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Heredity, process of transmitting biological traits from parent to offspring through genes, the basic units of heredity. Heredity also refers to the inherited characteristics of an individual, including traits such as height, eye color, and blood type.
Heredity accounts for why offspring look like their parents: when two dogs mate, for example, they have puppies, not kittens. If the parents are both Chihuahuas, the puppies will also be Chihuahuas, not great Danes or Labrador retrievers. The puppies may be a little taller or shorter, a little lighter or a lot heavier than their parents are. Their faces may look a little different, or they may have different talents and temperaments. In all the important characteristics, however—the number of limbs, arrangement of organs, general size, fur type—they will share the traits of their parents. The principles of heredity hold true not only for a puppy but also for a virus, a roundworm, a pansy, or a human.
Genetics is the study of how heredity works and, in particular, of genes. A gene is a section of a long deoxyribonucleic acid (DNA) molecule, and it carries information for the construction of a protein or part of a protein. Through the diversity of proteins they code for, genes influence or determine such traits as eye color, the ability of a bacterium to eat a certain sugar, or the number of peas in a pod. A virus has as few as a dozen genes. A simple roundworm has 5000 to 8000 genes, while a corn plant has 60,000. The construction of a human requires an estimated 50,000 genes.
If the DNA in a single human cell could be unraveled, it would form a single thread about five feet long and about 50 trillionths of an inch thick. To prevent this fine string of DNA from becoming knotted like a big tangle of yarn, parts of the strand are wrapped around proteins like a thread is wound around spools. These units of wrapped DNA are called nucleosomes, and they coil and fold into structures called chromosomes. Humans have 23 pairs of chromosomes. In each pair, one chromosome comes from the mother and the other from the father. Twenty-two of the pairs are the same in both men and women, and these are called autosomes. The twenty-third pair consists of the sex chromosomes, so called because they are the primary factor in determining the gender of a child. The sex chromosomes are known as the X and Y chromosomes.
Females have two X chromosomes, and males have one X and one Y chromosome. The Y chromosome is about one-third the size of the X chromosome. A sperm, the reproductive cell produced by the male, can carry either one X or one Y chromosome. An egg, the reproductive cell produced by the female, can carry only the X chromosome. When a sperm with an X chromosome unites with an egg, the result is a child with two X chromosomes—a female. When a sperm with a Y chromosome unites with an egg, however, the result is a child with one X and one Y chromosome—a male. Thus, the father determines the gender of the child.
Throughout the entire world of life, evolution has brought about only two types of reproduction—asexual and sexual. Asexual reproduction does not require a mate and is less complicated than sexual reproduction. It is used by simple life forms, such as bacteria; complex one-celled organisms, such as amoebas and diatoms; certain worms, such as flatworms; fungi; and many plants.
In asexual reproduction, one parent transmits all of its genetic information to the offspring, and the offspring is therefore identical to the parent. Asexual reproduction typically is a rapid and reliable method of reproduction. It is limited, however, because the genetic uniformity in the offspring makes them all equally susceptible to a change in the environment. If a new disease, a new predator, or a climate change is lethal to one individual, it is lethal to all genetically identical organisms. Such changes can effectively wipe out entire populations of genetically identical organisms. Sexual reproduction results in offspring with diverse traits, and is the predominant form of reproduction among plants, animals, and most other organisms.
In contrast to asexual reproduction, sexual reproduction requires two parents. Each parent creates sex cells, or gametes that contain half the parent’s genetic information. Human sex cells—sperm and eggs—contain 23 single, unpaired chromosomes rather than the 23 paired chromosomes found in all other body cells, or somatic cells. When egg and sperm unite in the process called fertilization, they form one cell that contains 23 pairs of chromosomes, the normal number for human body cells. The cell develops into a child that has a mixture of genetic information from both parents. As a result, the child is similar to each of the parents but not identical to either of them.
If these same parents have a second child, it is the product of fertilization of a different sperm and a different egg. Therefore the second child is unique, because each sperm and egg contains a unique set of chromosomes (see Meiosis). Scientists estimate that each person is capable of producing 223 or 8,388,608 unique sex cells. The total number of unique children possible from one couple is a phenomenal 223 × 223 or 246. This genetic diversity that results from sexual reproduction enables populations to withstand changing environments through evolution.
With the exception of the X and Y chromosomes, genes come in twos on the paired chromosomes, but the genes are not necessarily identical. The hair color gene from the father may carry information for black hair, but its partner on the chromosome from the mother may specify red hair. These different forms of genes that carry information for specific traits are called alleles. A person’s hair color depends on several alleles interacting in complex ways to determine the actual trait of the offspring.
A pattern of inheritance describes how alleles work together to produce traits. Understanding inheritance patterns enables geneticists to predict the probability that a child will inherit a certain trait. A variety of inheritance patterns influence the diverse traits found not only in humans, but in other animals, plants, fungi, and bacteria.
The dominant-recessive pattern of inheritance, a relatively simple pattern, involves paired alleles that influence one trait. In this pattern, one of the two alleles contains information for a certain characteristic—the lavender color of sweet pea flowers, for example—while the second allele directs the production of an alternate characteristic—the white flower color. In sweet peas, if these two alleles occur together, the allele for lavender flowers is expressed, and the flowers are lavender. The allele for lavender is therefore called the dominant allele. The allele for white is known as the recessive allele. Lavender flowers also occur when two alleles for lavender color are paired. Only when two alleles for the recessive characteristic are paired do white flowers appear. This genetic rule applies regardless of the organism or the trait. In the dominant recessive pattern, the recessive trait shows up only when two recessive alleles are paired.
In humans, several hundred genetic diseases and disorders follow the dominant-recessive pattern. These conditions result when a mutation, or a change in a normal allele, is found in a sperm or egg, and the mutation causes disease when the child inherits a pair of mutated alleles. If a child inherits one dominant allele and one recessive allele he or she typically does not have the disease. Such individuals are termed carriers, since although healthy, they carry the recessive allele. A carrier can pass either the dominant or recessive allele to their child. If both parents are carriers, these alleles can be passed along in four ways. The child can receive a normal allele from each parent, in which case it does not develop the disease. It can receive a mutated allele from the mother and a normal allele from the father, or a normal allele from the mother and a mutated allele from the father. In both of these cases, the child will be a carrier. The child develops the disease only if he or she receives a mutated allele from each parent. When both parents are carriers, there is a 25 percent chance that a child will be disease-free, a 25 percent chance that it will have the disease, and a 50 percent chance that it will be a carrier. Examples of genetic diseases that follow the dominant-recessive pattern include sickle-cell anemia, beta-thalassemia, cystic fibrosis, and severe combined immunodeficiency disease (see Genetic Disorders).
A significant number of human traits, such as eye color, skin color, height, weight, and muscle strength are typically regulated by more than one allele in a pattern known as polygenic inheritance. Several thousand alleles, for example, may combine to determine a person’s potential for pole-vaulting, and several hundred may play a role in establishing a person’s normal weight. Certain diseases may result from mutations in one or more alleles involved in polygenic inheritance. Researchers have identified nearly a dozen mutated alleles that are associated with diabetes mellitus, and a similar number are linked to asthma. Heart disease may be linked to two or three times that number. Some types of cancer may be correlated with more than 100 different genes. Polygenic inheritance is quite complex, and the ways in which multiple genes interact to produce traits are not fully understood.
X-Y linked, or sex-linked, inheritance results from the size differences between the X and Y chromosomes. The longer X chromosome carries an estimated 250 genes, which are responsible for critical biochemical functions such as normal blood clotting. The shorter Y chromosome carries 6 genes, which are responsible for other traits, such as producing significant amounts of testosterone, the male sex hormone.
X-Y linked conditions typically occur in a male when the single X chromosome carries a mutated allele, one that prevents normal blood clotting, for example. A male does not have a second X chromosome with a normal allele to override the mutation. As a result, the male in this case will have hemophilia, a disease in which blood does not clot normally. If one of the female’s X chromosomes carries the mutated allele, however, her second X chromosome is usually normal. The normal allele is the dominant allele, so the female does not have hemophilia. Thus, females are typically carriers of X-Y linked diseases but do not develop them unless they receive a mutated allele from each parent, an unusual event. Among the genetic disorders typically carried by females but inherited by males are hemophilia, color blindness, and Duchenne’s muscular dystrophy.
In most organisms, the chromosomes located in the cell nucleus contain the vast majority of the DNA. But another structure in the cell, called a mitochondrion, also holds a chromosome. The DNA on this chromosome is referred to as mitochondrial DNA. While both sperm and egg contain mitochondria, only the egg’s mitochondria are transmitted to the offspring. The sperm’s mitochondria are contained in the sperm’s tail, which never penetrates the egg.
Mutations in mitochondrial DNA have been implicated in a number of genetic diseases. These diseases include diabetes mellitus, deafness, heart disease, Alzheimer’s disease, Parkinson disease, and Leber’s hereditary optic neuropathy, a condition of complete or partial blindness resulting from degeneration of the optic nerve. Mitochondrial medicine is a relatively new specialty that seeks to explain the disorders and the patterns of inheritance associated with mitochondrial DNA.
Since mitochondrial DNA is inherited only from the mother—a type of inheritance known as maternal inheritance—scientists can trace these genes from one generation to the next, a simpler task than tracing genes that might come from either the mother or the father. The study of mitochondrial DNA has been employed to study human evolution. Recently scientists extracted mitochondrial DNA from Neandertal bones believed to be between 30,000 and 100,000 years old. They compared these ancient genes with those of hundreds of people around the world. As a result, they determined that Neandertals are a different species than humans and not their ancestors, as was formerly believed.
Alleles differ in the degree to which they determine traits. If a person inherits the alleles for Type A blood, for example, they have Type A blood from birth to death. Traits associated with some alleles, however, show up only under certain circumstances. For example, a specific allele might place a person at risk for developing diabetes mellitus, but only if they suffer a particular viral infection. Alleles that influence depression may make an individual more likely to become depressed, but only if they encounter life experiences that enhance the allele’s effects. Researchers increasingly find evidence that many alleles are associated only with a tendency toward particular traits. The expression of these alleles can vary during a person’s lifetime. Some alleles appear to be involved in an interplay with the environment: triggers such as toxins, light, certain nutrients, or stress may “turn on” an allele, resulting in expression of the trait.
Psychologists and biologists have long debated whether interaction with the environment—a person’s family and culture, for instance—is more important than genes in shaping disease, character, and behavior (see Animal Behavior). It is becoming more obvious that environment and genes have different degrees of influence, depending on the trait. Some traits such as eye color appear to depend on only a genetic component with little or no environmental input. However, others such as muscle strength or musical achievement seem to require contributions from both genes and the environment. If a person is born with the alleles for great athletic or musical potential, for example, those talents will not develop without practice. A child may be born with the alleles for potentially high academic intelligence, but lack of stimulation and limited exposure to new experiences in early childhood may keep the child from realizing that potential. Lack of nutrition during childhood can turn a person with the potential to be six feet tall into someone who barely clears five feet. Current research indicates that expression of alleles in certain individuals may also depend on their unique internal environment—their nervous system, hormone balance, or other aspects of their biochemistry.
Current knowledge of heredity is the result of more than 2000 years of contemplation of how inheritance works. The ancient Babylonians knew that pollen from a male date palm tree must be applied to the carpels of a female flower to obtain fruit, but they did not know about the reproductive cells in humans. The Greek scientist and philosopher Aristotle believed that inheritance was passed through the blood. This concept was embraced for centuries and persists today in such terminology as bloodlines, half bloods, and blue bloods.
The past few centuries have witnessed tremendous advances in understanding the role of reproductive cells in heredity. In 1651 the British scientist William Harvey proposed the idea, based on his experiments with embryos of different organisms, that all animals develop from eggs. In 1677 a different view was advocated by the Dutch naturalist Antoni van Leeuwenhoek, who was the first to observe human sperm under the microscope. Leeuwenhoek believed that sperm contained a child in miniature, which grew larger inside the female’s body. Two centuries of experiment and debate followed. Then in 1879, with the use of improved microscopes, German zoologists Herman Fol and Oscar Hertwig observed the union of egg and sperm in animals. This observation crystallized our understanding of the roles of male and female sex cells in reproduction.
Exactly how traits are transmitted to offspring from the sperm and egg was a topic of vigorous discussion in the 19th century. In 1866, the Austrian monk Gregor Mendel published his groundbreaking studies on inheritance in peas. At the time of his work, chromosomes, genes, and DNA were unknown. Even so, Mendel discovered a variety of genetic rules, including the concept of dominant and recessive genes. Mendel hypothesized that plants contain two factors for each plant trait, such as height, seed shape, and flower color, and that each plant received one factor from each parent. His work anticipated the discoveries that chromosomes are the factors that transmit heredity and that parents contribute one of each member of a pair of chromosomes to their offspring.
Mendel’s work was initially ignored, however, while other theories of heredity were advanced. French naturalist Jean-Baptiste Lamarck proposed that characteristics acquired during an individual’s lifetime are passed to offspring. This idea was embraced by many 19th-century scientists, including the British naturalist Charles Darwin. Darwin and others believed that particles in the body, called gemmules, reside in the limbs and organs. The gemmules become imprinted with any changes acquired by the body, such as development of a strong heart through exercise. The gemmules then move to the reproductive cells and transfer information about the body’s alterations to these cells. The reproductive cells transmit the acquired traits to the offspring through particles called pangenes. Darwin’s theory of heredity, known as pangenesis, attempted to account for both the process of heredity and for the variety of traits seen among offspring.
In 1889 the German biologist August Weismann published his opposition to this view. His experiments with reproduction in jellyfish and similar animals led him to believe that variations in offspring result from the union of a substance from the parents. He referred to this substance as germ plasm. Other scientists observed the movement of chromosomes in cell division and suggested that chromosomes transmit the hereditary information from parent to offspring. About the same time, Aristotle’s belief that blood transmitted inheritance was disproved by the British scientist Francis Galton. To do this, Galton transfused blood from black rabbits into white rabbits. If traits were indeed transmitted through blood, those white rabbits should have produced black offspring, but their offspring were, in fact, white.
In 1900 several biologists independently theorized that the union of sperm and egg, which resulted in a combination of the male and female chromosomes, corresponds to Mendel’s description of inheritance through factors. With the rediscovery of Mendel’s principles, genetics studies accelerated. In the early decades of the 19th century, the work of the American geneticist Thomas Hunt Morgan spearheaded these investigations. Working with fruit flies, Morgan proved that Mendel’s factors, or genes (shortened from the word pangenes), are transmitted from parent to offspring through the action of chromosomes. Morgan also found that genes for many traits are arranged in a linear fashion on each chromosome. He created the first chromosome maps, which laid the groundwork for modern genetics.
During the first few decades of the 20th century, researchers established that chromosomes are composed of DNA and protein. At that time, it was widely held that proteins contained the genetic information. In 1928, however, the British scientist Frederick Griffith carried out experiments that ruled out proteins as the genetic material. In 1944, the American geneticist Oswald T. Avery and his colleagues clearly demonstrated that DNA carried the genetic information in bacteria. Avery’s work was not generally accepted until 1952, when American scientists Alfred Hershey and Martha Chase showed that the hereditary material of the T2 virus, a virus that infects bacteria, is also DNA. The work of Avery, Hershey, and Chase led scientists to the understanding that DNA is the heredity molecule for all organisms. Related experiments were carried out in the early 1940s by the American biologist George Beadle and the American geneticist Edward Tatum. Their investigations with the fungus Neurospora demonstrated that mutations in genes result in defective enzymes, the specialized proteins that speed up biochemical reactions. Thus, the link between genes and proteins was established.
While many researchers accepted the role of DNA in inheritance, they did not understand how it could transmit genetic information from one generation to the next. In 1953, American biochemist James Watson and British biophysicist Francis Crick proposed the now-famous double-helix model of DNA. They offered compelling evidence that DNA consists of two parallel strands twisted like a spiral staircase. The “banisters” of this staircase are formed from sugar and phosphate molecules. Other molecules, called bases, form the “stairs.” Watson and Crick demonstrated that one “stair” consists of a base pair, which is either an adenine bonded to a thymine or a cytosine bonded to a guanine. Hundreds of thousands of these paired bases run the length of a DNA molecule. The Watson-Crick model suggested that during cell division, the bond between the base pairs is broken, causing the strands of the double helix to separate. Each of the two strands serves as a template to construct a second strand of DNA, and two new DNA molecules are formed. The two DNA molecules are exactly the same, and one goes to each new cell, resulting in cells with the same hereditary information. The dramatic discovery of DNA architecture stimulated a quest to uncover its precise role in determining heredity.