BACK TOTOP Browse A-ZSearchBrowse A-ZABCDEFGHIJKLMNOPQRSTUVWXYZ0-9 E-mail FormEmail ResultsName:Email address:Recipients Name:Recipients address:Message: Print-FriendlyBookmarksbookmarks-menuGeneticsHomozygous; Inheritance; Heterozygous; Inheritance patterns; Heredity and disease; Heritable; Genetic markersGenetics is the study of heredity, the process of a parent passing certain genes to their children. A person's appearance -- height, hair color, skin color, and eye color -- is determined by genes. Other characteristics affected by heredity are:GenesA gene is a short piece of DNA. Genes tell the body how to build specific proteins. There are about 20,000 genes in each cell of the human body. T...ImageRead Article Now Book Mark Article Likelihood of getting certain diseases Mental abilities Natural talentsAn abnormal trait (anomaly) that is passed down through families (inherited) may:Have no effect on your health or well-being. For example, the trait might just cause a white patch of hair or an earlobe that is longer than normal. Have only a minor effect, such as color blindness. Color blindnessColor blindness is the inability to see some colors in the usual way.Read Article Now Book Mark Article Have a major effect on your quality or length of life.For most genetic disorders, genetic counseling is advised. Many couples may also want to seek prenatal diagnosis if one of them has a genetic disorder.Genetic counselingGenetics is the study of heredity, the process of a parent passing certain genes on to their children. A person's appearance, such as height, hair co...ImageRead Article Now Book Mark Article Information Human beings have cells with 46 chromosomes. These consist of 2 chromosomes that determine what sex they are (X and Y chromosomes), and 22 pairs of nonsex (autosomal) chromosomes. Males are "46,XY" and females are "46,XX." The chromosomes are made up of strands of genetic information called DNA. Each chromosome contains sections of DNA called genes. The genes carry the information needed by your body to make certain proteins.ChromosomesChromosomes are structures found in the center (nucleus) of cells that carry long pieces of DNA. DNA is the material that holds genes. It is the bu...ImageRead Article Now Book Mark Article Each pair of autosomal chromosomes contains one chromosome from the mother and one from the father. Each chromosome in a pair carries basically the same information; that is, each chromosome pair has the same genes. Sometimes there are slight variations of these genes. These variations occur in less than 1% of the DNA sequence. The genes that have these variations are called alleles.Some of these variations can result in a gene that is not working properly. An abnormal gene may lead to an abnormal protein or an abnormal amount of a normal protein. In a pair of autosomal chromosomes, there are two copies of each gene, one from each parent. If one of these genes is abnormal, the other one may make enough protein so that no disease develops. When this happens, the abnormal gene is called recessive. Recessive genes are said to be inherited in either an autosomal recessive or X-linked pattern. If two copies of the abnormal gene are present, disease may develop.However, if only one abnormal gene is needed to produce a disease, it leads to a dominant hereditary disorder. In the case of a dominant disorder, if one abnormal gene is inherited from the mother or father, the child will likely show the disease.A person with one abnormal gene is called heterozygous for that gene. If a child receives an abnormal recessive disease gene from both parents, the child will show the disease and will be homozygous (or compound heterozygous) for that gene.GENETIC DISORDERSAlmost all diseases have a genetic component. However, the importance of that component varies. Disorders in which genes play an important role (genetic diseases) can be classified as:Single-gene defects Chromosomal disorders Multifactorial A single-gene disorder (also called Mendelian disorder) is caused by a defect in one particular gene. Single gene defects are rare. But since there are many thousands of known single gene disorders, their combined impact is significant.Single-gene disorders are characterized by how they are passed down in families. There are 6 basic patterns of single gene inheritance:Autosomal dominant Autosomal dominantAutosomal dominant is one of many ways that a trait or disorder can be passed down through families. In an autosomal dominant disease, if you get the...ImageRead Article Now Book Mark Article Autosomal recessive Autosomal recessiveAutosomal recessive is one of several ways that a trait, disorder, or disease can be passed down through families. An autosomal recessive disorder me...ImageRead Article Now Book Mark Article X-linked dominant X-linked dominantSex-linked dominant is a rare way that a trait or disorder can be passed down through families. One abnormal gene on the X chromosome can cause a se...ImageRead Article Now Book Mark Article X-linked recessive X-linked recessiveSex-linked diseases are passed down through families through one of the X or Y chromosomes. X and Y are sex chromosomes. Dominant inheritance occur...ImageRead Article Now Book Mark Article Y-linked inheritance Maternal (mitochondrial) inheritance The observed effect of a gene (the appearance of a disorder) is called the phenotype.In autosomal dominant inheritance, the abnormality or abnormalities usually appear in every generation. Each time an affected parent, either male or female, has a child, that child has a 50% chance of inheriting the disease.People with one copy of a recessive disease gene are called carriers. Carriers usually don't have symptoms of the disease. But, the gene can often be found by sensitive laboratory tests.In autosomal recessive inheritance, the parents of an affected individual may not show the disease (they are carriers). On average, the chance that carrier parents could have children who develop the disease is 25% with each pregnancy. Male and female children are equally likely to be affected. For a child to have symptoms of an autosomal recessive disorder, the child must receive the abnormal gene from both parents. Because most recessive disorders are rare, a child is at increased risk of a recessive disease if the parents are related. Related individuals are more likely to have inherited the same rare gene from a common ancestor.In X-linked recessive inheritance, the chance of getting the disease is much higher in males than females. Since the abnormal gene is carried on the X (female) chromosome, males do not transmit it to their sons (who will receive the Y chromosome from their fathers). However, they do transmit it to their daughters. In females, the presence of one normal X chromosome masks the effects of the X chromosome with the abnormal gene. So, almost all of the daughters of an affected man appear normal, but they are all carriers of the abnormal gene. Each time these daughters bear a son, there is a 50% chance the son will receive the abnormal gene.In X-linked dominant inheritance, the abnormal gene appears in females even if there is also a normal X chromosome present. Since males pass the Y chromosome to their sons, affected males will not have affected sons. All of their daughters will be affected, however. Sons or daughters of affected females will have a 50% chance of getting the disease.EXAMPLES OF SINGLE GENE DISORDERSAutosomal recessive:ADA deficiency (sometimes called the "boy in a bubble" disease) Alpha-1-antitrypsin (AAT) deficiency AATAlpha-1 antitrypsin (AAT) deficiency is a condition in which the body does not make enough of AAT, a protein that protects the lungs and liver from d...ImageRead Article Now Book Mark Article Cystic fibrosis (CF) Cystic fibrosisCystic fibrosis is a disease that causes thick, sticky mucus to build up in the lungs, digestive tract, and other areas of the body. It is one of th...ImageRead Article Now Book Mark Article Phenylketonuria (PKU) PKUPhenylketonuria (PKU) is a rare condition in which a baby is born without the ability to properly break down an amino acid called phenylalanine....ImageRead Article Now Book Mark Article Sickle cell anemiaSickle cell anemiaSickle cell disease is a disorder passed down through families. The red blood cells that are normally shaped like a disk take on a sickle or crescen...ImageRead Article Now Book Mark Article X-linked recessive:Duchenne muscular dystrophy Muscular dystrophyMuscular dystrophy is a group of inherited disorders that cause muscle weakness and loss of muscle tissue, which get worse over time.ImageRead Article Now Book Mark Article Hemophilia AHemophilia AHemophilia A is a hereditary bleeding disorder caused by a lack of blood clotting factor VIII. Without enough factor VIII, the blood cannot clot pro...ImageRead Article Now Book Mark Article Autosomal dominant:Familial hypercholesterolemia Familial hypercholesterolemiaFamilial hypercholesterolemia is a disorder that is passed down through families. It causes LDL (bad) cholesterol level to be very high. The condit...ImageRead Article Now Book Mark Article Marfan syndromeMarfan syndromeMarfan syndrome is a disorder of connective tissue. This is the tissue that strengthens the body's structures. Disorders of connective tissue affect...ImageRead Article Now Book Mark Article X-linked dominant:Only a few, rare, disorders are X-linked dominant. One of these is hypophosphatemic rickets, also called vitamin D -resistant rickets.RicketsRickets is a disorder caused by a lack of vitamin D, calcium, or phosphate. It leads to softening and weakening of the bones.ImageRead Article Now Book Mark Article Vitamin DVitamin D is a fat-soluble vitamin. Fat-soluble vitamins are stored in the body's fatty tissue and liver.ImageRead Article Now Book Mark Article CHROMOSOMAL DISORDERSIn chromosomal disorders, the defect is due to either an excess or lack of the genes contained in a whole chromosome or chromosome segment.Chromosomal disorders include:22q11.2 microdeletion syndrome Down syndrome Klinefelter syndrome Klinefelter syndromeKlinefelter syndrome, also called 47,XXY, is a genetic condition that occurs in males when they have an extra X chromosome. This can cause problems ...ImageRead Article Now Book Mark Article Turner syndromeTurner syndromeTurner syndrome is a rare genetic condition in which a female does not have the usual pair of X chromosomes.ImageRead Article Now Book Mark Article MULTIFACTORIAL DISORDERSMany of the most common diseases are caused by interactions of several genes and factors in the environment (for example, illnesses in the mother and medicines). These include:Asthma AsthmaAsthma is a chronic disease that causes the airways of the lungs to swell and narrow. It leads to breathing difficulty such as wheezing, shortness o...ImageRead Article Now Book Mark Article Cancer CancerCancer is the uncontrolled growth of abnormal cells in the body. Cancerous cells are also called malignant cells.Read Article Now Book Mark Article Coronary heart disease Coronary heart diseaseCoronary heart disease is a narrowing of the blood vessels that supply blood and oxygen to the heart. Coronary heart disease (CHD) is also called co...ImageRead Article Now Book Mark Article Diabetes DiabetesDiabetes is a long-term (chronic) disease in which the body cannot regulate the amount of sugar in the blood.ImageRead Article Now Book Mark Article Hypertension HypertensionBlood pressure is a measurement of the force exerted against the walls of your arteries as your heart pumps blood to your body. Hypertension is the ...ImageRead Article Now Book Mark Article StrokeStrokeA stroke occurs when blood flow to a part of the brain stops. A stroke is sometimes called a "brain attack. " If blood flow is cut off for longer th...ImageRead Article Now Book Mark Article MITOCHONDRIAL DNA-LINKED DISORDERSMitochondria are small structures found in most of the body's cells. They are responsible for energy production inside cells. Mitochondria contain their own private DNA.In recent years, many disorders have been shown to result from changes (mutations) in mitochondrial DNA. Because mitochondria come only from the female egg, most mitochondrial DNA-related disorders are passed down from the mother.Mitochondrial DNA-related disorders can appear at any age. They have a wide variety of symptoms and signs. These disorders may cause:Blindness Developmental delay Gastrointestinal problems Hearing loss Heart rhythm problems Metabolic disturbances Short stature Some other disorders are also known as mitochondrial disorders, but they do not involve mutations in the mitochondrial DNA. These disorders are most often single gene defects. They follow the same pattern of inheritance as other single gene disorders. Most are autosomal recessive. Open ReferencesReferencesFeero WG, Zazove P, Chen F. Clinical genomics. In: Rakel RE, Rakel DP, eds. Textbook of Family Medicine. 9th ed. Philadelphia, PA: Elsevier; 2016:chap 43.Korf BR. Principles of genetics. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 35.Scott DA, Lee B. The genetic approach in pediatric medicine. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 95.AllVideoImagesTogGenetics - illustration Genetics is the study of heredity and how traits are passed along from parents to offspring. Genes are contained within the chromosomes found in the egg and sperm. Each parent contributes one half of each pair (23 chromosomes -- 22 autosomal and 1 sex chromosome) to their child.GeneticsillustrationGenetics - illustration Genetics is the study of heredity and how traits are passed along from parents to offspring. Genes are contained within the chromosomes found in the egg and sperm. Each parent contributes one half of each pair (23 chromosomes -- 22 autosomal and 1 sex chromosome) to their child.GeneticsillustrationRelated Information Genes(Special Topic)Color blindness(Condition) Review Date: 1/6/2022 Reviewed By: Anna C. Edens Hurst, MD, MS, Associate Professor in Medical Genetics, The University of Alabama at Birmingham, Birmingham, AL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. No warranty of any kind, either expressed or implied, is made as to the accuracy, reliability, timeliness, or correctness of any translations made by a third-party service of the information provided herein into any other language. © 1997- A.D.A.M., a business unit of Ebix, Inc. Any duplication or distribution of the information contained herein is strictly prohibited. © 1997- All rights reserved. A.D.A.M. content is best viewed in IE9 or above, Firefox and Google Chrome browser.Content is best viewed in IE9 or above, Firefox and Google Chrome browser.
GeneticsHomozygous; Inheritance; Heterozygous; Inheritance patterns; Heredity and disease; Heritable; Genetic markersGenetics is the study of heredity, the process of a parent passing certain genes to their children. A person's appearance -- height, hair color, skin color, and eye color -- is determined by genes. Other characteristics affected by heredity are:GenesA gene is a short piece of DNA. Genes tell the body how to build specific proteins. There are about 20,000 genes in each cell of the human body. T...ImageRead Article Now Book Mark Article Likelihood of getting certain diseases Mental abilities Natural talentsAn abnormal trait (anomaly) that is passed down through families (inherited) may:Have no effect on your health or well-being. For example, the trait might just cause a white patch of hair or an earlobe that is longer than normal. Have only a minor effect, such as color blindness. Color blindnessColor blindness is the inability to see some colors in the usual way.Read Article Now Book Mark Article Have a major effect on your quality or length of life.For most genetic disorders, genetic counseling is advised. Many couples may also want to seek prenatal diagnosis if one of them has a genetic disorder.Genetic counselingGenetics is the study of heredity, the process of a parent passing certain genes on to their children. A person's appearance, such as height, hair co...ImageRead Article Now Book Mark Article Information Human beings have cells with 46 chromosomes. These consist of 2 chromosomes that determine what sex they are (X and Y chromosomes), and 22 pairs of nonsex (autosomal) chromosomes. Males are "46,XY" and females are "46,XX." The chromosomes are made up of strands of genetic information called DNA. Each chromosome contains sections of DNA called genes. The genes carry the information needed by your body to make certain proteins.ChromosomesChromosomes are structures found in the center (nucleus) of cells that carry long pieces of DNA. DNA is the material that holds genes. It is the bu...ImageRead Article Now Book Mark Article Each pair of autosomal chromosomes contains one chromosome from the mother and one from the father. Each chromosome in a pair carries basically the same information; that is, each chromosome pair has the same genes. Sometimes there are slight variations of these genes. These variations occur in less than 1% of the DNA sequence. The genes that have these variations are called alleles.Some of these variations can result in a gene that is not working properly. An abnormal gene may lead to an abnormal protein or an abnormal amount of a normal protein. In a pair of autosomal chromosomes, there are two copies of each gene, one from each parent. If one of these genes is abnormal, the other one may make enough protein so that no disease develops. When this happens, the abnormal gene is called recessive. Recessive genes are said to be inherited in either an autosomal recessive or X-linked pattern. If two copies of the abnormal gene are present, disease may develop.However, if only one abnormal gene is needed to produce a disease, it leads to a dominant hereditary disorder. In the case of a dominant disorder, if one abnormal gene is inherited from the mother or father, the child will likely show the disease.A person with one abnormal gene is called heterozygous for that gene. If a child receives an abnormal recessive disease gene from both parents, the child will show the disease and will be homozygous (or compound heterozygous) for that gene.GENETIC DISORDERSAlmost all diseases have a genetic component. However, the importance of that component varies. Disorders in which genes play an important role (genetic diseases) can be classified as:Single-gene defects Chromosomal disorders Multifactorial A single-gene disorder (also called Mendelian disorder) is caused by a defect in one particular gene. Single gene defects are rare. But since there are many thousands of known single gene disorders, their combined impact is significant.Single-gene disorders are characterized by how they are passed down in families. There are 6 basic patterns of single gene inheritance:Autosomal dominant Autosomal dominantAutosomal dominant is one of many ways that a trait or disorder can be passed down through families. In an autosomal dominant disease, if you get the...ImageRead Article Now Book Mark Article Autosomal recessive Autosomal recessiveAutosomal recessive is one of several ways that a trait, disorder, or disease can be passed down through families. An autosomal recessive disorder me...ImageRead Article Now Book Mark Article X-linked dominant X-linked dominantSex-linked dominant is a rare way that a trait or disorder can be passed down through families. One abnormal gene on the X chromosome can cause a se...ImageRead Article Now Book Mark Article X-linked recessive X-linked recessiveSex-linked diseases are passed down through families through one of the X or Y chromosomes. X and Y are sex chromosomes. Dominant inheritance occur...ImageRead Article Now Book Mark Article Y-linked inheritance Maternal (mitochondrial) inheritance The observed effect of a gene (the appearance of a disorder) is called the phenotype.In autosomal dominant inheritance, the abnormality or abnormalities usually appear in every generation. Each time an affected parent, either male or female, has a child, that child has a 50% chance of inheriting the disease.People with one copy of a recessive disease gene are called carriers. Carriers usually don't have symptoms of the disease. But, the gene can often be found by sensitive laboratory tests.In autosomal recessive inheritance, the parents of an affected individual may not show the disease (they are carriers). On average, the chance that carrier parents could have children who develop the disease is 25% with each pregnancy. Male and female children are equally likely to be affected. For a child to have symptoms of an autosomal recessive disorder, the child must receive the abnormal gene from both parents. Because most recessive disorders are rare, a child is at increased risk of a recessive disease if the parents are related. Related individuals are more likely to have inherited the same rare gene from a common ancestor.In X-linked recessive inheritance, the chance of getting the disease is much higher in males than females. Since the abnormal gene is carried on the X (female) chromosome, males do not transmit it to their sons (who will receive the Y chromosome from their fathers). However, they do transmit it to their daughters. In females, the presence of one normal X chromosome masks the effects of the X chromosome with the abnormal gene. So, almost all of the daughters of an affected man appear normal, but they are all carriers of the abnormal gene. Each time these daughters bear a son, there is a 50% chance the son will receive the abnormal gene.In X-linked dominant inheritance, the abnormal gene appears in females even if there is also a normal X chromosome present. Since males pass the Y chromosome to their sons, affected males will not have affected sons. All of their daughters will be affected, however. Sons or daughters of affected females will have a 50% chance of getting the disease.EXAMPLES OF SINGLE GENE DISORDERSAutosomal recessive:ADA deficiency (sometimes called the "boy in a bubble" disease) Alpha-1-antitrypsin (AAT) deficiency AATAlpha-1 antitrypsin (AAT) deficiency is a condition in which the body does not make enough of AAT, a protein that protects the lungs and liver from d...ImageRead Article Now Book Mark Article Cystic fibrosis (CF) Cystic fibrosisCystic fibrosis is a disease that causes thick, sticky mucus to build up in the lungs, digestive tract, and other areas of the body. It is one of th...ImageRead Article Now Book Mark Article Phenylketonuria (PKU) PKUPhenylketonuria (PKU) is a rare condition in which a baby is born without the ability to properly break down an amino acid called phenylalanine....ImageRead Article Now Book Mark Article Sickle cell anemiaSickle cell anemiaSickle cell disease is a disorder passed down through families. The red blood cells that are normally shaped like a disk take on a sickle or crescen...ImageRead Article Now Book Mark Article X-linked recessive:Duchenne muscular dystrophy Muscular dystrophyMuscular dystrophy is a group of inherited disorders that cause muscle weakness and loss of muscle tissue, which get worse over time.ImageRead Article Now Book Mark Article Hemophilia AHemophilia AHemophilia A is a hereditary bleeding disorder caused by a lack of blood clotting factor VIII. Without enough factor VIII, the blood cannot clot pro...ImageRead Article Now Book Mark Article Autosomal dominant:Familial hypercholesterolemia Familial hypercholesterolemiaFamilial hypercholesterolemia is a disorder that is passed down through families. It causes LDL (bad) cholesterol level to be very high. The condit...ImageRead Article Now Book Mark Article Marfan syndromeMarfan syndromeMarfan syndrome is a disorder of connective tissue. This is the tissue that strengthens the body's structures. Disorders of connective tissue affect...ImageRead Article Now Book Mark Article X-linked dominant:Only a few, rare, disorders are X-linked dominant. One of these is hypophosphatemic rickets, also called vitamin D -resistant rickets.RicketsRickets is a disorder caused by a lack of vitamin D, calcium, or phosphate. It leads to softening and weakening of the bones.ImageRead Article Now Book Mark Article Vitamin DVitamin D is a fat-soluble vitamin. Fat-soluble vitamins are stored in the body's fatty tissue and liver.ImageRead Article Now Book Mark Article CHROMOSOMAL DISORDERSIn chromosomal disorders, the defect is due to either an excess or lack of the genes contained in a whole chromosome or chromosome segment.Chromosomal disorders include:22q11.2 microdeletion syndrome Down syndrome Klinefelter syndrome Klinefelter syndromeKlinefelter syndrome, also called 47,XXY, is a genetic condition that occurs in males when they have an extra X chromosome. This can cause problems ...ImageRead Article Now Book Mark Article Turner syndromeTurner syndromeTurner syndrome is a rare genetic condition in which a female does not have the usual pair of X chromosomes.ImageRead Article Now Book Mark Article MULTIFACTORIAL DISORDERSMany of the most common diseases are caused by interactions of several genes and factors in the environment (for example, illnesses in the mother and medicines). These include:Asthma AsthmaAsthma is a chronic disease that causes the airways of the lungs to swell and narrow. It leads to breathing difficulty such as wheezing, shortness o...ImageRead Article Now Book Mark Article Cancer CancerCancer is the uncontrolled growth of abnormal cells in the body. Cancerous cells are also called malignant cells.Read Article Now Book Mark Article Coronary heart disease Coronary heart diseaseCoronary heart disease is a narrowing of the blood vessels that supply blood and oxygen to the heart. Coronary heart disease (CHD) is also called co...ImageRead Article Now Book Mark Article Diabetes DiabetesDiabetes is a long-term (chronic) disease in which the body cannot regulate the amount of sugar in the blood.ImageRead Article Now Book Mark Article Hypertension HypertensionBlood pressure is a measurement of the force exerted against the walls of your arteries as your heart pumps blood to your body. Hypertension is the ...ImageRead Article Now Book Mark Article StrokeStrokeA stroke occurs when blood flow to a part of the brain stops. A stroke is sometimes called a "brain attack. " If blood flow is cut off for longer th...ImageRead Article Now Book Mark Article MITOCHONDRIAL DNA-LINKED DISORDERSMitochondria are small structures found in most of the body's cells. They are responsible for energy production inside cells. Mitochondria contain their own private DNA.In recent years, many disorders have been shown to result from changes (mutations) in mitochondrial DNA. Because mitochondria come only from the female egg, most mitochondrial DNA-related disorders are passed down from the mother.Mitochondrial DNA-related disorders can appear at any age. They have a wide variety of symptoms and signs. These disorders may cause:Blindness Developmental delay Gastrointestinal problems Hearing loss Heart rhythm problems Metabolic disturbances Short stature Some other disorders are also known as mitochondrial disorders, but they do not involve mutations in the mitochondrial DNA. These disorders are most often single gene defects. They follow the same pattern of inheritance as other single gene disorders. Most are autosomal recessive. Open ReferencesReferencesFeero WG, Zazove P, Chen F. Clinical genomics. In: Rakel RE, Rakel DP, eds. Textbook of Family Medicine. 9th ed. Philadelphia, PA: Elsevier; 2016:chap 43.Korf BR. Principles of genetics. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 35.Scott DA, Lee B. The genetic approach in pediatric medicine. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 95.