In this slide set we will explore the molecular basis of genetic variation, including mechanisms and types of DNA sequence variations, the importance of DNA sequence variations, and the patterns of inheritance of genetic disorders.

Illustration:
The image on this slide is a photograph of the model of the DNA molecule built by Drs. James Watson and Francis Crick in 1953. Drs. Watson and Crick used this model to depict their proposed structure for the DNA double helix. The hypothesized structure was derived from X-ray diffraction data produced by Drs. Maurice Wilkins and Rosalind Franklin. The model was constructed from metal scraps obtained from a machine shop.

Drs. Watson and Crick published their proposed DNA structure in the journal Nature on April 2, 1953 (Volume 171, page 737). For their work, Drs. Watson, Crick and Wilkins were awarded the Nobel Prize in Physiology or Medicine in 1962. Dr. Franklin died before 1962. Since Nobel Prizes are awarded only to living individuals, she could not be honored.

This video was produced by The National Human Genome Research Institute (www.genome.gov) with co-sponsorship of the Office of Biological and Environmental Research, US Department of Energy (www.doegenomes.org), Howard Hughes Medical Institute (www.hhmi.org), Pharmaceutical Research and Manufacturers of America (http://science.bio.org), Nature (www.nature.com/genomics), Science (www.scienceonline.org), and the American Society of Human Genetics (www.faseb.org/genetics/ashg/ashgmenu.htm). This video (accessed 06-28-2005) is available free through:
http://www.genome.gov/Pages/EducationKit/online.htm

Human genetic disorders can be caused by a variety of mechanisms. Chromosomal disorders are caused by alterations in the number or gross structure of chromosomes. Examples of chromosomal disorders include Down syndrome and Turner syndrome. Contiguous gene deletion syndromes result from large continuous deletions of the genetic material of a chromosome that cause several to many genes to be lost. Examples of contiguous gene deletion syndromes include DiGeorge syndrome (commonly characterized by a history of recurrent infection, heart defects, and characteristic facial features) and Williams syndrome (causes a wide range of medical and developmental symptoms).

The signs and symptoms of genetic disorders caused by chromosomal abnormalities including contiguous gene deletion syndromes are extremely variable. In the case of structural rearrangements of chromosomes and contiguous gene deletion syndromes, most physical effects of the abnormality are likely due to disrupted expression or regulation of genes at or near the sites of the abnormality. In the case of chromosomal aneuploidies (abnormal number of chromosomes), alteration of the normal copy number of genes on the involved chromosome is likely the cause of most of the physical effects.

Single gene disorders occur when a mutation is present in a single gene. The symptoms of the disorder are typically tied specifically to the function of the altered gene. Examples of single gene disorders include cystic fibrosis (CF), Duchenne muscular dystrophy, Fragile X syndrome, and Huntington disease. Single gene disorders can be autosomal, occurring due to mutations in genes located on chromosomes 1-22; X-linked, occurring due to mutations in genes located on the X chromosome; Y-linked, occurring due to mutations in genes located on the Y chromosome; or mitochondrial (maternally inherited), occurring due to mutations in genes located on the mitochondrial chromosome.

Common complex disorders are disorders in which multiple genetic and/or environmental factors act together to influence the expression and severity of a disorder. Examples of common complex disorders include high blood pressure, obesity, and diabetes.

The genetic location of the gene(s) altered in association with a genetic disorder affects the pattern of inheritance of the disorder (the way a disease is inherited in a family). The pedigree shown in this slide illustrates an autosomal dominant disorder caused by mutation of a single gene that is being passed from one generation to the next throughout a family. Knowing the pattern of inheritance of the disorder allows prediction of genetic risks for family members. Knowing the gene involved in the disorder allows genetic testing to identify individuals at risk for the disease or for passing the disease gene to their children. In pedigree drawings, boxes are males, circles are females. Shaded shapes are affected individuals, open shapes are unaffected individuals. The diamond at the bottom of the pedigree represents a fetus of unspecified gender. In the autosomal dominant pedigree drawn here, the fetus would be expected, prior to genetic testing, to have a 50% chance of inheriting the disease gene. If a genetic test were available for the disorder in question, the test could determine whether this fetus inherited the disease-associated copy of the gene or the normal copy of the gene from his/her affected mother.

Mendelian disorders follow Mendel's rules of inheritance. They can be inherited in an autosomal dominant, autosomal recessive, X-linked, or Y-linked manner. Autosomal dominant inheritance is typically characterized by the appearance of a disorder in every generation of a family, passed down from parent to child for several generations. Autosomal dominant inheritance was sometimes described as vertical inheritance although this term is not commonly used today. The word trait is typically used by geneticists to describe genetically inherited physical characteristics or features that are not considered to be diseases or disorders. Autosomal dominant traits are inherited in a manner similar to that of autosomal dominant disorders.

Humans typically carry two copies of each autosomal chromosome (chromosomes 1-22) and two copies of each autosomal gene. Most individuals affected by autosomal dominant disorders carry one affected and one normal copy of the gene associated with the disorder. These individuals are said to be heterozygous for the disease gene. Homozygous affected individuals carrying two affected copies of a gene associated with a disorder can occur, especially if the disorder is common. For some autosomal dominant disorders, homozygotes (persons with two copies of an affected gene) are more severely affected than heterozygotes (persons with one affected and one normal copy of the gene). Achondroplasia, the most common form of short-limb dwarfism, is an example of this phenomenon.

In general, males and females affected by autosomal dominant disorders will be affected with equal frequency and severity, but there may be exceptions in cases where a particular disorder affects the genders differently. Male to male transmission of the disorder can occur in families with affected males. The children of affected heterozygous individuals typically have a 50% chance of inheriting the disease associated gene and being affected, unless the disorder demonstrates reduced penetrance. Reduced penetrance is a phenomenon in which some gene carriers remain unaffected despite the fact that they carry a gene associated with a disorder. One explanation for this phenomenon is that the gene is necessary but not sufficient to cause the disease. In these cases, the child has a 50% risk of inheriting the gene from an affected parent but may not show the disorder. Some autosomal dominant disorders show reduced penetrance. Others do not. The degree of reduced penetrance associated with a particular disorder depends upon the disorder in question.

Persons not affected by autosomal dominant disorders generally do not have affected children. However, there are exceptions, such as when a new mutation in a gene occurs at some point during transmission of the gene from parent to child. The child could be affected by the disorder while the parent remains unaffected. The percentage of cases of a particular disease that arise from new mutations varies from disorder to disorder.

Autosomal dominant disorders also frequently show a high degree of variability. Variability means that individuals who are affected by the disorder can display a variety of symptoms and a range of severity of the symptoms. A heterozygous affected parent has a 50% chance of passing the gene associated with the disease to each of his or her children, but if the disorder is highly variable it may be difficult to predict the severity to which a child will be affected. In most cases, variability is thought to represent the influence of other genetic and/or environmental factors on the expression of the disorder.

Examples of autosomal dominant disorders include: Marfan syndrome, neurofibromatosis, and Waardenburg syndrome. In pedigree drawings, boxes are males, circles are females. Shaded shapes are affected individuals, open shapes are unaffected individuals. The drawing on this slide illustrates an autosomal dominant disorder. In this family, the disease is passed from a man to his son, and then from the son to his daughter.

Mendelian disorders, which follow Mendel's rules of inheritance, can be inherited in an autosomal dominant, autosomal recessive, X-linked, or Y-linked manner. Humans typically carry two copies of each autosomal chromosome (chromosomes 1-22) and two copies of each autosomal gene. Individuals affected by autosomal recessive disorders carry two copies of the gene associated with the trait or disorder. The word trait is typically used by geneticists to describe genetically inherited physical characteristics or features that are not considered to be diseases or disorders. Autosomal recessive traits are inherited in a manner similar to that of autosomal recessive disorders.

Autosomal recessive inheritance is frequently observed as the appearance of a disorder in only one generation of a family as isolated cases or affected siblings and/or cousins. Autosomal recessive inheritance is sometimes described as horizontal inheritance although this term is not commonly used today. In the case of common recessive disorders or disorders that lead to non-random mating, such as deafness, which is common and frequently leads to deaf by deaf matings, a pseudo-dominant pattern of inheritance may be observed. For genetic counseling and risk assessment, it is important to recognize the possibility that pseudo-dominant inheritance might be occurring in a family.

In general, males and females affected by autosomal recessive disorders will be affected with the same frequency and severity, but there may be exceptions in cases where a disorder affects the genders differently. In general, autosomal recessive disorders do not show reduced penetrance or the considerable variability that can be associated with autosomal dominant disorders. However, there are some autosomal recessive disorders for which variability in the severity of symptoms among affected individuals is observed. Reduced penetrance is a phenomenon in which some gene carriers remain unaffected despite the fact that they carry a gene associated with a trait or disorder. One explanation for this phenomenon is that the gene is necessary but not sufficient to cause disease.

Most often, children affected by autosomal recessive disorders are born to unaffected parents. However, for certain autosomal recessive disorders, there may be very mild or atypical manifestations of the disease in a carrier parent. Frequently in autosomal recessive disorders, there is no family history of the disease and no reason to suspect an affected child could be born. Carrier couples (in which both partners are carriers of the same autosomal genetic disorder) have a 25% risk of an affected child with each pregnancy. Examples of autosomal recessive disorders include cystic fibrosis (CF), Tay-Sachs disease, and sickle cell disease.

Under what conditions widespread screening should be undertaken for the identification of unaffected carriers of autosomal recessive disorders is a topic of much debate. At present, most carrier screening programs focus on high risk groups such as screening for cystic fibrosis (CF) carriers among Caucasians and screening for CF, Tay Sachs and other genetic disease carriers among Ashkenazi Jews.

In pedigree drawings, boxes are males and circles are females. Shaded shapes are affected individuals and open shapes are unaffected individuals. The drawing in this slide illustrates an autosomal recessive disorder. In this case, the affected child is born to unaffected parents with no family history of the disease.

Mendelian disorders, which follow Mendel's rules of inheritance, can be inherited in an autosomal dominant, autosomal recessive, X-linked, or Y-linked manner. X-linked dominant disorders are characterized by the appearance of a disease in both males and females in several generations of a family. True X-linked dominant disorders are few in number. There are many more X-linked recessive disorders. The word trait is typically used by geneticists to describe genetically inherited physical characteristics or features that are not considered to be diseases or disorders. X-linked traits are inherited in a manner similar to that of X-linked disorders.

X-linked recessive disorders are typically observed as affected males born to unaffected parents. However, it may be more accurate to describe X-linked disorders simply as X-linked and not as recessive or dominant because with many disorders historically described as X-linked recessive, carrier females can demonstrate variable degrees of signs or symptoms of the disease. For X-linked disorders, there can be a family history of the disorder in the form of affected uncle-nephew pairs, affected brothers, or affected male cousins, depending upon the carrier status of the females in the family. For common conditions, a pseudo-dominant pattern of inheritance observed as an affected male with an affected father can also be seen. However in such cases, the affected son actually would have had to inherit the trait from his mother because males inherit their father's Y chromosome, not their father's X chromosome. It is important to understand the pattern of inheritance and genetic location of the associated gene, so that accurate risk analysis and genetic counseling can be performed for families.

In the case of X-linked disorders, father to son transmission is never seen because fathers give sons their Y chromosome. Each son of a female carrier of an X-linked disorder has a 50% chance of being affected; daughters have a 50% chance of being a carrier. All daughters of an affected male will be carriers because fathers have just one X chromosome to contribute to a daughter. Examples of X-linked disorders include Duchenne muscular dystrophy and hemophilia A and B.

In addition, the classic red-green colorblindness trait is inherited in an X-linked recessive manner. Because it is so common, colorblind females who carry a copy of the gene associated with colorblindness on both of their X chromosomes are sometimes seen, as are colorblind father-son pairs. However, because we know the inheritance pattern of the trait, we know that a son inherits the trait from his mother, not his father. Roughly 8% of US Caucasian males are colorblind, while less than 1% of females are colorblind.

In pedigree drawings, boxes are males and circles are females. Shaded shapes are affected individuals and open shapes are unaffected individuals. In the drawing in this slide, an uncle-nephew pair affected by an X-linked disorder is shown. In this case, the women in the pedigree with dots in the center of the circles representing them are obligate carriers of the disorder. The sister of the affected boy in the third generation of the pedigree has a 50% chance of being a carrier.

Diseases of the mitochondria are inherited in one of two ways: through the maternal lineage or as autosomal disorders. Mitochondria, which are located inside the cell but outside the nucleus, carry their own small circular genome that encodes 13 proteins, 2 rRNAs and 22 tRNAs. Mitochondria also import proteins encoded by nuclear genes. Diseases of the mitochondria caused by mutations in genes carried on the mitochondrial chromosome are inherited through the maternal lineage, passed from affected mothers to all of their children, but never from affected fathers, because only eggs contribute mitochondria to the embryo. Diseases of the mitochondria caused by mutations in genes carried on the nuclear chromosomes demonstrate an autosomal pattern of inheritance that is dependant upon the gene or mutation involved.

Individual mitochondria frequently carry multiple copies of their genomes. A single cell can carry many mitochondria. Not all of the mitochondrial genomes a person carries will necessarily carry a particular mutation. When all of the mitochondrial DNA present in an individual carries a particular mutation, that individual is said to be homoplasmic for the mutation. If some normal and some abnormal mitochondrial genomes are present in an individual, that person is said to be heteroplasmic for the mutation.

Because of the complexity of the underlying genetics, maternally inherited mitochondrial disorders are remarkable for their variability. This variability is believed to be based on the relative percentage of mutant mitochondrial genes a person carries, the number of abnormal mitochondria that result, and the tissue distribution of those abnormal mitochondria. As a result, it can be difficult to predict the severity of maternally inherited mitochondrial disorders in children of affected women. Examples of maternally inherited mitochondrial disorders include aminoglycoside-induced ototoxicity (deafness) and MELAS (mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes).

Multifactorial disorders, which include common complex disorders, are characterized by the interaction of multiple genetic and/or environmental factors that influence the expression of a disorder. Although common complex disorders may not show a clear pattern of inheritance like Mendelian disorders do, there is typically a familial aggregation of the disorder. That is, close relatives of an affected individual may be at increased risk for the disorder, compared to the general population. Examples of multifactorial diseases include some forms of diabetes, high blood pressure, and obesity.

Disorders associated with a new mutation occur when a mutation happens prior to or during gametogenesis, or after fertilization. The mutation is found in the child but not in either parent. Timing of occurrence of the mutation is important to the tissue distribution of the mutated gene, the symptomatic expression of the disease, the risk for additional affected offspring of the parents, and the risk for future offspring of the affected individual. For example, if a mutation occurs in the gonadal tissues of an individual with a new mutation, there is an increased risk to future offspring of that person. An advanced paternal age effect is characteristic of some new mutation genetic diseases. Examples of diseases with a high new mutation rate include achondroplasia, Marfan syndrome, Waardenburg syndrome, and neurofibromatosis. These all are dominant disorders. Although recessive new mutations surely occur, it is far more likely that both parents of an individual affected by a recessive disorder are carriers of the recessive mutations.

Imprinting is a form of gene silencing whereby certain genes are methylated during gametogenesis (formation of egg and sperm) to impart parent of origin effects on gene expression. For example, if a gene is maternally imprinted (that is, silenced in the maternal copy during female gametogenesis), only the paternal copy of the gene is expressed in the child. Paternal imprinting of genes also occurs. During gametogenesis, parental imprints are erased and reset, in a manner consistent with whether the gametogenesis is occurring in a male or a female.

A number of human genetic diseases occur because of mutations in imprinted genes or in the control sequences responsible for setting, resetting or controlling imprinted regions. For example, if a person carries a mutation in an imprinted gene, the disease will be expressed only if the mutation is inherited from the parent whose gene is expressed. Imagine a mutation in a gene that is imprinted in the maternal genome. If a mutation in this gene is inherited from one's mother, the disease will not occur because it is the father's gene that is expressed. However, if the mutation is inherited from one's father, the disease will be evident.

Another source of abnormalities in imprinted genes involves the imprinting centers (DNA sequences) that control the establishment of an imprint and the switching of imprints in gametogenesis. Mutations in imprinting centers can result in disease because of failure to erase the grandparental imprint during gametogenesis. Examples of disorders associated with imprinted genes include Angelman syndrome and Prader-Willi syndrome. These disorders can also be caused when errors in meiosis result in a child who inherits both copies of his or her chromosome 15 from a single parent, resulting in inheritance of only imprinted copies of the gene. This phenomenon is called uniparental disomy, or UPD.

The appearance of a genetic disease at progressively earlier ages of onset and with increasing severity in successive generations of a family is called anticipation. The molecular genetic phenomenon behind anticipation is a length expansion of repetitive DNA sequences within a gene such as triplet (three nucleotide) repeats during transmission of the gene from generation to generation. These expansions cause increased severity of the disease with growing repeat length. Examples of diseases that show anticipation include myotonic dystrophy, Fragile X syndrome and Huntington disease.

There are two types of genetic heterogeneity: allelic and locus. Allelic heterogeneity occurs when different mutations in a single gene cause a particular disease. One disease that shows allelic heterogeneity is cystic fibrosis (CF). More than 1,000 different mutations have been identified in the gene associated with CF. Understanding allelic heterogeneity is important for interpreting DNA test results and predicting disease severity. Locus heterogeneity occurs when a particular disorder can be caused by mutations in more than one gene. Understanding locus heterogeneity is important for selecting  the proper DNA tests to order for a patient, interpreting DNA test results, predicting disease severity based on DNA test results, and predicting disease risk for family members.

X-inactivation, sometimes called Lyonization, occurs during embryogenesis and is the process by which most genes on one of the X chromosomes of a female are inactivated. This mechanism provides dosage compensation and equates the level of gene expression of most X-linked genes in females to that of males who have only one X chromosome. There are some X-linked genes in females that escape inactivation. X-inactivation usually occurs randomly within the cells of a female such that approximately half of the cells of a female express the genes from one of her X chromosomes and the other half of her cells express the genes from her other X chromosome. However, skewing can occur in X-inactivation. Skewed X-inactivation is observed when one of the X chromosomes of a female is preferentially inactivated in substantially more than half of her cells. In some cases, skewing of X-inactivation is extreme and may be caused by variations in the survival of cells in which a particular X chromosome is inactivated. Skewing of X inactivation can, in rare cases, cause X-linked diseases to be expressed in females.