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.
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.
AND SEXUAL REPRODUCTION
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.
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.
D Mitochondrial Inheritance 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’s 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 10,000-year-old mitochondrial DNA from Neandertal bones. 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.
IV OTHER PRINCIPLES OF HEREDITY 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.
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 Baptist de Monet 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.
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.
Drawing on the work of Beadle and Tatum, and using the Watson-Crick model of DNA, scientists determined that DNA must be a code that directs the construction of proteins. Proteins are built of small molecules called amino acids, which link together to form the protein. The amino acids must be lined up in a particular order, like letters in a correctly spelled word, for the protein to form correctly. Scientists inferred that DNA instructs the cell to link amino acids in the proper order. They further determined that a unique sequence of three bases on DNA, a triplet, is a code for one amino acid, and that unique triplets code for each of the twenty amino acids. In 1961 American biochemist Marshall W. Nirenberg and his colleagues began to unravel the code. Using an artificial mixture of amino acids and ribonucleic acid (RNA), a molecule similar to DNA, they showed that the base adenine repeated three times in a row is the code for the amino acid phenylalanine.
By 1967 scientists had translated the genetic code for all twenty amino acids. They had also confirmed that one gene, a section of DNA, is a code for one protein or part of a protein. Within 15 years, researchers had developed the capability of inserting genes from one organism into another, a breakthrough that ushered in the field of biotechnology. In the not-too-distant future, scientists may perfect the technology for inserting or removing genes from an egg, sperm, or embryo. This development may drastically alter the traditional principles of heredity, opening the door to a new array of rules governing the transmission of traits from parent to offspring.
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