term refering to the biological information that is transmitted from parents to offspring in every generation. Nowadays, the field of genetics is responsible for the scientific study of heredity and its mechanisms, and the main focus of genetic research is the examination of the gene as carrier of information on the structure, function, and biological attributes of the organism, and its transmission to subsequent generations.

The term ‘heredity’ was introduced into the English language in the 1860s from the French hérédité, as a noun referring to the properties and characters considered as hereditary. The term ‘heredity’ was preferred over the existing term ‘inheritance’ by biologists of the time, because it was not loaded with the Lamarckian overtones of the latter. Borrowed from landed gentry and used to refer to old family property as well as to that acquired during a particular lifetime, the term ‘inheritance’ was associated with notions of acquired characteristics. Francis Galton, an active spokesman for the importance of heredity in the human make-up, and founder of the science of eugenics, claims in his autobiography to have been the first to use the term ‘heredity’ in the 1860s. However, other biologists, such as Charles Darwin, had started using the term some years earlier.

In 1900, Gregor Mendel's 1866 paper on the study of hybrids of the edible pea was independently ‘rediscovered’ in Europe. Although Mendel's experiments were part of his interests on the origin of new species by hybridization (rather than by variation), and were thus not directly concerned with the elucidation of the laws of heredity, they were interpreted in 1900 as the first systematic study unravelling the mechanisms of heredity.

Gregor Mendel (1822–1884), an Augustinian monk at Brno, Moravia (now part of the Czech Republic), performed his classic experiments using varieties of the edible pea (Pisum sativum) grown in the monastery garden. By artificial fertilization, he crossed two pure varieties of peas and followed the inheritance of seven pairs of character differences (yellow or green seeds; round or angular seeds; white or grey-brown seed coats; green or yellow pods; smooth or ridge pods; tallness or shortness; axillary or terminal flowers). He reported that, in the first hybrid generation (F1), only one character in each pair of character differences would be manifested. He used the word ‘factor’ to refer to the determing agent responsible for each character, and described their effects as either dominant or recessive. Through self-fertilization, he crossed the F1 to produce the second hybrid generation (F2) and reported the reappearance of the recessive characters in a 1:3 ratio. Mendel explained his results by describing the characters studied as distinct, stable factors, which were passed on independently and unchanged from parent to offspring. Although the recessive characters would be masked in the F1, their independent transmission from parent to offspring could be confirmed by observing their reappearance in the F2. The reappearance of hidden recessive characters in the F2 disagreed with prevailing notions on ‘blending’ inheritance, postulating the blending and dilution of parental traits in the offspring. Mendel also carried out the self-fertilization of the F2, from which he confirmed the existence in the F2 of three types of plants: two pure parental types and one hybrid type.

Mendel's hybridization experiments are theoretically formulated in the figure. As example, this shows the cross between two varieties of peas displaying seed colour as character difference.

In 1900, with the international recognition of Gregor Mendel's work as the basis for a new science of heredity, a new wave of experimentation with hybrid formation began that appealed to the breeding interests of botanists and zoologists. In 1906, the Cambridge zoologist William Bateson introduced the word ‘genetics’ to refer to the expanding new field of research. Bateson became a vocal defender of the validity of Mendel's conclusions as the scientific foundation for the new discipline. He encouraged the use of Mendelian principles not only for the study of the plant and animal world, but also for the examination of heredity in humans. On February 1, 1906, he addressed the Neurological Society of London on the topic of Mendelian heredity and its application to man. In this lecture, Bateson presented to an audience of physicians a new picture of human heredity in which human physical traits were treated as Mendelian segregating characters, and he reformulated human hereditary disease as being caused by single genetic factors obeying Mendelian principles. He explained brachydactyly, congenital cataract, albinism, alcaptonuria, haemophilia, and colour blindness as being caused by Mendelian factors (dominant or recessive) of heredity.

Bateson spoke extensively about the behaviour of Mendelian factors, but was unable to provide a material mechanism guiding their operation. He refused to accept ideas associating the gene with a particular stretch of chromosomal material. However, between 1910 and 1915, Thomas Hunt Morgan and his students, working at Columbia University, New York, gathered enough data to support successfully the chromosomal theory of the gene, which firmly established the Mendelian genetic factors as material unities, or ‘genes’, embedded in the chromosome. The use of the chromosomal theory of the gene gave rise to a very productive area of experimentation, now known as ‘classical genetics’ which produced the first genetic maps, showing the relative positions of genes on the chromosome, and a gave clear notion of the nature of mutations.

Outside the laboratory, the concept of heredity occupied a crucial role in debates on the importance of nature over nurture and on the possibilities of using biological norms to guide social reform during the end of the nineteenth century and the first decades of the twentieth. Hereditarian theories, considering heredity as the central factor determining human character, were used by biologists, physicians, and social activists to explain human temperament, family pathology, and the structure of society. Francis Galton, a strong believer in the hereditarian position, founded the discipline of eugenics, which sought to improve the quality of human heredity by manipulating human reproduction. The field of eugenics developed into a breeding programme proposing a series of measures to prevent the reproduction of those labelled as ‘unfit’ or ‘feebleminded’. As a counterpart, such programmes sought to promote the reproduction of those harbouring in their heredity ‘superior’ human qualities. Eugenic thought became highly influential during the first decades of the twentieth century in the US and in Britain, Germany, and other parts of Europe. It started losing its pre-eminence in the 1930s and 1940s, when it was highly criticized by scientists and the public for its scientific inaccuracy, for its class and race bias, and for the excesses to which it could lead, as exemplified by the horrors taking place during the implementation of state controlled reproductive policies in Nazi Germany.

Silvia Frenk

Heredity

The process by which the genetic code of parents is passed on to their children.

There are certain traits that parents pass on to their children, including eye color, hair color, height, and other physical characteristics. The coding for these traits are contained inside DNA molecules that are present within all human cells. Since the discovery of DNA by James Watson (1928-) in the 1950s, the science of genetics has focused on the study of DNA and the ways in which physical traits are passed on from generation to generation. Within genetics, a special branch of DNA science—called quantitative, or biometrical, genetics— has emerged, which studies the heritability of such traits as intelligence , behavior, and personality . This branch focuses on the effects of polygenes in the creation of certain phenotypes. Polygenes, as the name implies, refer to the interaction of several genes; and phenotypes are certain variable characteristics of behavior or personality. Quantitative geneticists, therefore, study the effects of groups of genes on the development of personality and other abstract variables. They rarely, it should be noted, are able to pinpoint a behavior's genesis to a specific gene. Specific genes have been found to cause a small number of diseases, however, such as Huntington's disease and other degenerative disorders.

In studying personality traits and intelligence, the latest research in quantitative genetics suggests that the heritability rate for many characteristics hovers around 50 percent. In 1988 a study of twins reared apart revealed the heritability of 11 common character traits. The findings, published in the Journal of Personality and Social Psychology, reported that social potency is 61% influenced by genes; traditionalism, 60%; stress reaction, 55%; absorption (having a vivid imagination ), 55%; alienation , 55%; well-being, 54%; harm avoidance (avoiding dangerous activities), 51%; aggression , 48%; achievement, 46%; control, 43%; and social closeness, 33 percent.

Other recent studies have compiled lists of traits most influenced by heredity. Physical characteristics that are most genetically determined include height, weight, tone of voice, tooth decay, athletic ability , and age of death, among others. Intellectual capabilities include memory , IQ scores, age of language acquisition, reading disabilities, and mental retardation . Emotional characteristics found to be most influenced by heredity were shyness , extroversion , neuroses, schizophrenia , anxiety, and alcohol dependence. It is important to note that these are tendencies and not absolutes. Many children of alcoholics, for instance, do not become alcoholics themselves. Many social and cultural factors intervene as humans develop, and the child of an alcoholic, who may be genetically vulnerable to acquiring the disease, may avoid drinking from witnessing the devastation caused by the disease. (For a fuller discussion of the role of environment , see Nature-Nurture Controversy.)

Recent work has shown that genes can both be influenced by the environment and can even influence the environments in which we find ourselves. A 1990 study found that animals raised in environments requiring significant motor activity actually developed new structures in the brain that were significantly different from the brain structures of animals raised in environments lacking motor stimuli. Observations from such experiments have revealed that complex environments actually "turn on" sets of genes that control other genes, whose job it is to build new cerebral structures. Therefore, living in an environment that provides challenges can genetically alter a person's makeup. Additionally, a genetic predisposition to introversion can cause people to isolate themselves, thus changing their environment and, in the process, altering their development of social skills. This, then, contributes further to their genetic predisposition to introversion.

There also appears to be universal, inherited behavior patterns in humans. Common behaviors across diverse cultures include the patterns of protest among infants and small children at being separated from their mothers. A study conducted in 1976 found that separation protests emerge, peak, and then disappear in nearly identical ways across five widely diverse cultures. Other studies have found universal facial expressions for common emotions, even among pre-literate hunter-gatherer cultures that have had no exposure to media. It used to be thought that the human smile was learned through observation and imitation , but a 1975 study found that children who had been blind from birth began smiling at the same age as sighted children. Many of these behaviors are thought to be instinctual. Aside from the infant/developmental behaviors already mentioned, other inherited behavior patterns in humans include sex, aggression, fear , and curiosity/exploration.

Further Reading

Beal, Eileen. "Charting the Future? Researching Heredity Quotient in African American Families." American Visions (October-November 1994): 44.

Berkowitz, Ari. "Our Genes, Ourselves?" BioScience (January 1996): 42.

Metzler, Kristan. "The Apple Doesn't Fall Far in Families Linked to Crime." Insight on the News (29 August 1994):17.

Tellegen, A. "Personality Similarity in Twins Reared Apart and Together." Journal of Personality and Social Psychology 54 (1988): 1031.

204. Heredity

See also 15. ANCESTORS ; 44. BIOLOGY ; 147. EVOLUTION ; 307. PARENTS ; 341. RACE .

abiogenesis

generation of living organisms from inanimate matter. Also called spontaneous generation .

anencephaly

the congenital absence of the brain and spinal cord in a devel-oping fetus.

biotypology

the science or study of biotypes, or organisms sharing the same hereditary characteristics —biotypologic, biotypological , adj.

blastogenesis

the theory that hereditary characteristics are transmitted by germ plasm. Cf. pangenesis . —blastogenetic , adj.

cytoplasm

the entire substance of a cell excluding the nucleus.

deoxyribonucleic acid (DNA)

the complex substance that is the main carrier of genetic information for all organisms and a major component of chromosomes.

DNA

deoxyribonucleic acid.

dysgenesis

lack of or partial fertility, as found in hybrids like the mule, which cannot breed amongst themselves but only with the parent stock. —dysgenetic , adj.

geneagenesis

alternation of generations. —geneagenetic , adj.

genetics

1. Biology. the science of heredity, studying resemblances and differences in related organisms and the mechanisms which explain these phenomena.

2. the genetic properties and phenomena of an organism. —geneticist , n. —genetic , adj.

hereditist

a believer in the theory that heredity, more than environment, determines nature, characteristics, etc.

homogenesis

the normal course of generation in which the offspring resembles the parent from generation to generation. —homogenetic , adj,

Mendelism

the laws of inheritance through genes, discovered by Gregor J. Mendel. —Mendelian . n., adj.

pangenesis

the theory advanced by Darwin, now rejected, that transmission of traits is caused by every cell’s throwing off particles called gemmules, which are the basic units of hereditary transmission. The gemmules were said to have collected in the reproductive cells, thus ensuring that each cell is represented in the germ cells. Cf. blastogenesis . —pangenetic , adj.

perigenesis

Haeckel’s theory of generation and reproduction, which assumes that a dynamic growth force is passed on from one generation to the next. —perigenetic , adj.

prepotency

the capacity of one parent to impose its hereditary characteristics on offspring by virtue of its possessing a larger number of homozygous, dominant genes than the other parent. —prepotent , adj.

radiogenetics

a division of radiobiology that studies the effects of radioactiv-ity upon factors of inheritance in genetics. —radiogenic , adj.

recombinant DNA

a DNA molecule in which the genetic material has been artificially broken down so that genes from another organism can be intro-duced and the molecule then recombined, the result being alterations in the genetic characteristics of the original molecule.

ribonucleic acid (RNA)

a nucleic acid found in cells that transmits genetic instructions from the nucleus to the cytoplasm.

RNA

ribonucleic acid.

telegony

the supposed transmission of hereditary characteristics from one sire to offspring subsequently born to other sires by the same female. —telegonic , adj.

Weismannism

the theories of development and heredity asserted by August Weismann (1834-1914), esp. that inheritable characteristics are carried in the germ cells, and that acquired characteristics are not hereditary. —Weismannian , n., adj.

xenogenesis

1. abiogenesis; spontaneous generation.

2. metagenesis, or alternation of generations.

3. production of an offspring entirely different from either of the parents. Also xenogeny . —xenogenic, xenogenetic , adj.

xenogeny

xenogenesis.