heredity: Definition and Much More from Answers.com
- ️Wed Jul 01 2015
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.
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.
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
See also eugenics; gene; genetics, human.