Data science involves the study of large, complex data sets that arise from various types of research projects. With respect to genomic studies, such work requires expertise in quantitative scientific disciplines such as bioinformatics, computational biology and biostatistics.

Deletion is a type of mutation involving the loss of genetic material. It can be small, involving a single missing DNA base pair, or large, involving a piece of a chromosome.

Deletion really means that something is missing. And as a geneticist talking about deletion it means something is missing of the genetic material. And it can be something small, just a base pair; it can be something larger; it can be part of a gene; it can be even larger; it can be an entire gene; or yet larger again, it can be part of the chromosome. And depending upon what it is, you have to look at it in different ways. You can find a deletion in a chromosome just by doing a cytogenetic or chromosome analysis, or a deletion in a gene you can find out by sequencing the DNA. So when you have a deletion, depending upon the size, it can have different effects. What was the most surprising to me was that just by having a deletion of one base pair, you can have the most severe birth defect, and sometimes by missing an entire chromosome, you don't even see all that much compared to just having a deletion of a small base pair. Different deletions can lead to different findings, and they can affect just behavior; they can affect how a child, how a person looks; they can affect a very severe problem that the child may die at birth; or they can affect something that just has to do with eye color, hair color, with weight or height of the person.

Maximilian Muenke, M.D.

Deoxyribonucleic acid (abbreviated DNA) is the molecule that carries genetic information for the development and functioning of an organism. DNA is made of two linked strands that wind around each other to resemble a twisted ladder — a shape known as a double helix. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G) or thymine (T). The two strands are connected by chemical bonds between the bases: adenine bonds with thymine, and cytosine bonds with guanine. The sequence of the bases along DNA’s backbone encodes biological information, such as the instructions for making a protein or RNA molecule. 

Is there a more amazing molecule than DNA? It makes each of us who we are. The more scientists understand it, the more we all understand ourselves, one another, and the world around us. For example, did you know that we are all far more alike than we are different? In fact, the DNA from any two people is 99.9% identical, with that shared blueprint guiding our development and forming a common thread across the world. The differing 0.1% contains variations that influence our uniqueness, which when combined with our environmental and social contexts give us our abilities, our health, our behavior. How can one, single molecule contain so much mystery and wonder? We are only beginning to understand the answer to that question, which is what makes the study of DNA so exciting.

Diploid is a cell or organism that has paired chromosomes, one from each parent. In humans, cells other than human sex cells, are diploid and have 23 pairs of chromosomes. Human sex cells (egg and sperm cells) contain a single set of chromosomes and are known as haploid.

Diploid is the term that refers to the number of each type of chromosome that an organism has. And diploid specifically means every cell in that organism has two copies of each type of chromosome. So humans are diploid, and each human has, then, in the nucleus of their cells two copies of Chromosome 1, two of number two, two of number three, all the way out to two of number 22, and then if you're a female you have two copies of the X chromosome, if you're a male you have an X and a Y. So diploid means you have two of each type of chromosome.

DNA fingerprinting is a laboratory technique used to determine the probable identity of a person based on the nucleotide sequences of certain regions of human DNA that are unique to individuals. DNA fingerprinting is used in a variety of situations, such as criminal investigations, other forensic purposes and paternity testing. In these situations, one aims to “match” two DNA fingerprints with one another, such as a DNA sample from a known person and one from an unknown person.

DNA fingerprinting. I think a lot of people are first introduced to DNA fingerprinting while watching crime shows. An officer collects some samples from the crime scene. They put it in a tube. And then an hour later, they hold up a brightly colored gel, squint at it, and say, aha, we have a match for the killer's DNA. Then the show is over. Of course, that isn't exactly how things work in real life. But DNA fingerprinting is an important part of forensic science. Although it can't really tell you exactly who committed a crime, it can be used to help narrow down a list of suspects based on how well their DNA matches the samples that were found at the crime scene. Investigators can also use the DNA results to search specific databases to find other potential suspects.

DNA replication is the process by which a molecule of DNA is duplicated. When a cell divides, it must first duplicate its genome so that each daughter cell winds up with a complete set of chromosomes.

DNA replication is probably one of the most amazing tricks that DNA does. If you think about it, each cell contains all of the DNA you need to make the other cells. And we start out from a single cell and we end up with trillions of cells. And during that process of cell division, all of the information in a cell has to be copied, and it has to be copied perfectly. And so DNA is a molecule that can be replicated to make almost perfect copies of itself. Which is all the more amazing considering that there are almost three billion base pairs of DNA to be copied. And replication uses DNA polymerases which are molecules specifically dedicated to just copying DNA. Replicating all of the DNA in a single human cell takes several hours of just pure copying time. At the end of this process, once the DNA is all replicated, the cell actually has twice the amount of DNA that it needs, and the cell can then divide and parcel this DNA into the daughter cell, so that the daughter cell and the parental cell in many case are absolutely genetically identical.

DNA sequencing is a laboratory technique used to determine the exact sequence of bases (A, C, G, and T) in a DNA molecule. The DNA base sequence carries the information a cell needs to assemble protein and RNA molecules. DNA sequence information is important to scientists investigating the functions of genes. The technology of DNA sequencing was made faster and less expensive as a part of the Human Genome Project.

DNA consists of a linear string of nucleotides, or bases, for simplicity, referred to by the first letters of their chemical names--A, T, C and G. The process of deducing the order of nucleotides in DNA is called DNA sequencing. Since the DNA sequence confers information that the cell uses to make RNA molecules and proteins, establishing the sequence of DNA is key for understanding how genomes work. The technology for DNA sequencing was made faster and less expensive as a part of the Human Genome Project. And recent developments have profoundly increased the efficiency of DNA sequencing even further.

Eric D. Green, M.D., Ph.D.

Dominant refers to the relationship between two versions of a gene. Individuals receive two versions of each gene, known as alleles, from each parent. If the alleles of a gene are different, one allele will be expressed; it is the dominant gene. The effect of the other allele, called recessive, is masked.

Dominant refers to a relationship between two versions of a gene. If one is dominant, the other one must be not dominant. In that case, we call it recessive. A dominant gene, or a dominant version of a gene, is a particular variant of a gene, which for a variety of reasons, expresses itself more strongly all by itself than any other version of the gene which the person is carrying, and, in this case, the recessive. Now, it usually refers to inheritance patterns frequently used in conjunction with a Punnett square where, if an individual has two versions of a gene, and one is observed to frequently be transferred from one generation to another, then it is called dominant. Biochemically, what is going on in this case is that the genetic variation, for a variety of reasons, can either induce a function in a cell, which is either very advantageous or very detrimental, which the other version of the gene can't cover up or compensate for. In that case, you're going to have a dominant mutation, and that dominant mutation can be benign. It can refer to eye color of one sort or another; that can be can a dominant mutation. Or it can refer to a disease. Huntington's disease, for instance, is a dominant mutation where, if one is carrying that version of the Huntington gene, that mutation, that dominant mutation, will give the individual the disease regardless of what that person's other Huntington's disease gene allele is. That other Huntington's disease gene allele can be perfectly normal, but the person still has the disease because of that one copy of the Huntington's disease gene that is mutated. That is dominance.

Christopher P. Austin, M.D.

Dominant, as related to genetics, refers to the relationship between an observed trait and the two inherited versions of a gene related to that trait. Individuals inherit two versions of each gene, known as alleles, from each parent. In the case of a dominant trait, only one copy of the dominant allele is required to express the trait. The effect of the other allele (the recessive allele) is masked by the dominant allele. Typically, an individual who carries two copies of a dominant allele exhibits the same trait as those who carry only one copy. This contrasts to a recessive trait, which requires that both alleles be present to express the trait.

Dominant traits and alleles. Dominant refers to the inheritance of traits that are typically passed vertically from parent to child where both the parent and the child are affected by the trait or disorder that is related to that gene. The most common form is autosomal dominant, where the associated gene is on one of the 22 non-sex chromosomes, and where the affected individual harbors two alleles of that gene, one pathogenic and one benign. In this inheritance pattern, harboring one pathogenic allele is sufficient to cause the person to have the trait. Then, an affected person having only one of two copies of the gene being pathogenic is what leads to the 50% chance of inheritance of the trait in their offspring.

Leslie G. Biesecker, M.D.

Double helix is the description of the structure of a DNA molecule. A DNA molecule consists of two strands that wind around each other like a twisted ladder. Each strand has a backbone made of alternating groups of sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G), or thymine (T). The two strands are held together by bonds between the bases, adenine forming a base pair with thymine, and cytosine forming a base pair with guanine.

A double helix has become the icon for many, many kinds of discussions about where science has been and where it's going. This really is an amazing structure. You can't stare at the double helix for very long without having a sense of awe about the elegance of this information molecule DNA, with its double helical form basically being the way in which all living forms are connected to each other, because they all use this same structure for conveying that information. Of course, this is Watson and Crick's incredible realization back in 1953, but it will stand in history as probably one of the most significant scientific moments of all time.

Francis S. Collins, M.D., Ph.D.

Down syndrome is a genetic disease resulting from a chromosomal abnormality. An individual with Down syndrome inherits all or part of an extra copy of Chromosome 21. Symptoms associated with the syndrome include mental retardation, distinctive facial characteristics, and increased risk for heart defects and digestive problems, which can range from mild to severe. The risk of having a child with Down syndrome rises with the mother's age at the time of conception.

Down syndrome is a condition that was first described in the mid-1860s by Dr. Langdon Down in England who noticed that there are number of individuals who had some of the same sorts of facial features and also tended to have impaired intellectual abilities. Back in the 19th century, there were no good ideas to what caused Down syndrome. Actually, it took almost 100 years after Dr. Down first noticed this condition for us to figure out that it's caused by having an extra copy of Chromosome 21, which is often referred to as Trisomy 21 for the fact that there are three, or "tri", copies of Chromosome 21 rather than the usual two copies of the chromosome that most people have. We know that by having this extra chromosome that it causes some of the problems with brain development that can lead to the impaired intellectual development that's common in individuals with Down syndrome, as well as some of the typical facial and other features that we've seen in individuals with Down syndrome. But we've also come to learn that while there are some commonalities amongst individuals with Down syndrome, there is great diversity amongst people with Down syndrome. Great diversity in terms of intellectual abilities and great diversity in terms of physical features and other sorts of things.

Duplication is a type of mutation that involves the production of one or more copies of a gene or region of a chromosome. Gene and chromosome duplications occur in all organisms, though they are especially prominent among plants. Gene duplication is an important mechanism by which evolution occurs.

Duplications occur when there is more than one copy of a specific stretch of DNA. This can occur in several different contexts. During a disease process, extra copies of the gene can contribute to a cancer. Genes can also duplicate through evolution, where one copy can continue the original function and the other copy of the gene produces a new function. On occasion, whole chromosomes are duplicated. In humans this causes disease. Throughout evolution, there have been several occasions, both in fish and plants, where whole genomes have been duplicated.

Lawrence C. Brody, Ph.D.

Electrophoresis is a laboratory technique used to separate DNA, RNA, or protein molecules based on their size and electrical charge. An electric current is used to move molecules to be separated through a gel. Pores in the gel work like a sieve, allowing smaller molecules to move faster than larger molecules. The conditions used during electrophoresis can be adjusted to separate molecules in a desired size range.

Electrophoresis is a very broadly used technique which, fundamentally, applies electric current to biological molecules, whether--they're usually DNA, they can be protein or RNA, too...and separates these fragments into pieces which are larger or smaller. It's used in a variety of applications... Everything from forensics for determining the identity of individuals that may have been involved in a crime, by linking their DNA pattern, their electrophoresis pattern, to one that's in a database. The whole basis by which the human genome was done is by something called capillary electrophoresis, by separating DNA into shorter pieces and then running them on these electrophoresis gels which allow the patterns of As, Cs, Ts, and Gs to be elucidated. They're also very important in protein research, and then genetic mutation research, because when proteins or DNA are mutated, they are frequently longer or shorter, and they therefore show up on an electrophoresis gel differently than normal, so many diagnostic tests are still done using electrophoresis, so it's a very widely used basic research technique, was very important for the understanding of gene and protein function, but it's now gotten into the area of clinical diagnostics and forensics as well. Electrophoresis is usually done in what looks like a box which has a positive charge at one end and a negative charge at the other. And as we all learned in basic physics, when you put a charged molecule into an environment like that, the negative molecules go to the positive charge, and vice versa. In looking at proteins in a gel, in one of these boxes, you usually take the entire protein, and you're looking at the entire length of the protein and seeing how big it is, and the bigger it is, the shorter it will migrate into the gel, so that the small proteins will end up at the bottom of the gel, because they have migrated the farthest, and the biggest ones will wind up staying at the top. In the case of DNA, DNA's a very long molecule, so you wouldn't want to run, for the most part, a whole DNA molecule from a cell onto a gel. It's just so big that it would never get into the gel, so what scientists do, and what people do in classrooms these days, is to chop up that DNA using things like inscription enzymes, which chop up the DNA into more manageable pieces in a reproducible way. And then those pieces, depending on how big the pieces are, migrate more or less into the gel father down the box from top to the bottom.

Christopher P. Austin, M.D.

Environmental factors, as related to genetics, refers to exposures to substances (such as pesticides or industrial waste) where we live or work, behaviors (such as smoking or poor diet) that can increase an individual’s risk of disease or stressful situations (such as racism). Genetic studies often take environmental factors into consideration, as these exposures can increase an individual’s risk of genetic damage or disease.

Environmental Factors. Many things in the environment impact our health. This includes pollution in the air we breathe, carcinogens in the food we eat, pesticides, lead, increased computer screen time, you name it. Understanding environmental factors is important for understanding genomics. Environmental factors can lead to genetic damage that causes disease. There are also often gene-environment interactions, or a complex interplay between genes and environment, that underlie the risk and development of disease. Since the end of the Human Genome Project, we have seen dramatic improvement in our ability to measure genetic variation. Accuracy has gone up, costs have gone down. Meanwhile, the environmental factors remain numerous, complex, difficult to standardize, and highly variable over time. A key challenge to understanding the influence of genomics on disease will involve figuring out how to best measure, integrate, and analyze environmental data as part of our genomic studies.

Carolyn M. Hutter, Ph.D., Director, Division of Genome Sciences

Epigenetics is an emerging field of science that studies heritable changes caused by the activation and deactivation of genes without any change in the underlying DNA sequence of the organism. The word epigenetics is of Greek origin and literally means over and above (epi) the genome.

Epigenetics is the study of changes in gene function that are heritable and that are not attributed to alterations of the DNA sequence. The term epi means above. It's a Greek prefix. It's also defined as on top of the basic DNA sequence. In general terms you can think of them like accent marks on words where the DNA is the language and the modifications are the accent marks. Epigenetic marks change the way genes are expressed. The promise of epigenetics is that it tells us about the cell, it's a way to define the cell that's different than just looking at gene expression levels. We could look at any kind of cell and it will have specialized epigenetic patterns. There are two types of modifications: DNA methylation and histone modification. DNA methylation goes awry in cancers so if we knew the normal pattern of methylation and then looked at the pattern of methylation in a tumor we could see what changes were taking place and we could see which genes were being affected.

Laura Elnitski, Ph.D.

Epistasis is a circumstance where the expression of one gene is affected by the expression of one or more independently inherited genes. For example, if the expression of gene #2 depends on the expression of gene #1, but gene #1 becomes inactive, then the expression of gene #2 will not occur. In this example, gene #1 is said to be epistatic to gene #2.

Epistasis is actually a complicated term that's misused quite a bit in the literature. But it refers to a circumstance where the expression of one gene is affected by the expression of one or more independently inherited genes. Okay, that doesn't really make it much easier. Let me give you an example. If the expression of gene 2 depends upon the expression of gene 1, but gene 1 becomes inactive, it actually also effects the expression of gene 2. So you can't really tell what's going on with gene 2 if gene 1's altered. Gene 1 is then said to be epistatic to gene 2. Okay, that also sounds kind of complicated. Let me give you an example. If you look at the color of animals... One of the genes that's needed to make color is tyrosinase, and if you don't have tyrosinase, if it's mutated, it results in a condition call albinism. So the fur or the skin's completely white because you can't make the pigment. If there's another gene involved in making pigment--for example, making brown pigment--if you're wild-type for tyrosinase, you can see the brown mutation, and you have a brown animal. But if you have a tyrosinase mutation and a brown mutation, it still looks like tyrosinase mutation; it's all white. You don't see whether it has the brown mutation or not. So therefore, tyrosinase is epistatic to brown.

William Pavan, Ph.D.

Eugenics is a discredited belief that selective breeding for certain inherited human traits can improve the “fitness” of future generations. For eugenicists, “fitness” corresponded to a narrow view of humanity and society that developed directly from the ideologies and practices of scientific racism, colonialism, ableism and imperialism.

Eugenics. Using unscientific ideas and immoral and unethical means, eugenics sought to solve the problems of society by eliminating the “unfit” and encouraging the “fit” to increase in number. The American Eugenics Movement was one of the most active and extreme manifestations, inspiring aspects of eugenics ideologies and practices in Germany, Czechoslovakia, and elsewhere around the world. While eugenics reached its peak in popularity during the Second World War, eugenics practices remained widespread globally until the 1970s. And in many cases, sadly, continue today.

Christopher R. Donohue, Ph.D., Historian, NHGRI History of Genomics Program

Evolution is the process by which organisms change over time. Mutations produce genetic variation in populations, and the environment interacts with this variation to select those individuals best adapted to their surroundings. The best-adapted individuals leave behind more offspring than less well-adapted individuals. Given enough time, one species may evolve into many others.

Evolution, as Darwin described it in "The Origin of Species" in 1859, had several components; three to be exact. First of all, evolution proposes that all organisms are descended from a common ancestor, or at least a very small set of common ancestors--still some debate about that, but probably a single common ancestor. Evolution says that there is gradual change over time, which we know is due to mutations in DNA. And then third, and very important, natural selection operates upon, over those long time periods, the changes that occur, to result in emergence of species that have particular abilities to survive in a niche that the environment has provided. So put those three things together...you've got evolution.

An exome is the sequence of all the exons in a genome, reflecting the protein-coding portion of a genome. In humans, the exome is about 1.5% of the genome.

Proteins are encoded by genes. And genes consist of two major components, exons and introns. Exons contain the nucleotides that directly encode for proteins, whereas introns are stretches of DNA between the exons and do not encode for proteins. The entire collection of all the exons from all the genes in a genome is called an exome. In the case the human genome, the exome only corresponds to about 1.5% of the genome’s roughly 3 billion nucleotides. Genome scientists have developed laboratory methods that allow them to just sequence a genome’s exome; in other words, just the part of the genome that directly encodes for proteins. So, you will often hear genomicists talk about an exome sequence, which is a very small part of the overall genome (or whole-genome) sequence.

Eric Green, M.D., Ph.D., Director, Office of the Director

An exon is the portion of a gene that codes for amino acids. In the cells of plants and animals, most gene sequences are broken up by one or more DNA sequences called introns. The parts of the gene sequence that are expressed in the protein are called exons, because they are expressed, while the parts of the gene sequence that are not expressed in the protein are called introns, because they come in between--or interfere with--the exons.

Exons are that part of the RNA that code for proteins. Now, RNA, when it first gets transcribed, is a very, very long piece of RNA molecule. And really, the important parts of that RNA are the exons. There are large, large chunks of RNA that get excised out. Now, it's important to remember that because I use the term excised doesn't mean that exons go away. The exons are what stay in the mature mRNA and eventually code for amino acids. Many times, including medical students like my wife, will forget whether it's the exons that code for the amino acids or the introns that code for the amino acids. Let me set the record straight that it's the exons that code for the amino acids, because sometimes people try to remember that exons get excised, but that's not true. It's that introns interfere. So you always have to remember that introns interfere, and the introns get excised out of the RNA to leave a string of exons together that will eventually code for the amino acids.

A family history is a record of medical information about an individual and their biological family. Human genetic data is becoming more prevalent and easy to obtain. Increasingly, this data is being used to identify individuals who are at increased risk for developing genetic disorders that run in families.

When you think about your family's genealogy, you often think about past generations and maybe even future generations. When you come to talk to a healthcare provider, instead of asking you for your genealogy, they're going to ask you for your family history. And what they mean by family history is to get information about people's health, both in relatives who are no longer living, living individuals who are closely and less closely related to you, so that they can help you predict risks to your own health and perhaps even to your future offspring.

A fibroblast is the most common type of cell found in connective tissue. Fibroblasts secrete collagen proteins that are used to maintain a structural framework for many tissues. They also play an important role in healing wounds.

A fibroblast is a specific type of connective tissue cell that is found in skin and tendons and other tough tissues in the body. It secretes collagen. In genetics research, it's particularly important because it is a type of cell that is isolatable from people because it is a skin cell, and it can be grown in a laboratory and therefore examined for genotypes and some phenotypes particularly associated with diseases.

A first degree relative is a family member who shares about 50 percent of their genes with a particular individual in a family. First degree relatives include parents, offspring, and siblings.

When you visit with a genetic counselor, one of the first things that they might ask you about is your first degree relatives. And by that they mean your parents, your sisters and brothers, and your children. Each of your first degree relatives shares half of their genetic information in common with you, and so we would expect that things that occur in families are likely to be revealed when we look at relatives who are closely related.

Fluorescence in situ hybridization (FISH) is a laboratory technique for detecting and locating a specific DNA sequence on a chromosome. The technique relies on exposing chromosomes to a small DNA sequence called a probe that has a fluorescent molecule attached to it. The probe sequence binds to its corresponding sequence on the chromosome.

One method for localizing a piece of DNA within a genome is called fluorescence in situ hybridization, abbreviated FISH. In this approach, a fluorescent dye is attached to a purified piece of DNA, and then that DNA is incubated with the full set of chromosomes from the originating genome, which have been attached to a glass microscope slide. The fluorescently labeled DNA finds its matching segment on one of the chromosomes, where it sticks. By looking at the chromosomes under a microscope, a researcher can find the region where the DNA is bound because of the fluorescent dye attached to it. This information thus reveals the location of that piece of DNA in the starting genome.

The founder effect is the reduction in genetic variation that results when a small subset of a large population is used to establish a new colony. The new population may be very different from the original population, both in terms of its genotypes and phenotypes. In some cases, the founder effect plays a role in the emergence of new species.

Founder effect is a phenomenon in the work that we do that refers to the migration of a small group of people from a larger population to go settle in another environment. And they carry along with them a subset of genetic information that existed in the larger population. And because of that, carrying the subset, they actually reduce the amount of genetic variation that exists within a new population now. And as a result of that, they may emphasize certain phenotypes or certain genes, and all this may be deemphasized. So a founder effect can sometimes impact a population in such a way that they may have less of a particular type of gene or more of a particular type of gene. And it can change what we call phenotype, which is the things that we look at like your height, you know, your weight, or having a particular disease or not having a particular disease. So in effect that is what founder effect is.

Charles N. Rotimi, Ph.D.

Fragile X syndrome is a hereditary disorder affecting mostly males. Symptoms include mental retardation, distinctive facial features, and poor muscle tone. The syndrome is caused by mutations in a gene on the X chromosome. Since males have a single copy of the X chromosome, they show symptoms if gene on their X chromosome is mutated. Females have a second, usually normal, copy of the gene on their other X chromosome. Consequently, they are less likely to show symptoms of the syndrome.

Fragile X syndrome is a hereditary syndrome that tends to affect males more severely than it does females. That's because the etiology, the cause of Fragile X syndrome, is having some extra material on the X chromosome. Females have two copies of the X chromosome in the cells of their bodies so that the abnormality that's in one copy of their X chromosome is to some degree counterbalanced by their other X chromosome. Men, however, have only one copy of the X chromosome in their cells, and therefore the extra material in the X chromosome tends to create more severe problems for men than it does for women. The typical issues that are faced by Fragile X syndrome include some degree of intellectual impairment, sometimes so severe that it's labeled as mental retardation. They can often be rather mild. There are also some particular facial features that people who are trained specifically to notice these things, such as larger ear size, can notice them, but that varies between individuals with Fragile X syndrome.

Alan E. Guttmacher, M.D.

A frameshift mutation is a type of mutation involving the insertion or deletion of a nucleotide in which the number of deleted base pairs is not divisible by three. "Divisible by three" is important because the cell reads a gene in groups of three bases. Each group of three bases corresponds to one of 20 different amino acids used to build a protein. If a mutation disrupts this reading frame, then the entire DNA sequence following the mutation will be read incorrectly.

A frameshift mutation is a particular type of mutation that involves either insertion or deletion of extra bases of DNA. Now, what's important here is the number three. The number of bases that are either added or subtracted can't be divisible by three. And that's important because the cell reads a gene in groups of three bases. Every group of three bases corresponds to one of the 20 different amino acids that are used by your body to make proteins. And keep in mind your body has a lot of proteins; everything from the material that makes up your skin, to the material that makes up your hair, to the digestive juices that help you digest that yummy lunch you just had. If a mutation disrupts one of those reading frames, so that the wrong amino acid is put in place, then the entire DNA sequence following the mutation will be disrupted or read incorrectly. Very often, what we see is a premature termination. Instead of the encoded protein being of a certain particular size, it'll end up being much shorter, and it won't be able to accomplish the role that's been set out for it.

Elaine A. Ostrander, Ph.D.

Fraternal twins are also dizygotic twins. They result from the fertilization of two separate eggs during the same pregnancy. Fraternal twins may be of the same or different sexes. They share half of their genes just like any other siblings. In contrast, twins that result from the fertilization of a single egg that then splits in two are called monozygotic, or identical, twins. Identical twins share all of their genes and are always the same sex.

Fraternal twins are also called dizygotic twins. And the difference between fraternal and identical twins is that fraternal twins derive from two different eggs. Fraternal twins may be the same gender, they may have many of the same characteristics, but also may be very different from each other and, in fact, share half of their genes just like their sisters and brothers. It's important to recognize that the difference between fraternal twins and monozygotic, or identical twins, is that monozygotic twins result from the fertilization of a single egg with a single sperm and then during the embryonic development, or during the cell splits, those massive eggs split into two individuals, which later develop into two offspring.