"If we examine the accomplishments of man in his most advanced endeavors, in theory and in practice, we find that the cell has done all this long before him, with greater resourcefulness and much greater efficiency."-Albert Claude (1898-1983)
Have you ever looked at two siblings and known right off that they were related? Their body structure, facial features, colors of the skin, eyes, and hair are almost the same. It is clear that they have the same parents. But they may have a third sibling whose appearance is greatly different. Why would two children with the same parents look almost identical in some cases but extremely different in others?
The first clues to the answer came in the 1800s when Gregor Mendel began studying heredity in pea plants. He found that there are rules to inheritance of characteristics from parent to child, not just in humans but in every organism that reproduces sexually.
Since the time of Mendel, we have learned a lot about why traits are inherited. We know now that the rules explaining what features are passed from one generation to the next are all stored, as if they were a written code, in molecules of DNA. Genetics is the science that studies heredity and variation in organisms and how DNA carries information through one generation after another.
What is the purpose of DNA?
The cells of every living organism, from the simplest bacterium to humans, contain DNA molecules. These are very long strands (the DNA molecules in a human cell would measure about 1 meter long if stretched out) made up of chains of smaller molecules. What is the purpose of these DNA molecules?
Inherited traits are passed from one generation to the next through DNA. It stores the instructions a cell needs in order to make proteins. These proteins are the keys to a wide range of chemical reactions that direct the metabolism, growth, and specialization of living cells.
A molecule of DNA has a backbone structure with a sequence of four different molecules, called bases, attached to it. The sequence of the bases codes information about the organism in the same way that the sequence of letters of the alphabet codes the meaning of this sentence.
Every function of a cell and each trait of the organism is controlled by a gene, or sometimes a combination of genes, located on a particular section of DNA. Each gene is one segment of the DNA sequence, described by the pattern of hundreds or thousands of bases.
A simple bacterium may have a single strand of DNA holding about 1,000 genes. Plants and animals, including humans, have more genes— about 25,000 for humans. Plant and animal DNA molecules are arranged as two paired strands in the familiar double helix form. A human cell has 46 molecules of DNA, each of which has 50 to 250 million bases.
A gene is a region of a DNA molecule that controls the production of one or more specific proteins. All of the gene’s information is contained in the sequence of bases in its part of the DNA molecule. Each gene makes up one unit of inheritance.
Why can’t two parents with blue eyes have a child with brown eyes?
Many aspects of your appearance are the result of heredity—traits that you inherited from your parents. Frequently, features like hair color, eye color, skin tone, height, and general build run in families. Siblings often have similar appearances. Although there can be great variations within a family, there are some combinations of traits that cannot occur. For example, two parents with blue eyes never have a child with brown eyes. Why is this so?
In the 1800s, Gregor Mendel determined the basic principles of inherited traits by studying how specific traits are passed from one generation of plants to the next. In organisms that reproduce sexually, offspring inherit many traits from the parents. Each organism inherits two possible characteristics for a given trait—one from each parent—and passes on one of those two traits to each of its own offspring.
If the two inherited traits are different, one of them is dominant, which means the gene that controls it is activated. The other gene for the trait is recessive. Although it is not activated, it can be passed on to the next generation.
The gene for brown eyes is dominant over the gene for blue eyes. That means that a person with a gene for brown eyes and a gene for blue eyes will have brown eyes. If both parents have blue eyes, then neither of them has a gene for brown eyes, so none of their children can have brown eyes. On the other hand, a person with brown eyes can have a recessive gene for blue eyes. If both parents have a dominant gene for brown eyes and a recessive gene for blue eyes, their child can inherit two genes for blue eyes. Therefore, two parents with brown eyes can have a child with blue eyes but two parents with blue eyes cannot have a child with brown eyes.
Fast Facts
Inheritance is actually not quite as simple as one gene being dominant and the other recessive. Sometimes both genes can be activated at the same time. You can inherit a gene for straight hair or a gene for curly hair. A person with one of each gene is likely to have wavy hair, somewhere between straight and curly. To complicate things even further, some traits are controlled by a combination of two or more genes, each of which exists in the DNA in two forms, one from each parent.
Do all dogs belong to the same species?
A lion and a tiger are similar in size and if you look at their features, both are clearly cats. The two animals, however, come from two different species. A Great Dane and a Chihuahua are both dogs but one is many times larger than the other and their features differ from one another much more than the features of the two big cats. The two dogs, however, are considered to be from the same species. Why are very different dogs from the same species while similar cats represent different species?
Part of the answer lies in the definition of species. The original concept of species was introduced as an organizing tool when scientists thought that life on Earth fell cleanly into well-defined categories that were completely separate from one another. Unfortunately (or maybe fortunately), the natural world is not always so clear. Scientists do not completely agree on a current definition of species.
"I look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other … it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, for convenience’s sake." —Charles Darwin (1809-1882)
In the most familiar definition, a species is a group of organisms capable of interbreeding and producing fertile offspring. Other definitions look at the isolation of one group from another in nature, considering groups that would have no opportunity to mate as different species, even if they could physically produce offspring. Recently, more precise definitions probe similarities in DNA to classify relationships.
Practically speaking, a Great Dane and a Chihuahua cannot mate due to the extreme size difference. However, the Great Dane can mate with a smaller dog, such as a German Shepherd, which can produce offspring with a collie. The collie in turn could fall for a beagle, producing cute and fertile offspring. And who wouldn’t want a beagle-Chihuahua mixed-breed pup?
There is always variation in a species: each zebra has its own pattern of stripes; polar bears vary in size within a certain range; each gorilla in a band has its own personality and ways of interacting with others. No other species, however, shows the wide range of sizes, shapes, and behaviors as you find with dogs. All of the dogs you know can trace their ancestry back to wolves, but today very few of them could ever be mistaken for a wolf. Why would this particular species be so varied compared to others?
In part, it’s because dogs live in a wide environmental range. Wild dogs adapted to cold regions would have longer fur and bigger bodies than those from warm places. The major cause of the diversity, however, appears to be us—humans. People have been breeding dogs for at least 12,000 years. Many dogs were bred for specific reasons: chasing fleet game animals, following small pests into holes, detecting and warning against intruders. Within the last few centuries, appearance became a primary driver among dog breeders. Each breed has an ideal appearance that drives the pairings.
Fast Facts
Appearance in not the best indicator of how closely related one species is to another. To determine relationships between species, scientists must compare DNA—the more similar the DNA, the more closely the species are linked genetically. Although they all look and act in similar ways, DNA analysis has shown that all dogs are descendants of wolves, but not of coyotes or jackals. The coyotes and jackals are distant cousins.
Most of the dogs that you see around you owe their appearance, not to natural selection, but to an artificial selection process driven by humans. The species adapts by filling many different niches tied to human wants and needs rather than by adapting to pressures of the natural environment.
What was the purpose of the Human Genome Project?
The Human Genome Project (HGP), sponsored by the U.S. Department of Energy and National Institutes of Health, lasted for 13 years, studying the basis of human heredity—the entire set of genes that determines the characteristics of an individual. The goal was a complete map, showing the location of each gene on the human DNA molecules. What is the value of this map and how could it be used?
The HGP had several goals: to identify all of the genes in human DNA, determine the sequence of the approximately 3 billion bases, develop tools to analyze the vast amount of information, transfer the technologies to the private sector, and address ethical, legal, and social issues. The mapping of the genome was completed in 2003 but the analysis will continue for many years.
An organism’s genome is its complete set of DNA. The human genome contains about 6 billion base units. Most of the genome is identical for all humans, but a small part of it differs. Within these differences are the genes that make each person unique.
Because each individual has different DNA, the map was made using samples from several individuals. It does not show the exact sequence from any one person, but identifies the general sequence of human DNA and the specific areas that correspond to genes.
Scientists expect to develop many benefits from knowledge of the human genome. Many diseases are either caused or affected by our genes.
Each person has a unique sequence and the differences in those sequences can be correlated to the risk of developing certain diseases. One practical benefit of knowing which genes are related to diseases is the ability to determine which people are more likely than others to contract the diseases. Tests have already been developed to predict a genetic tendency to develop breast cancer, cystic fibrosis, and Alzheimer’s diseases in some cases. Knowing that you have a genetic tendency toward one of these diseases can help determine treatments or prevention measures.
Understanding the human genome is not a magic bullet that will allow us to understand the causes and prevent all diseases. Many diseases can be traced to environmental, not hereditary sources. Some diseases that appear to run in families may be caused by shared habits and environmental conditions, rather than genetic causes. Even in cases where specific genes are associated with a particular disease, for example, breast cancer, the genetic code generally only shows a tendency toward the disease. Usually other factors determine whether a person is affected or not.
Different people respond to drugs in different ways. Some drugs are extremely effective in certain people but ineffective or even dangerous to others. Knowing how genetic differences influence the way drugs interact with people’s bodies could allow doctors to find the right drug and the right dose for treatment of a specific person.
Beyond determining the best drug for treatment, there is a possibility of applying genetic information to create and adapt drugs. Ultimately, genetic therapy may provide a way to cure, or at least manage, genetic diseases by making changes in a patient’s DNA itself.
How does cloning work?
In 1997, Scottish scientists announced the results of an experiment in which they had developed a sheep named Dolly by manipulating cells in a laboratory. Unlike sheep with two parents, Dolly had genes that were identical to those of her mother. How can a new organism be created by cloning?
When we talk about cloning an organism, we mean creating a genetic duplicate of the organism—that is, one that has the same DNA. Although Dolly made a lot of news, cloning was around long before the appearance of the first cloned sheep. In nature, single-celled organisms reproduce by cloning when the cell splits into two new cells with identical genes. Gardeners love to make copies of a favorite plant by rooting a stem cutting. Identical twins start out as a single fertilized cell, which splits to form two genetically identical embryos—clones of one another.
Fast Facts
Although articles about cloned animals often refer to them as identical copies, they are not exactly the same. The DNA that controls inherited characteristics is located in the nucleus, but there is also a second source of DNA, the mitochondria. The mitochondria are small structures in the cell that are involved in energy production. Mitochondria are always inherited from the mother because they are part of egg cells but not sperm cells. Mitochondrial DNA represents only a small part of the DNA in a cell. A clone created by nuclear transfer is actually not as closely related to its nuclear parent as identical twins are related to one another.
If you know any identical twins, you are probably aware that they are not identical. Twins can have very different personalities and, although their physical features are generally quite similar, they are not absolutely identical. It is their DNA that is the same, but many other factors influence individual people or individuals of any other organism.
Dolly, the sheep, however, was created by a different process with the same result.
Scientists removed the nucleus from an egg cell of a sheep, keeping the rest of the cell intact. They then isolated a single cell from another adult sheep and removed its nucleus. When they transferred this nucleus to the egg cell, they created a new complete cell with genetic information identical to that of the adult sheep. This cell then divided to form an embryo, which was implanted into the uterus of a surrogate mother. Dolly then developed into a clone of her parent, the donor of the nucleus.
Can humans be cloned to produce copies of people?
The advances in cloning technology that led to the creation of Dolly raised many questions about cloning that had not previously been discussed. One particular question—whether the same techniques can be used for humans—has raised a number of other technical and ethical questions. Is it possible to clone humans, and if so, should we do so?
Reasons that have been proposed for cloning humans include helping infertile couples produce genetically related offspring and bringing deceased relatives back to life. The process of cloning humans should be the same as that of cloning sheep, so biologists believe that humans could be cloned for these purposes, or others.
There are some ethical concerns, though, that do not necessarily apply to the cloning of animals (although not everyone agrees on application to animal cloning). Most of the time, cloning does not work. Many embryos die after being implanted in the uterus. In addition, a large percentage of the offspring die before, or shortly after, birth. Furthermore, many of the animals that survive suffer from defects in their hearts, lungs, or other organs and malfunctioning immune systems.
There is a second type of human cloning that avoids some, but not all, of the ethical issues. In therapeutic cloning, a nucleus taken from a person is inserted into a donor egg to form an embryo with that person’s DNA. The embryo is not implanted into a woman’s uterus to grow into a baby, but instead is allowed to divide several times to produce stem cells. The stem cells are removed and used to grow any type of tissue, which can be implanted to the donor of the original nucleus. Because they are genetically identical, these cells will not be rejected by the immune system, a major problem in organ or tissue transplants. Therapeutic cloning has been proposed as a possible cure for many diseases, including Alzheimer’s and Parkinson’s.
The main ethical concern of therapeutic cloning is the creation of an embryo specifically for destruction as the stem cells are harvested.
How does human DNA compare to animal and plant DNA?
All plants and animals have DNA that forms double strands. The organisms that are described by the DNA molecules are very different, though. How much difference is there in the DNA of different organisms?
In many ways, the DNA molecules of all organisms are the same. Bacteria, plants, spiders, and humans all have DNA that is coded with the same four bases. These bases provide a code for manufacturing proteins. Some of the processes that occur inside our bodies also occur in other organisms—even plants. For example, when we eat foods containing sugars from plants, the sugar molecules provide energy to our bodies. The same process breaks down the sugars inside the plant itself. Some of the genes used by plants to produce proteins used during the conversion of sugar to energy may be identical to the human genes that produce the same proteins.
Comparisons of the genomes of humans and those of several apes indicate that the chimpanzee is humanity’s closest relative. A comparison of the genes in chimpanzee DNA and human DNA show that about 98.7 percent of the genes are the same. All of the other apes appear to be more distant relatives of the chimps. A study of the genome of chickens shows that we even share about 60 percent of our genes with them.
Fast Facts
Is a mushroom more closely related to a marigold or a sparrow? This may seem like a strange question, but the answer is even stranger. While fungi and plants may seem to be closely related to one another, genetic studies have shown that fungi actually have a closer relationship to animals. It is true that the relationship is very distant, but certain DNA sequences and protein production in cells have shown that fungi are more like animals (including humans) than plants. This may explain why certain fungal infections are very tenacious. It is hard to find substances that will attack these fungi but not their hosts.
Why are some genetic diseases more common in men than in women?
During the eighteenth and nineteenth centuries, a number of European nobles suffered from a disease called hemophilia, in which blood does not clot normally. The disease can be very dangerous, particularly if injuries lead to uncontrolled internal bleeding. Hemophilia is a hereditary disease, caused by a defective gene. Hemophilia is common among all races. Children with hemophilia are almost always boys, although it is inherited from their mothers’ families. Why would a disease affect one gender substantially more often than the other?
Hemophilia is an example of an X-linked hereditary disease. It is caused by a defective gene on a particular chromosome that differs by gender. Humans have 23 pairs of chromosomes, each pair containing one strand of DNA from each parent.
However, in one of the chromosome pairs— the one that determines the gender of the offspring—the two strands do not contain the same number of genes. In the twenty-third chromosome pair, females have two completely matched DNA molecules. In males, however, the DNA strand from the mother is longer than the one from the father. The female chromosome is labeled as X and the male chromosome is labeled Y (based on their shapes as observed through a microscope).
In all other chromosome pairs, there are two genes and the dominant gene determines the person’s characteristic. In males, all of the genes in one end of the chromosome pair are donated by the egg cell from the mother. If one or more of the genes on this segment are defective, the trait will be observed, even if it is recessive. This is because there is no corresponding gene from the father.
Several hereditary diseases are linked to the X chromosome, including hemophilia and color blindness. In general, these diseases appear in males. A girl will only have the disease if her father has it and her mother carries the recessive gene for it. Half of the boys whose mothers carry the gene will have the disease, regardless of whether their fathers have it or not.
A chromosome is a single strand of DNA, packaged together with certain proteins. Every human cell, except for sex cells, has 46 chromosomes arranged in 23 pairs. Each chromosome has hundreds or thousands of genes. The sperm and egg cells in humans each contain 23 chromosomes, 23 pairs when the egg is fertilized.
How do DNA tests work?
Every crime show has a lab that performs a DNA test in order to find absolute evidence of the identity of the criminal. A sample is placed in the machine and it spits out the name of someone whose DNA is an exact match within seconds. While the results actually take much longer to obtain than shown on TV, DNA testing is a powerful identification tool. It has been used both to convict and to exonerate people accused of a crime, establish paternity with very close to 100 percent accuracy, and identify victims of accidents who could not be identified by any other means. How does DNA testing work?
Most of your DNA (in fact, more than 99 percent of it) is exactly the same as that of your parents, your neighbors, and even a random stranger from the other side of the world. There are some sections, however, that vary from person to person. It is this small fraction, spread throughout the total genome, that makes you the unique person that you are. No one else has the same sequence of bases in all of these sections of DNA unless you have an identical twin.
DNA analysis looks at some of these specific sections, called markers. A small sample of DNA is taken from cells found in body fluids, skin, hair follicles, or the inside of the cheek. After the DNA is isolated from the cells, millions of copies are made using an enzyme that speeds DNA reproduction. Other reactions then break the DNA molecules apart at specific locations to isolate particular markers. These markers are compared to the unknown sample.
Any one specific marker is shared by many people, but the chance of two people having two identical markers is much smaller. In an ideal situation, we would look at the whole DNA sample, comparing all the possible markers to make identification. With today’s technology, though, this is not possible. DNA analysts look at a number of different markers to create a "DNA fingerprint." The more markers identified, the better the chances that the match between two samples is accurate.
A DNA test using a blood sample is no more accurate than a test using cells from a cheek swab. The swab does not collect saliva for the tests. It actually picks up loose cells that were shed by the tissue of the inside of the mouth. Because all cells contain the same DNA, the swab will provide the same DNA as a blood sample. If a hair sample is used, it must be pulled, not cut, because hair is not made of living cells. When a hair is pulled, however, it carries with it some cells from the living follicle.
Without looking at the whole of a person’s DNA, it is not possible to absolutely identify a particular person as the source of a DNA sample. In the case of identical twins, it is never possible. However, by using many markers, the odds of a correct identification are extremely favorable. Normally, between 6 and 13 markers are compared. At the higher end of this range, it is extremely unlikely that an incorrect identification will occur, but not absolutely impossible.
On the other hand, it is easy to identify a mismatch in DNA analysis. If any of the markers is different, then the DNA does not match. In a criminal investigation, therefore, a suspect can be eliminated as the source of a sample based on the absence of a single marker. All DNA from a particular person matches from one end of each molecule to the other.
Because samples can remain useful for a very long time, DNA analysis has been used to reopen cases that were once considered solved. The results have far greater reliability than other forms of identification, including eyewitness testimony. A number of people have been cleared of crimes of which they had been convicted, sometimes decades earlier, by analysis of evidence held in storage.
Can DNA analysis be used for anything other than criminal investigation?
The most familiar (thanks to television) use for DNA testing is forensic analysis of crime scene samples for evidence to use in the identification of a criminal. While this is an important application of the ability to analyze DNA, it is only one of many investigations where DNA information is useful. In what other ways do scientists use DNA?
Even within forensic analysis, there are other ways to use DNA. In some cases, it is not only the suspect who must be identified but the victim as well. Identification of human remains if they are severely decomposed is a bit tricky. Many of the features we use day to day to tell one person from another, such as facial shape, are made of soft tissue and are destroyed over time after death. Dental records are often used but they are useful only when the jaws and teeth are available and when a record exists for comparison.
DNA analysis provides another tool for identification. Only a tiny sample of DNA is necessary for the analysis. Comparison samples can come from the possible victim’s toothbrush, hairbrush, or other personal sources. Even when such a sample is not available, markers can be compared to those of close relatives. The largest single DNA identification project took place after the attack on the World Trade Center in 2001. More than 1,600 victims of that disaster have been identified by DNA analysis of bodies and parts of bodies recovered from the scene.
Archaeologists rely on DNA to establish relationships among ancient cultures. Samples from archaeological sites have shed new light on human migrations, traced interactions among cultures, and identified mummies and other human remains. Information about the changes in DNA over time provides new insight into the origins of modern humans and the relationships between modern humans and earlier people, such as the Neanderthal people.
Of course, because DNA is a substance common to all life, DNA analysis is not limited to humans. There are an almost countless number of applications where it is useful to know relationships between living organisms or the source of materials in which DNA analyses are powerful tools. Examples include detecting bacteria and other organisms that cause pollution, tracing the source and development of diseases, and identifying poaching of endangered wildlife species.
Although it is not possible to build dinosaurs from ancient DNA, as shown in Jurassic Park, scientists have found that some DNA lasts a long time. Ice core samples in Siberia have yielded DNA from large mammals that lived 30,000 years ago and from plants that grew as long as 400,000 years ago. Although a few older samples have been reported, there is a possibility that the samples became contaminated over time and the DNA that was recovered was actually produced in more recent times.
How do paternity tests work?
Before the development of DNA testing, paternity testing often depended on matching of blood types. Because so many people share each of the blood types, however, the test can really only be used to eliminate the possibility of paternity rather than to identify the father. DNA paternity testing is much more accurate. How can DNA be used to determine whether people are related?
A paternity test is based on the fact that your entire DNA comes from your parents. One half of your DNA exactly matches that of your mother and the other half exactly matches that of your father. DNA samples are prepared from the child, the mother, and the possible father following the same procedures used to match samples in criminal investigations.
For a paternity test, however, we are not looking for an exact match. On average, half of the markers will be found in the mother’s DNA. These markers are ignored. The other markers are compared to the DNA sample from the man who may be the child’s father. If any of the markers do not match, then it is not possible that he is the father. The confidence of a positive result, in which all markers match, depends on the number of markers tested. In general, paternity tests identify enough markers to report a 99.9 percent probability that the positive identification is accurate.
"The time with which we have to deal is of the order of two billion years. What we regard as impossible on the basis of human experience is meaningless here. Given so much time, the ‘impossible’ becomes possible, the possible probable, and the probable virtually certain. One has only to wait: time itself performs the miracles."
