Heredity: Understanding the Passing of Traits - Definition & Examples

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Introduction to Heredity

Heredity, the passing of traits from parent to offspring. The natural laws of heredity account for the fact that one generation of a particular plant or animal resembles preceding generations. Chicks hatch from chicken eggs and develop into chickens. Human babies are born of human beings and grow to human adulthood. More specifically, large chickens tend to have large offspring, and persons with fair skin usually have fair-skinned children.

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Not all hereditary traits, however, are as obvious as body size and shape or skin color. For example, some persons inherit color blindness, but may not even be aware of the condition. Others may inherit the tendency to develop a certain disease under certain environmental conditions.

Although heredity produces characteristics through successive generations, there are usually limited differences, or variations, between one generation and the next. Sometimes an individual will possess a hereditary trait that has not been evident for several generations. The process that makes this possible is called atavism, or reversion, and the individual is called a throwback.

Genetics is the study of the biological mechanisms by which an organism's traits are passed from generation to generation. It is concerned both with the transmission of traits and with other characteristics of genetic material.

Knowledge gained from the study of hereditary factors in disease and disease resistance has been valuable in diagnosing and treating many illnesses. Improved methods of agriculture also owe a great deal to the study of heredity. The development of hardy, disease-resistant, and high-yielding hybrid crops, such as corn, has been of immense economic value.

Heredity termsAlleles are different forms of the same gene.Chromosomes are tiny threadlike structures inside each cell. Chromosomes carry the genes.DNA stands for deoxyribonucleic acid. It is the substance within the chromosomes that carries the hereditary instructions for producing proteins and RNA.Gene expression is the process by which a cell makes a protein or RNA according to the instructions carried by a gene.Genes are tiny biochemical units inside each cell that determine particular hereditary traits, such as eye color and blood type. Each gene is a segment of DNA that carries instructions for producing the chainlike molecules called RNA.Genetic variation refers to the differences in inherited traits that exist among the members of a species.Genetics is the scientific study of heredity.Genome is a set of all the genes a species has on its chromosomes. Scientists believe the human genome consists of 20,000 to 30,000 genes.Genotype is the underlying genetic makeup of a trait or the overall genetic makeup of an individual.Mutation is a change in a gene. It may produce a new trait that can be inherited.Phenotype is the observable appearance of a trait or the overall appearance of an individual.Protein is a chemical building block in the body. Proteins exist in every cell.RNA stands for ribonucleic acid. Similar to DNA, it plays a key role in the production of proteins.Trait is a characteristic, such as hair color.

How Heredity Works

In the simplest type of reproduction, a one-celled organism divides into two new cells and the offspring closely resemble the parent. In other types of reproduction, however, the process that provides for the passing on of traits from one generation to the next is not always apparent. This is especially true of sexual reproduction.

The offspring of some organisms that reproduce sexually do not at first even resemble their parents. Tadpoles do not look like frogs, nor caterpillars like butterflies. Eventually, however, they reach a stage in their development where they look like their parents and other adults of their species. It is apparent, therefore, that what they have inherited is a growth pattern.

All organisms inherit a pattern of growth. Sexually reproduced organisms inherit a pattern from each parent, and the combined patterns determine how the organism will develop. The information necessary to produce these patterns is found in the hereditary, or genetic, material that exists in all living cells. This material is usually found in a cell's nucleus, where it is arranged into tiny bodies called chromosomes. Each chromosome contains several types of proteins and deoxyribonucleic acid (DNA). DNA consists of two strands, each composed of a long chain of subunits called nucleotides, of which there are four kinds. Inheritable traits are determined by DNA segments, called genes, that contain specific sequences of the four kinds of nucleotides. The body, or somatic, cells of all organisms carry a number of paired, homologous (similar) chromosomes. Sexual reproduction occurs with the union of two specialized cells, called gametes, one from each parent. A gamete, such as an egg or sperm, contains only half the number of chromosomes as a somatic cell of the same species. The process by which gametes are produced is called meiosis (mī-ω'si$s).

Only certain cells (such as those in the testes and ovaries) undergo meiosis. In meiosis the homologous chromosomes duplicate, then pair up and interconnect in a process called synopsis. The cell then divides into two, with each chromosome of a homologous pair going to a new cell. The result is four gametes, each containing half the number of chromosomes as the the parent cell. .

Principles of Heredity

The effects of chromosome activity upon the inherited traits of organisms have been formulated into a number of principles, or laws. Some of these principles were observed long before anything was known about chromosomes and genes.

An Austrian monk, Gregor Mendel (1822-1884), made the first truly scientific study of heredity and published his findings in 1866. Mendel's method was to crossbreed various kinds of garden peas that had opposing pairs of obvious traits. The principles he established—segregation and dominance, and independent assortment—became the basis for the science of genetics. The four other principles discussed here were discovered by later geneticists.

1. Segregation and Dominance

In his first experiments, Mendel crossbred varieties that differed in only one pair of opposing traits. He crossed red-blossomed peas with white-blossomed peas; smooth-seeded with wrinkle-seeded peas; tall-stemmed with dwarf-stemmed peas. Altogether, nine alternative traits were investigated by a great number of crossings. (Three of the nine—flower color, seed-coat color, and seedling-axil color—were later considered aspects of the same trait.) The results in each case followed the same general mathematical pattern, which can be simplified by using symbols for the successive generations.

P1 is the parent generation; F1 is the first filial, or hybrid, generation; F2 is the second filial generation, produced by crossing the hybrid F1 plants. The results were as follows: F1 resembled only one of the P1; F2 resembled both members of the P1 in a proportion of approximately three to one.

For example, red-blossomed peas crossed with white-blossomed peas produced only red-blossomed plants. When the hybrid red-blossomed plants were crossed, the result was about three red-blossomed to one white-blossomed.

Mendel concluded that each plant carried a pair of factors (what are today called genes), one inherited from each parent, which controlled such traits as blossom color. (In the illustration, the factors are indicated by the letters R for red and r for white.) When the gametes were formed, the paired factors segregated, or separated, and each gamete received only one factor from each parent. The arrows in the illustration show how the various factors combine in different patterns. In F1, for example, the left R combines with the right r to produce Rr; the left R combines with the right R to produce RR. Similarly, the left r combines with the right R (Rr) and the right r (rr).

Mendel observed that a plant carrying both red and white factors developed red blossoms, not pink ones, as might have been expected. He concluded that one factor was dominant over the other factor, which he called recessive. For example, the red-blossom factor was dominant; the white-blossom factor, recessive. Therefore only those plants (about one-fourth of the F2) that had both factors for white would be white.

Later experiments, however, showed that one factor is not always completely dominant over the other. Some paired factors show no dominance. Therefore, although the factors do not blend, there may be a blending effect of traits. For example, when certain yellow-blossomed plants are crossed with white-blossomed ones, the offspring may have cream-colored blossoms.

In modern terms, the paired genes (factors) are called alleles (a$-lēlz') of each other. If they are identical, the organism is said to be pure, or homozygous, for the pair. If they are different (as are the genes for the red and white blossoms), the organism is said to be hybrid, or heterozygous, for the pair.

2. Independent Assortment

In other experiments Mendel crossed plants having two pairs of obvious opposing traits. The F2 generation consisted of four different kinds of individuals in the ratio 9:3:3:1. One-sixteenth of the F2 showed both recessive traits. Nine-sixteenths showed both dominant traits. Three-sixteenths showed one dominant trait but not the other. Another three-sixteenths showed the second dominant but not the first. This ratio is the expected one according to the mathematical laws of combinations and indicates that the two pairs of genes were on two different pairs of homologous chromosomes, and segregated, or assorted, independently.

3. Linkage and Crossing Over

By chance, all the plants with which Mendel experimented showed, in the F2 generation, traits resulting from independent assortment of genes. Later research by other geneticists indicated that independent assortment does not always take place. In many cases the two pairs of genes are on the same pair of homologous chromosomes and stay together during sexual recombination. Such genes are said to be linked.

Traits controlled by linked genes will appear together in the F2 generation and frequently in successive generations. However, they do not always appear together because genes originally linked are sometimes separated during meiosis by a process called crossing over. This separation occurs if parts of a homologous pair of chromosomes break during synapsis and exchange places.

4. Sex Determination

In animals and certain plants, there is an exception to the rule that chromosomes of somatic cells appear as homologous pairs. The male and female of such species differ to some extent in chromosome structure and function. For example, the human male has only 22 paired homologous chromosomes; he has two unpaired chromosomes, called X and Y. Each somatic cell of the female has 23 pairs, including a pair of X chromosomes.

Half the human male gametes contain X chromosomes, and half contain Y chromosomes. At fertilization, if a male gamete containing an X chromosome unites with a female gamete, a female organism is produced. If a male gamete containing a Y chromosome unites with a female gamete, the new organism will be male. The male parent, therefore, determines the sex of the offspring.

Techniques have been developed that can separate sperm (male gametes) containing X chromosomes from sperm containing Y chromosomes. It is possible to predetermine the sex of a child by artificial insemination with sperm containing either X chromosomes or Y chromosomes.

5. Sex Linkage

Traits controlled by genes found on X and Y chromosomes are said to be sex linked. Such traits may be exhibited in the successive generations in different ways than traits controlled by other genes. For example, recessive traits appear more frequently in males than in females, if the traits are carried on X chromosomes. This higher frequency is due to the fact that the male organism receives only one X chromosome, from the female parent. Since there is apt to be no allele on the Y chromosome to counteract the recessive gene found on the X chromosome, the recessive trait will appear. A female will not exhibit the trait unless the recessive gene is on both her X chromosomes.

Such diseases and conditions as hemophilia (a disease of the blood), color blindness, and muscular dystrophy are recessive, sex-linked traits that are more common in men than in women.

6. Interaction of Genes

Although genes are called units of heredity, this does not mean that one gene controls one trait. The genes in an organism interact. One gene may affect many traits, and one trait may be determined by many genes.

Genomic Imprinting

According to Mendelian principles, it makes no difference whether a given gene (or group of genes) for a hereditary trait—color in a flower, for example—is inherited from the male parent or from the female parent. However, scientists have discovered that certain genes, depending on whether they are inherited from the male or female parent, may be inactive. The process that causes the genes to be inactive is called genomic imprinting. Scientists believe the genes are inactivated either temporarily or permanently by chemical modifications that occur during the formation of gametes. These genes cause the genetic process to act unpredictably, leading, in some cases, to the conditions that cause genetic-linked diseases.

Mutation

A mutation is any change in genetic material. A change in the structure of a gene is called a gene mutation. A change in the number or arrangement of genes on a chromosome is a type of mutation called a chromosomal aberration. Gene mutations and chromosomal aberrations occur spontaneously in nature from causes not fully understood. They have also been produced artificially by exposing chromosomes to radiation or to certain chemicals.

Some mutations in the gametes cause harmful abnormalities that lead to the death of the offspring. Others are less harmful, or are beneficial to the new organism, and are passed on to the next generation. In time, physical variations produced by mutations lead to new species and thus carry on the process of evolution.

Heredity and Environment

A popular question for many years was: which is more important in determining the characteristics of an individual, heredity or environment? Scientists have learned that both play important parts, and that most qualities are the result of the interaction of heredity and environment.

The genetic makeup, or genotype, of an organism cannot be altered by its environment except through mutation. However, the way in which the organism develops is constantly affected by external influences. Its phenotype, or gross structure and function, undergoes changes throughout its life, because the phenotype is determined by both its genotype and its environment.

Traits vary widely in their susceptibility to environment. The human blood groups seem not to be affected at all. Human skin color can be altered by irradiation, chemicals, and disease. Animal bone structure may be modified by diet and by glandular disease. The size and productivity of plants are affected by the soil and climate in which they are grown.

Certain diseases, such as diabetes mellitus and some forms of gout, are hereditary, but will develop only under certain conditions. On the whole, mental and emotional traits seem to be more readily influenced by environment than are physical traits.

Genetic Defects

Some diseases are caused by genetic, or hereditary, defects in which there is an abnormal number of chromosomes or the chromosomes have an abnormal structure. Down syndrome, for example, is usually caused by the presence of an extra chromosome in the nucleus.

If a parent has a dominant gene for a disease, each child has a 50 per cent chance of receiving the defective gene. Examples of dominant-gene disorders are Huntington's disease (a progressive degeneration of the nervous system), achondroplasia (a form of dwarfism), and hypercholesterolemia (high levels of blood cholesterol). If both parents are carriers of a recessive trait, each child has a 25 per cent chance of inheriting a genetic disease. Examples of recessive-gene diseases are cystic fibrosis (a malfunction of the mucus and sweat glands), phenylketonuria (a deficiency of an essential liver enzyme), sickle-cell anemia (a blood disorder primarily affecting blacks), and Tay-Sachs disease (a fatal brain disorder affecting children of Eastern European Jewish ancestry).

In genetic screening, the presence of a genetic disorder is determined by analyzing a person's chromosomes and DNA. The material to be analyzed is isolated from cells that are usually obtained from blood, bone marrow, skin, testes, or placental tissue. Microscopic examinations can reveal flaws in the chromosomes. A chemical test called restriction fragment length polymorphism, or RFLP, analysis is commonly used to detect segments of DNA associated with specific diseases.

Genetic screening helps to determine the probability that individuals or their existing or future offspring will have health problems caused by genetic defects. It is most commonly used by persons with a family history of genetic disorders. If a gene for a disorder is present, certain steps (such as dietary changes) can sometimes be taken to lessen the chance that the disorder will develop; other steps (such as frequent checkups) can ensure that the disorder, should it develop, will be detected in an early, treatable stage. Also, a person with a known genetic defect may decide not to have children. When it has been determined that a pregnant woman's unborn child will probably have a genetic disorder (and when abortion is not an option), counseling can help prepare the parents to accept the child and provide appropriate care.

History of the Study of Heredity

Early Theories. A number of erroneous ideas about heredity date to ancient times. They were discredited only in the last few centuries.

The theory that living organisms were produced from nonliving matter by spontaneous generation received a setback through the work of the French bacteriologist Louis Pasteur (1822-1895). At one time it was thought that human beings could be born of other mammals. The belief that certain cereal plants could turn into others was held as late as the middle of the 20th century by some Russian horticulturists.

For centuries people believed in “blood inheritance,” a blending of bloods in half-and-half portions from the parents, quarter portions from grandparents, and so on. The notion persists in remarks about persons inheriting good or bad blood.

The theory of acquired characteristics was introduced by the ancient Greeks. According to this theory, traits acquired by an organism during its lifetime can be passed on to its offspring. For example, a man who developed his muscles by exercise would have children with strong muscles. This theory was supported by the French biologist Jean Baptiste Lamarck (1744-1829). It was officially approved in 1948 by the Communist party in the Soviet Union and did not lose favor with the party for several years. Intensive research in the early half of the 20th century by the United States biologist Theodosius Dobzhansky has helped to discredit the theory of acquired characteristics.

Mendel's work was neglected until 1900, when his principles were rediscovered by three biologists working independently: Karl E. Correns, in Germany; Hugo de Vries, in the Netherlands; and Erich Tschermak, in Austria. Mendel's papers were republished, and his findings became the basis for the science of genetics.