INTRODUCTION

Humans take great pride in identifying distinguishing traits from one generation to the next. We enjoy speculating on the resemblance of children to their parents and question which child has, for example, the father’s eyebrows or the mother’s chin. With such observations begins the study of genetics and the submicroscopic structures known as genesA segment of DNA that contains the instructions to make a specific protein (or part of a protein). Genes are contained on chromosomes. Chromosomes, and the genes on those chromosomes, are passed on from parent to child. Errors in the DNA that make up a gene are called variants and can lead to diseases.…There is probably some geneticRelating to (or due to) genes and heredity or the field of studying genes and heredity. component in almost all disease processes, but the extent of this component varies.” – Jerry L. Northern & Marion P. Downs

Nearly 3000 genetic disorders have been identified. Of the…babies born in the United States each year, 2-3% have a major genetic or congenitalThis means ‘present at or before birth.’ It usually refers to health conditions or birth defects that are present in a baby at or prior to birth. disease. The average person has 4-8 potentially harmful genes.” – Jerry L. Northern & Marion P. Downs

Identification of genes that are responsible for inheritedAcquiring a trait from one’s parents. Most traits, such as eye color or hair color, are inherited from a parent through genes. disorders has become commonplace, if not mundane; and genetic factors in common disorders are coming to light. The promise of genetics in medicine is still largely unappreciated, but this…is changing. Genetics used to be viewed as the discipline that studied rare disorders. Now genetics is recognized as an integral part of oncology,… cardiology and neurology; eventually it will leave no area untouched.” – Bruce R. Korf

Genetics affects each and every one of us. As health care professionals, the chance that genetics will become part of our practice and patient care is increasing every day.

This brief genetics review has been designed to provide an overview of certain genetics terminology and concepts that will likely come up throughout this website, in your work, and in each of our lives.

GENERAL GENETICS

Genetics

Genetics refers to the study of genes and chromosomesA strand of DNA contained within a cell. Each chromosome contains many thousands of genes. In humans, there are a total of 46 chromosomes, half of which come from each parent. The combined total of all chromosomes in a cell is the genome., and how traits and conditions are inherited and passed through families.

Genes and DNA

Genes are the instructions that tell the cellsThe smallest living unit. Cells make up all organs and tissues in multi-cellular organisms, like humans. They can also live independently, as in bacteria and other microorganisms. At a minimum, a cell is surrounded by a membrane, contains DNA at some stage in its life, and is able to replicate itself into two equal parts. in our bodies how to grow and function. Genes are often thought of as an instruction manual or blueprint for how we develop and who we are. We all have two copies of almost all of the genes in our bodies, and each of our millions of cells contains one full set of all of our genes. We receive one copy of each of our genes from our mother, and one copy of each of our genes from our father. Similarly, if an individual has offspring, they pass one member of each gene pair onto each child.

Genes are made of a molecule called deoxyribonucleic acid (DNA)Deoxyribonucleic acid (DNA) is a molecule found in the chromosomes that carries genetic information. DNA is composed of four units (called bases) that are designated A, T, G, and C. The sequence of the bases spell out instructions for making all of the proteins needed by an organism. A gene is a section of DNA that holds the instructions for a specific protein. A change in one or more of the DNA bases making up a gene is called a mutation. Some mutations change the protein instructions and can lead to particular health problems or disorders. Each parent passes half of their chromosomes, and thus half of their DNA instructions, onto their children. It is these instructions that cause certain traits, such as eye or hair color, to be inherited.. DNA is the basic unit of heredity. DNA is composed of four units (called base pairs): Adenine (A); Cytosine (C); Guanine (G); and Thymine (T). The four units that make up DNA are like the 26 letters of the English alphabet. The sequence, or specific order, of these bases spells out instructions for the body to make certain proteins, or carry out certain functions (like how the 26 letters of the English alphabet are put together in specific orders to create words and sentences). The letters of DNA make up our genetic codeThe genetic code determines how the sequence of bases in a gene code for the sequence of amino acids in a protein. A gene is made of bases that are designated A, T, G, and C. Each series of three bases is essentially a word that codes for one of the 20 amino acids that make up all proteins. For example, the sequence AAG within a gene tells a cell to insert the amino acid lysine into a growing protein..

The figure below shows the relationship between a cell, chromosome, DNA, gene, and bases. Chromosomes will be discussed later in this review.

DNA has a double helix structure, which is sometimes described as a twisted ladder. The “sides” of the DNA ladder are made of phosphate and sugar, and the “rungs” are made of base pairs. A always pairs with T, and C always pairs with G. Base pairs are also used as a unit of measure to indicate a length of DNA, or the size of a gene. For example, a gene or piece of DNA that is 10bp long consists of 10 base pairs, whereas a gene or piece of DNA that is 2Kb long consists of 2000 base pairs.

Genes and Proteins

The genetic code of DNA tells the body to make certain proteins. From DNA, a complimentary molecule called ribonucleic acid (RNA) is made, and from RNA, proteins are made. Thus, RNA is the intermediate molecule between DNA and proteinA molecule that makes up many parts of every cell in the body. Examples of proteins include hormones, enzymes, hair, and antibodies. Proteins are made up of 20 different types of individual units called amino acids. It is the order of these amino acids in a protein that determines what form and function a protein has. Each gene holds the instructions for making a single protein.; in other words, it helps to translate the instructions of DNA into real proteins.

Proteins are the molecules in our body that make up the parts of our body, and that carry out jobs in our body. For example, our tissues and organs are all made up of proteins. In addition, enzymesA molecule that helps chemical reactions take place. For example, enzymes in the stomach speed up the process of breaking down food. Each enzyme can participate in many chemical reactions without changing or being used up., hormonesHormones include many different types of chemicals that act as messengers around the body. Hormones are made by specific endocrine glands and are secreted into the blood when needed by other parts of the body. Hormones travel to other organs and tissues in the body to signal them to do something. For example, insulin is a hormone that signals muscle and fat cells to remove glucose from the blood and enter the cells. and antibodiesAs part of the immune system, antibodies are found in the blood and help to detect and destroy invaders like bacteria and viruses. are all specific types of proteins. The smaller units of protein are called amino acidsAmino acids are small molecules that make up proteins. There are over 100 different amino acids, but our body uses only 20 amino acids to make all of its proteins. Our genes determine the sequence of amino acids in a protein. This sequence determines what shape the protein takes, and what function that protein serves in the body.. There are over 100 different amino acids in nature, but our bodies use only 20 amino acids to make all of their proteins.

Alleles, Genotypes and Phenotypes

Recall that we have two copies of each of our genes. Alleles refer to the different forms of a gene. For example, a difference in the sequence of bases between two copies of a gene would mean that these two copies are different alleles (different forms of the gene). Different alleles of the same gene may code for different forms of a protein. Sometimes, however, different alleles will not affect the protein they code.

When a person is a heterozygote for a certain gene (heterozygous for a gene), this means that their two copies of this gene are different from each other. When a person is a homozygote for a certain gene (homozygous for a gene), this means that their two copies of this gene are the same.

Genotype refers to all of the alleles of all of the genes that a person has. More broadly, the genotype of an individual is their total genetic makeup. Phenotype, on the other hand, refers to the physical characteristics of an individual. These characteristics may be ones that are visible to the eye (such as hair color, eye color, and height), or they may be internal or biochemical (such as blood pressure and IQ). The phenotype of a person results from their genotype, often in combination with their environment.

The terms alleleName for different forms of a gene. Different alleles of a gene may produce different forms of a protein. Some alleles of a gene may form altered proteins that do not function properly. These can often lead to disease., heterozygous, homozygous, genotype and phenotype are explained in the following figure. Dominant and recessive will be reviewed later. Please note that this illustration depicts a simplified example of eye color inheritance.

GENE ALTERATIONS

Mutations

A mutationA change or alteration that occurs in the DNA. Mutations can be caused by the environment (sun, radiation, or chemicals), aging, or chance. Often the causes of mutations are never known. Some mutations do not affect the information contained in the DNA. Other mutations have serious consequences on how that gene functions. is any change in the usual sequence of DNA. For example, suppose part of a gene usually has the sequence GTAC. If the sequence in a copy of the gene was GTTC, this change from A to T would be considered a mutation. Some mutations cause conditions, others contribute to the healthy diversity between all people, and still other mutations do not cause any change and do not affect the person who has them at all. Whether or not a mutation has any effect depends upon whether it affects the form or function of the protein it codes for.

The causes of mutations are often unknown. Mutations in the genes of a person’s germline (eggs and sperm, as opposed to mutations in non-sex cells), can be passed down to their offspring. On the other hand, some of the mutations we are born with likely occurred just by chance. Still other mutations may be caused by things such as the environment (sun, radiation, or chemicals) or aging.

Polymorphisms

Polymorphisms refer to a change (mutation) in the DNA sequence that is present in at least one percent of the population. Polymorphisms are generally considered to be “normal” variations in the sequence of DNA, and are generally not considered harmful. One example of a polymorphismA change in the DNA sequence (variant) that is present in at least 1 percent of the population and is not considered harmful. One example of a polymorphism is in the hair color gene. Slight changes in the DNA sequences make the hair blond or brown. is in the hair color gene. Slight changes in the DNA sequence code for different hair. Other polymorphisms do not cause any visible or significant change in the people who have them.

INFORMATION ABOUT CHROMOSOMES

Chromosomes

Chromosomes are composed of DNA, and are tiny structures in the nucleus of our cells. Our genes are packaged into chromosomes, and so genes are located all along each of our chromosomes.

In humans, there is usually a total of 46 chromosomes (23 pairs) in each cell. 44 of these chromosomes (22 pairs) are called autosomes, which means that they are the same in both males and females. The final pair of chromosomes are called the sex chromosomes. Females have two X chromosomes, and males have one X chromosomeOne of the two chromosomes that are responsible for determining the sex of an organism. The other sex chromosome is called the Y chromosome. Both the X and the Y chromosome contain several genes, only some of which are involved in determining sex. In humans, the X chromosome is the larger of the two sex chromosomes. Females usually have two X chromosomes and males usually have one X chromosome and one Y chromosome. and one Y chromosomeOne of the two sex chromosomes that is responsible for determining the sex of an organism. The other sex chromosome is called the X chromosome. Both the X and the Y chromosome contain several genes, only some of which are involved in determining sex. In humans, the Y chromosome is the smaller of the two sex chromosomes. Most females have two X chromosomes and most males have one X chromosome and one Y chromosome..

Karyotypes

When chromosomes are studied in the laboratory, they are usually put into order (by pair, size and shape) to make an organized chromosome picture called a karyotype. A karyotypeAn arranged picture of the chromosomes from a single cell. The chromosomes are arranged in order (by numbered pair, size and shape) from a picture of chromosomes taken under a microscope. A karyotype allows cytogeneticists (scientists who study chromosomes) to see whether an individual has any extra or missing genetic material, or any rearrangements that are large enough to be seen. allows cytogeneticists (scientists who study chromosomes) to see whether an individual has any extra or missing genetic material, or any rearrangements in their chromosomes that are large enough to be seen. A karyotype does not allow for changes in individual genes to be seen.

Normal Female Karyotype: 46,XX

Normal Male Karyotype: 46,XY

Chromosome Segregation

As with gene pairs, we each receive one member of each chromosome pair from our mother, and the other member of each pair from our father. Similarly, when we have offspring, we pass one member of each of our chromosome pairs to each of our offspring. When a male passes on an X chromosome, the offspring will be female, and when he passes on a Y chromosome, the offspring will be male.

The figure to the right shows the transmission of chromosomes (and therefore genes) from parents to children.

In this figure, three pairs of chromosomes are shown:
pair #1 (green);
pair #2 (yellow);
pair #3 – sex chromosomes (pink and blue).

The father’s chromosomes are shown in solid color, and the mother’s are striped. Children randomly get one member of each chromosome pair from their mother (striped) and one member of each pair from their father (solid). Daughters get an X from their mother (striped) and an X from their father (solid). Sons get an X from their mother (striped) and a Y from their father (solid).

Chromosome Nomenclature

When karyotypes are complete, the chromosome makeup of an individual is written as:
46, XX for a normal female; and 46, XY for a normal male. The number refers to the total number of chromosomes in each cell, and the letters refer to the sex chromosomes. Additional notation is added when there is a difference from the usual chromosome complement.

There is also a specific way in which a particular spot or region on a chromosome is written; this is used to refer to the particular location of a gene on a chromosome.

The figure on the left is called an ideogram;
this simply means that it is a drawing of a
chromosome with locations labeled. The shorter
arm of the chromosomes is labeled p, and the
longer arm is labeled q. Along each arm are
numbers that indicate specific spots along the
chromosome.

 

An example of a gene location on a chromosome is 17q21. This refers to a gene located on the long (q) arm of chromosome 17. The 21 following the q refers to the exact spot of the gene on that arm of the chromosome. 17q21 is the location of the BRCA1 gene, which is associated with an increased risk of breast, ovarian and prostate cancers.

The Human GenomeAll of the genetic material (DNA) contained in a full set of chromosomes in an organism. In humans, about three billion base pairs make up our genome. Project has been working towards the goal of mapping (determining the location of) all genes in the human genome. A genetic mapA map of where genes are located relative to each other on the chromosome. Genetic maps are also called linkage maps. is a map of the location of genes relative to each other on the chromosome. (Genetic maps are also called linkage maps.)

INHERITANCE PATTERNS

Inheritance patterns describe the ways in which traits or conditions are passed through families. The various patterns of inheritance are described in the following pages.

Autosomal RecessiveMost of the metabolic disorders that can be detected by newborn screenings are inherited in an “autosomal recessive” pattern. Autosomal recessive conditions affect both boys and girls equally. How autosomal recessive inheritance works: Everyone has a pair of genes for each enzyme in the body. A separate pair of genes is responsible for making each enzyme. A person with a metabolic disorder has one enzyme that is either missing or not working properly. The problem is caused by a pair of “recessive” genes that are not working correctly. They do not make the needed enzyme. A person has to have two non-working “recessive” genes in order to have an autosomal recessive metabolic disorder. A person with an autosomal recessive disorder inherits one non-working gene from his or her mother and the other from his or her father. The parents are called carriers for that condition. Parents of children with a metabolic disorder rarely have the disorder themselves. Instead, for that pair of genes, each parent has one that is working correctly and one that is not working (called the “recessive” gene). People with a single non-working gene are called carriers. If one gene of the pair is working correctly, it makes up for the recessive non-working gene. Therefore, carriers usually will not have the condition. Inheritance

Autosomal means that the changed gene is located on an autosomeOne of the two types of chromosomes found in most animals. Autosomes contain many thousands of genes, but do not contain genes that determine the sex of an organism. Sex chromosomes, on the other hand, do contain sex-determining genes. Humans have a total of 46 chromosomes; 44 autosomes (chromosome pairs 1-22) and two sex chromosomes (X and Y). (non-sex chromosome), so males and females are equally likely to be affected. Recessive means that both copies of the gene must be changed in order for a person to have the condition. In autosomal recessive inheritance, a person must inherit two copies of a particular form of a gene in order to show the trait or have the condition. PKU is an example of a disorder that is inherited in an autosomal recessive manner.

The figure to the right shows that for conditions inherited in an autosomal recessive manner, both copies of the gene must be altered (mutated) and not working in order for a person to have the condition. If a person has a mutation in only one copy of the gene, they are a carrier and are not affected with the condition.

The figure above shows that two parents who are carriersA person who has one copy of a gene mutation for a particular autosomal recessive disorder (remember genes come in pairs). Carriers are not affected by the disorder. However, they can pass on the gene variant to their children. Children who inherit two such gene variants will be affected by the disorder. The term variants is now used in place of the term mutation. of an autosomal recessive condition have a 25% (1 in 4) chance of each child inheriting the condition.

Autosomal DominantGenes come in pairs. If one gene of a pair causes, by itself, a particular trait or disorder to be present, it is called ‘dominant.’ If the gene is found on one of the first 22 pairs of chromosomes, it is called autosomal dominant (chromosomes 1 through 22 are called ‘autosomes’). Although genes are always in pairs, in autosomal dominant inheritance, you only need to inherit one copy of a dominant gene in order to show a particular trait.Autosomal dominant conditions can be inherited from one parent, who also has that condition or the gene for that condition, or can occur in one person in the family for the first time (called a ‘new mutation’). If neither parent has the autosomal dominant gene change that the child has, it is a ‘new mutation’ just present in that child, and there is a low risk for it to affect other siblings. However, the person with the autosomal dominant gene change has a 50% chance to pass it on to each of his or her children. Inheritance

Autosomal means that the changed gene is located on an autosome (non-sex chromosome), so males and females are equally likely to be affected. Dominant means that only one copy of the “dominant” gene needs to be changed in order for a person to have the condition. In other words, although genes are always in pairs, a person needs to inherit only one copy of a particular form of a gene in order to have an autosomal dominant condition. Huntington’s disease is an example of a disorder inherited in an autosomal dominant manner.

The figure to the right shows that for conditions inherited in an autosomal dominant manner, only one copy of the gene needs to be altered (mutated) in order for a person to have the condition.

 

The figure above shows that when an individual with an autosomal dominant condition has a child with an individual who does not have the condition, each of their children has a 50% (1 in 2) chance of having the condition. 

X-linkedA mode of inheritance. X-linked genes are found on the X chromosome. They have a different inheritance pattern than other genes because women have two X chromosomes while men only have one. Any mutation on the X chromosome may not cause a disease in women if the gene on the other chromosome is normal. However, that same mutation on one X chromosome in men will cause the disease because they have no second copy of the gene to compensate. Inheritance

X-linked inheritance refers to conditions or traits for which the gene is located on the X chromosome. Recall that females have two X chromosomes while males have only one X chromosome.

The majority of X-linked conditions are X-linked recessiveThis is a pattern of inheritance in which a gene for a particular trait or disorder is located on the X chromosome. Genes that are ‘recessive’ cause traits or conditions only when they are paired with a dominant gene. Genes usually come in pairs, except on the sex chromosomes in males. Males have one X chromosome that they inherit from their mothers and one Y chromosome that they inherit from their fathers. If a gene on the X chromosome causes a particular trait or disorder, a male will always show that trait or condition as they do not have another gene to cover up its effects. Females usually have a normal copy of that gene on their other X chromosome which covers up the effects of the recessive gene. Some common X-linked recessive disorders include hemophilia, Duchenne muscular dystrophy and color blindness., meaning that one normal (working) copy of the gene would compensate for a non-working copy. It is much more common for males to have X-linked recessive conditions than females, since males do not have a second copy of their X chromosome to compensate if their one copy has a mutation. There are some cases when females can be affected with an X-linked recessive condition, but this is much less common.

X-linked dominant inheritance is quite rare.

The figure to the right shows that females will only be affected with an X-linked recessive condition if both copies of the gene on their X chromosome have mutations (this is quite rare). If a female has a mutation in only one copy of the gene on their X chromosome, they will not have the condition. Males, on the other hand, will have the condition when the gene on their one X chromosome has a mutation, since they do not have a second copy to compensate.

The figure above shows that when a women is a carrier of an X-linked recessive condition and has a child with a man who does not have the condition, each of their male children has a 50% (1 in 2) chance of having the condition and each of their female children has a 50% (1 in 2) chance of being a carrier of the condition.

When a female is a carrier of an X-linked recessive condition (i.e., has a mutation in one copy of an X-linked gene), each of her offspring will have a 50% (1 in 2) chance of inheriting the working copy, and a 50% (1 in 2) chance of inheriting the non-working copy. Thus, daughters of a female carrier have a 50% chance of being a carrier and a 50% chance of being a non-carrier, and sons have a 50% chance of being affected and a 50% chance of not being affected.

When a male has an X-linked condition, all of his daughters will be carriers (since all female offspring receive an X from their father), and none of his sons will be affected (since he will pass on his Y chromosome to all of his sons).

Mitochondrial / MaternalHaving to do with the mother. Inheritance

The normal 46 chromosomes in our body are contained in the center of the cell, which is called the nucleus. MitochondriaThese are the parts within each cell that make energy for the body. are structures located in the cytoplasm of the cell outside of the nucleus. Mitochondria also contain genes that are separate from the ones in the nucleus, although the mitochondrial DNA is one long string of genes and is not arranged as chromosomes. Mitochondria are organelles that provide much of the energy cells used for the work they do. All of the mitochondria in a person’s cells descend from the mitochondria present in the original egg from that person’s conception. The sperm does not contribute any mitochondria to the baby.

Thus, an individual’s mitochondria are only inherited from his or her mother. This pattern of inheritance is called mitochondrial or maternal inheritance. An abnormality in one of the mitochondrial genes can therefore be passed by the mother in her egg cells. Because mitochondria can be inherited only from a mother’s egg, mitochondrial genes show a very distinct pattern of inheritance: Both males and females can be affected with a mitochondrial disease gene, but only females can transmit that mitochondrial disease gene to children.

Multifactorial Inheritance

Many conditions are caused by a combination of genes and other factors, such as the environment. These conditions are said to be “multifactorial.”  People who have such a condition are often born into families with no other affected people. Parents of a child with the condition have a greater chance of having another child with the condition than couples who do not have a child with the condition.

Mutifactorial inheritance is actually the most common form of inheritance; most traits and characteristics are inherited in a multifactorial fashion.

This figure above shows that a certain combination of genetic and environmental factors is needed in order for a person to be affected with a multifactorial condition.

GENETIC TESTING & SCREENING

Genetic Testing

A genetic test involves looking at the letters that make up the instructions of a person’s code of DNA (genes) to see if certain mutations (changes) are present. A person’s genes are usually looked at from a small sample of blood. Genes can also be examined from other body tissues, such as a cheek swabThis is a cotton covered stick that is used to painlessly collect cells from the inside of the mouth. The cotton tip is swirled on the inside of the cheek and then placed in a sterile tube. The cells collected are used for specific genetic tests., or a tissue sample.

There are two overall types of genetic testing: DNA SequencingThe process of determining the order of the bases in a region of DNA.; and Mutation Analysis.

In “DNA sequencing,” the DNA code of letters is read along the entire gene (or for a certain part of it). In this way, any changes in the spelling of the gene’s instructions will be seen. In some cases, a change may be found that is known to be associated with a condition or an increased risk for a condition. In other cases, there may not be any changes found. In still other cases, a change may be found, but the meaning of this change may not be known at this time.

In “mutation analysis,”  testing is done to look at a specific region of a gene, rather than reading the code all along a gene. Mutation testing is usually done if the mutation in a family is already known, or if there are certain mutations that are usually associated with a condition.

What are Some Benefits of Genetic Testing?

If a mutation (change) is detected, it may explain why the person has the condition. In some cases, knowing what mutation a person has will allow doctors to predict how severe the condition might become and what other symptoms may be expected. This may allow the person’s medical care to be adjusted accordingly. Knowing the mutation responsible will also predict the chances that future children may inherit the condition from the parent. Prenatal testing (testing a baby before it is born to see if it has the mutation) may also be possible.

What are Some Limitations of Genetic Testing?

Not all of the genes that are involved in conditions are known, so even if a condition runs in the family, it may not be possible to find the mutation involved.

Likewise, a negative result (no change is found) does not guarantee that the person will not get the condition. The person may have a different mutation that was not detectable by the test used, or the person may have a mutation in a different gene that also causes the same condition.

Genetic tests are different from other medical tests in that the results may provide information about other members of the family, and not just the person being tested. In addition, some people are concerned about keeping the results of their genetic testing private. Test results should not be seen by anyone who is not involved in the testing, unless permission is obtained.

Genetic ScreeningThe process of testing for disease in a person who does not show signs of having the disease (nonsymptomatic or asymptomatic person). The goal of screening is to catch the disease in its early stages.

Screening is the process of testing for disease in a person who does not show signs of having the disease (nonsymptomaticNot showing any of the signs of the condition or disease. or asymptomaticNot showing any of the signs of the condition or disease. person). The goal of screening is generally to catch the disease in its early stages. It is important to note that with most screening, follow up testing is required to confirm a diagnosis.

Below are the possible results of screening

Possible Results	True Positive	True Negative	False PositiveA test result indicating that a person has a mutation or a disease when, in fact, they do not have a mutation or disease.	False NegativeA test result indicating that a person does not have a mutation or change predisposing them to a disease when, in fact, they do have this mutation or disease.
Person’s health status	Person HAS the disease	Person does NOT have the disease	Person does NOT have the disease	Person HAS the disease
Test Result	Confirms that person HAS the disease	Confirms that person does NOT have the disease	Test results say that person HAS the disease.	Test says that the person does NOT have the disease

Genetics Evaluation

Below are definitions of a few terms related to the Genetics Evaluation.

A geneticistA person who specializes in genetics. is a medical doctor with a specialty in genetics. A genetic counselorThese are health care providers who have special training in genetic conditions. They help families understand genetic disorders and how they are passed down. Genetic counselors offer information and support to people who have genetic conditions in their families or are concerned that they may have a child with an inherited disorder. is a healthcare professional who provides information and support to individuals and families who have a genetic disorder, might be at risk for developing an inherited condition, or are concerned that they may have a child with an inherited disease. The geneticist and genetic counselor work together to obtain and analyze family medical histories, and calculate and explain people’s risks, options, and testing results.

A pedigreeA medical drawing that includes all of a person’s close relatives, the relationship between family members, and health information. A pedigree is used by health care professionals to analyze a family for genetic diseases. is a medical drawing of a family tree that includes all of a person’s close relatives, the relationship between family members, and health information. A pedigree is used by health care professionals to analyze a family for conditions which certain family members may have or be at risk for.

Pedigrees also show the relationship between individuals in a family. Males in a pedigree are indicated by squares, and females are indicated by circles. The lines between individuals enable us to see the relationship between them.

A Few Final Terms

• Congenital refers to features or conditions that a baby is born with, as opposed to conditions that develop later in life. For example, congenital hearing loss is hearing loss which a baby is born with compared to hearing loss due to old age.
• Acquired means that a feature or condition developed later in life.
• Syndromic refers to a group of symptoms and clinical findings that, when found together in a single individual, make up a particular condition or disease.
• Nonsyndromicmeans that a person does not have any other symptoms other than an isolated clinical finding. Nonsyndromic cleft lip, for example, is an isolated birth defect without any other symptoms.
• Familial indicates a disease or clinical finding that runs in the family. In other words, a familial condition is one in which more that one family member is affected.
• SporadicOccurring occasionally or randomly. In medical terms, a sporadic disease is one in which the disease occurs in people with no family history and no inherited cause. refers to a condition that does not run in the family. Its occurrence is isolated and is limited to only one family member.