Genetics
Homozygous; 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 influenced by genes. Other characteristics influenced by heredity are:
Genes
A 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...
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- Mental abilities
- Natural talents
An abnormal trait (variant) 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 blindness
Color 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 conditions, genetic counseling is advised for couples who want to become pregnant.
Genetic counseling
Genetics 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...
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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.
Chromosomes
Chromosomes 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...
Read Article Now Book Mark ArticleEach pair of 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. A variant 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 variant 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 variant gene are present, disease may develop.
However, if only one variant gene is needed to produce a disease, it leads to a dominant hereditary condition. In the case of a dominant condition, if one variant gene is inherited from the mother or father, the child will likely show the disease.
A person with one variant gene is called heterozygous for that gene. If a child receives a variant recessive disease gene from both parents, the child will show the disease and will be homozygous (or compound heterozygous) for that gene.
GENETIC DISORDERS
Almost 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 variants
- Chromosomal conditions
- Multifactorial
A single-gene condition (also called Mendelian disorder) is caused by a variant in one particular gene. Many conditions caused by single gene variants are rare. But since there are many thousands of known single gene conditions, their combined impact is significant.
Single-gene conditions are characterized by how they are passed down in families. There are 6 basic patterns of single gene inheritance:
- Autosomal dominant
Autosomal dominant
Autosomal dominant is one of many ways that a genetic trait or disorder can be passed down through families. In an autosomal dominant disease, if you...
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Autosomal recessive
Autosomal recessive is one of several ways that a genetic trait, disorder, or disease can be passed down through families. An autosomal recessive dis...
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X-linked dominant
Sex-linked dominant is a rare way that a genetic trait or disorder can be passed down through families. One abnormal gene on the X chromosome can ca...
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X-linked recessive
Sex-linked diseases are passed down through families through one of the X or Y chromosomes. X and Y are sex chromosomes. Dominant inheritance occur...
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- Maternal (mitochondrial) inheritance
The observed effect of a gene (the appearance of a condition) is called the phenotype.
In autosomal dominant inheritance, the variations 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 variant 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 being affected with the disease is much higher in males than females. Since the variant 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 variant gene. So, almost all of the daughters of an affected man appear normal, but they are all carriers of the variant gene. Each time these daughters bear a son, there is a 50% chance the son will receive the variant gene.
In X-linked dominant inheritance, the variant gene appears in females, causing the associated condition, 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. Many of these diseases are fatal in males.
EXAMPLES OF SINGLE GENE DISORDERS
Autosomal recessive:
- ADA deficiency (sometimes called the "boy in a bubble" disease)
- Alpha-1-antitrypsin (AAT) deficiency
Alpha-1-antitrypsin
Alpha-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...
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Cystic fibrosis
Cystic 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...
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Phenylketonuria
Phenylketonuria (PKU) is a rare condition in which a baby is born without the ability to properly break down an amino acid called phenylalanine....
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Sickle cell anemia
Sickle 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...
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X-linked recessive:
- Duchenne muscular dystrophy
Muscular dystrophy
Muscular dystrophy (MD) is a group of inherited disorders that cause muscle weakness and loss of muscle tissue, which get worse over time.
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Hemophilia A
Hemophilia A is a hereditary bleeding disorder caused by a lack of blood clotting factor VIII. Without enough factor VIII, the blood cannot clot pro...
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Autosomal dominant:
- Familial hypercholesterolemia
Familial hypercholesterolemia
Familial hypercholesterolemia is a disorder that is passed down through families. It causes LDL (bad) cholesterol level to be very high. The condit...
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Marfan syndrome
Marfan syndrome is a disorder of connective tissue. This is the tissue that strengthens the body's structures. Disorders of connective tissue affect...
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X-linked dominant:
Only a few, rare, disorders are X-linked dominant. One of these is hypophosphatemic rickets, also called vitamin D -resistant rickets.
Rickets
Rickets is a disorder caused by a lack of vitamin D, calcium, or phosphate. It leads to softening and weakening of the bones.
Read Article Now Book Mark ArticleCHROMOSOMAL DISORDERS
In chromosomal disorders, the variant 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 syndrome
Klinefelter syndrome, also called 47,XXY, is a genetic condition that occurs in males when they have an extra X chromosome. This can cause problems ...
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Turner syndrome
Turner syndrome is a rare genetic condition in which a female does not have the usual pair of X chromosomes.
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MULTIFACTORIAL DISORDERS
Many 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
Asthma
Asthma 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...
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Cancer
Cancer is the uncontrolled growth of abnormal cells in the body. Cancerous cells are also called malignant cells.
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Coronary heart disease
Coronary 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...
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Diabetes
Diabetes is a long-term (chronic) disease in which the body cannot regulate the amount of sugar in the blood.
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Hypertension
Blood 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 ...
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Stroke
A 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...
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MITOCHONDRIAL DNA-LINKED DISORDERS
Mitochondria 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 (variations) 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 variations in the mitochondrial DNA. These disorders are most often single gene variants. They follow the same pattern of inheritance as other single gene disorders. Most are autosomal recessive.
References
Feero 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, Limdi NA. Principles of genetics. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 31.
Scott DA, Lee B. Genetics in pediatric medicine. In: Kliegman RM, St. Geme JW, Blum NJ, et al, eds. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia, PA: Elsevier; 2025:chap 95.
Genetics - 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.
Genetics
illustration
Genetics - 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.
Genetics
illustration
Review Date: 3/31/2024
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 C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.