A

Absolute Risk (AR)

is defined as the chance of a person developing a specific disease over a specified time-period. For example, a woman’s lifetime absolute risk of breast cancer is one in nine. That is to say, one woman in every nine will develop breast cancer at some point in their lives. Absolute risk can also be expressed as a percentage (e.g. a woman’s absolute risk of breast cancer can be written as 11% instead of one in nine) or as a decimal (one in nine becomes 0.11).
Related terms: Adjusted life-time risk, Allelic odds ratio, Relative risk, Genotype relative risk, Life-time risk, Multiplicative risk model, Overall genetic risk

Adenine

One of the four nucleotides in DNA that make up the letters ATGC, adenine is the “A”. The others are thymine, guanine, and cytosine. Adenine always pairs with thymine in the rungs of the ladder within the DNA double helix.
Related terms: Cytosine, DNA, Guanine, Nucleotides, Thymine

Adjusted life-time risk

is the life-time risk of an individual for a given disease adjusted with his identified genetic risk based on the knowledge of his genotypes (disease risk SNPs). The adjusted life-time risk is calculated by multiplying the life-time risk with the identified genetic risk which is expressed relative to the overall general population risk. The unadjusted life-time risk is based on the overall population risk of known and unknown causes and risk factors like environmental and life-style factors and the average or overall genetic contribution in the reference population.
Related terms: Absolute risk, Relative risk, Genotype relative risk, Life-time risk, Multiplicative risk model, Overall genetic risk

Alleles

are different versions of the same gene. An allele is the version of a particular SNP or chromosome segment that you inherited from either your mother or father. Your cells carry 23 pairs of chromosomes, where one was inherited from your mother and the other from your father. This means that for any nucleotide or SNP located on an autosomal chromosome you have inherited two versions (one maternal and the other paternal). These are usually referred to as your two alleles for that particular location in the genome.

Allelic odds ratio (allelic OR)

is defined as [ s / (1-s) ] / [ f / (1-f) ] where ‘ s ’ is the frequency of a disease-associated variant in cases and ‘ f ’ is its frequency in controls. Most genetic association studies report allelic OR to summarize the magnitude of risk conferred by the tested markers.
Related terms: Absolute risk, Adjusted life-time risk, Relative risk, Genotype relative risk, Life-time risk, Multiplicative risk model, Overall genetic risk

Amino acids

are the building blocks of proteins made in the body. In humans there are 20 amino acids. Each amino acid is encoded by a sequence of 3 nucleotides that instructs the cell to insert that amino acid in a specific position as the protein is assembled.
Related terms: Nucleotides, Protein

Antibody

are gamma globulin proteins that are found in blood or other bodily fluids, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses. Recognition of an antigen by an antibody tags it for attack by other parts of the immune system.
Related terms: Antigen

Antigens

are substances that prompt the generation of antibodies and can cause an immune response. Antigens are usually proteins or polysaccharides. These can be parts of bacteria, viruses, or other microorganisms. Antigens are also defined as substances that bind to specific antibodies. Recognition of an antigen by an antibody tags it for attack by other parts of the immune system.
Related terms: Antibody

Autosomal

refers to the location on any chromosome other than a sex chromosome (X- or Y-chromosome). Humans have 22 pairs of autosomes.

Autosomal dominant

refers to the effect of a genetic variant with only one copy on either the maternal or the paternal chromosome of any of the 22 autosomal chromosome pairs (chromosomes other than the X or Y sex chromosomes) needed for the disease or characteristic to show. Each child of an individual with an autosomal dominant trait or disorder therefore has a 50% risk of inheriting it. Affected individuals either have an affected parent or have a new mutation. Lactase persistence is an example of an autosomal dominant trait. A genetic test for Lactose Intolerance is included in the deCODEme complete scan.

Autosomal recessive

refers to the effect of a genetic variant with two copies needed for the characteristic or disease to show, one on the maternal and one on the paternal chromosome of any of the 22 autosomal chromosome pairs (chromosomes other than the X or Y sex chromosomes). Those carrying one copy of a recessive variant are said to be carriers or heterozygous where as those who have two copies are homozygous and have an affected status. Only when both parents are carriers is there a risk (25%) of having an affected child. Hemochromatosis which is included in the deCODEme complete scan, is in its most common forms, an example of an autosomal recessive trait.

B

Base pair

Two nitrogenous bases paired together in the rungs of the double-stranded DNA double helix by weak bonds; specific pairing of these bases (adenine with thymine and guanine with cytosine) ensures accurate DNA replication.

Bayes’ rule

The rule that expresses the joint probability in terms of the conditional probability is:

Pr (A , B) = Pr (A | B) Pr ( B ) = Pr ( B | A) Pr ( A )

For independent events where the conditional probability is the same as the probability of the event, i.e.

Pr (A | B) = Pr (A) and Pr (B | A) = Pr (B),

we have that the joint event probability, Pr (A , B), is simply Pr (A) Pr (B).
Related terms: Bayes’ theorem, Probability.

Bayes’ theorem

expresses the conditional probability in relation to the total probability of mutually exclusive events:

Pr (A _i | B) = Pr (B | A _i) Pr (A _i) / ( Pr (B | A _i) Pr (A _i) + ... + Pr (B | A _n) Pr (A _n) ) = Pr (B | A _i) Pr (A _i) / Pr (B)

Related terms: Bayes’ rule, Probability.

BRCA1 / BRCA2

are the first breast cancer genes to be identified. BRCA1 is a gene on chromosome 17 and BRCA2 is on chromosome 13. Both are genes that normally help to suppress cell growth, also known as tumor suppressor genes. Mutated forms of these genes are believed to be responsible for about half the cases of inherited breast cancer, especially those that occur in younger women. A person who inherits a mutated BRCA1 or BRCA2 gene also has a higher risk of getting ovarian (females), or prostate cancer (males). deCODEme does not include tests for these high-risk variants.

C

Candidate gene

a gene, located in a chromosome region suspected of being involved in a disease.

Carrier

an individual who has one copy of a genetic variant (is heterozygous) for a single recessive gene (that causes disease only when two copies are present). Although carriers are not affected by the recessive disease gene, two carriers can have a child with the disease, if the child receives one copy of the recessive gene from each parent.

Chromosomes

are compact packages of DNA contained within single cells. Unraveled, the ultra-thin strands of DNA from a single human cell are about 3 meters long. The only way for each and every cell of the body to carry such vast amounts of DNA is by winding it into the complex bundles, known as chromosomes, thereby taking up less space inside the cell nuclei. We inherit 46 chromosomes from our parents, which are divided into 23 pairs. One chromosome from each pair comes from our father and the other from our mother. It is this mix of chromosomes from our parents that determines our sex and our characteristics, including our propensity to develop various diseases.

Chromosome 1

Of the 23 pairs of chromosomes within each human cell, chromosome 1 is the largest, spanning about 247 million base pairs and representing about eight percent of the total DNA in human cells. Currently, researchers estimate that there are more than 3,000 genes located on Chromosome 1.

Chromosome 2

is the second largest human chromosome, spanning more than 243 million base pairs and representing about eight percent of the total DNA in human cells. Chromosome 2 is estimated to contain over 1,300 genes.

Chromosome 3

spans almost 200 million base pairs and represents approximately 6.5 percent of the total DNA in human cells. The estimated number of genes on chromosome 3 is between 1,100 and 1,500.

Chromosome 12

spans about 132 million DNA building blocks (base pairs) and represents between 4 percent and 4.5 percent of the total DNA in cells. Chromosome 12 likely contains between 1,200 and 1,400 genes, some of them found to associated with certain health conditions, for example the SLCO1B1 gene, a variant of which predisposes people to statin-induced myopathy.
For more on chromosome 12, visit the U.S. Library of Medicine´s Genetic Home Reference.

Chromosome 21

is the smallest human chromosome, spanning about 47 million base pairs (the building blocks of DNA) and representing approximately 1.5 percent of the total DNA in cells. The number of genes on chromosome 21 is currently estimated between 300 and 400. Chromosome 21 is the only chromosome that can be present in three copies in a healthy human body rather than two. Children who are born with an extra chromosome 21 have Down syndrome, also known as Trisomy 21.
For more on chromosome 21, visit the U.S. Library of Medicine´s Genetic Home Reference.

Congenital

Present at birth. A congenital condition is one that an individual is born with, but is not necessarily genetic.

Compound heterozygous

describes those who are heterozygous (have different alleles on each chromosome pair) at two different locations on genes which both have to do with the disease or condition.

Creatine Kinase

(CK), also known as phosphocreatine kinase (CPK), is a type of protein called an enzyme. The normal function of CK in our cells is to add a phosphate group to creatine, turning it into the high-energy molecule phosphocreatine, an energy source for various cells, including muscle cells. A CPK test measures the amount of creatine kinase that leaks into the blood in the event of cell damage, and can therefore used to diagnose a heart attack (indication of damage to heart muscle cells), determine if or how badly a muscle is damaged, detect early muscle disease, reveal muscular dystrophy (Duchenne) and more.

Cytosine

One of the four nucleotides in DNA that make up the letters ATGC, cytosine is the “C”. The others are adenine, thymine, and guanine. Cytosine always pairs with guanine in the rungs of the ladder within the DNA double helix.
Related terms: Adenine, DNA, Guanine, Nucleotides, Thymine

D

Deletion

a specific type of genetic mutation involving the absence of a segment of the genetic material from a chromosome. Deletions can lead to diseases and disorders depending on where they occur. The deleted sequence may be as small as a single nucleotide base or as large as one or more genes.

De Novo (new) mutation

is a term used for a genetic variant that appears for the first time within the family as a result of a mutation in the egg from the mother, sperm from the father, or in the early embryo after fertilization of the egg.

Deoxyribonucleic Acid or DNA

is the genetic material which contains the genetic code and is found within the nuclei of every cell in your body. It is made of long chains of building blocks called nucleotides. These chains form the iconic twisting ladder structure (the DNA double helix) that was discovered by the Nobel laureates Francis Crick and James Watson in 1953. DNA is like an organic blueprint or book of recipes that contains all the instructions needed to make an organism like you, to renew your cells and to keep your body functioning properly.

Diagnostic test

any kind of medical test performed to aid in the diagnosis or detection of disease. The results provided in deCODEme are not diagnostic in that they do not diagnose or detect disease. deCODEme results however provide genetic risk assessment results based on scientific studies on the association of specific genetic variants and common diseases and traits. The genetic risk assessments can supplement overall risk assessments based on family history, lifestyle and environmental risk factors.

Diploid

the number of chromosomes in most cells except the eggs or sperm cells (which contain a haploid number of chromosomes (half the diploid or 23). In humans, the diploid number is 46.

Dizygotic twins

are twins derived from separate eggs independently fertilized by two different sperm cells. Such twins are also known as fraternal twins and share about 50% of their chromosomes, so they can be of different sexes. Fraternal twins are more common than identical twins.

Duplication

a particular kind of a genetic mutation in which production of one or more copies of any piece of DNA occurs, from a single gene to an entire chromosome. Duplications can lead to diseases or disorders.

E

Enzymes

are proteins that speed up chemical reactions in the body. The body produces and uses thousands of different types of enzymes, including metabolic enzymes, which run our bodies; digestive enzymes, which digest our food; and food enzymes from raw foods, which start food digestion.

Exon

The region of a gene that contains the code for producing that genes specific protein. Each exon codes for a certain part of the complete protein. In humans, a gene’s exons are separated by long regions of DNA (called introns or sometimes ‘junk DNA’) that have no apparent function.

F

Female line testing

Mitochondrial DNA or mtDNA is inherited only from mother to child. Due to its peculiar mode of inheritance through the mother, mtDNA can be used to determine the genealogical relationship between two or more individuals (men or women) living today through the direct female line. Ultimately, all humans can trace their mtDNA through the direct female line to a single female ancestor, playfully named “mitochondrial Eve”.

First-degree relative

a relative with whom you share about half (50%) of your genes, such as your parents, full siblings, and your children.

Fraternal twins

are twins derived from separate eggs independently fertilized by two different sperm cells. Also known as dizygotic twins, they share about 50% of their chromosomes as do siblings in general and they can be of different sexes. Fraternal twins are more common than identical twins.

G

Genes

are especially important segments of the chromosomes:/that directly influence one or more traits. They are relatively small segments of chromosomes, where the sequence of DNA nucleotides encodes a recipe for making a protein. The human genome has approximately 20,000 – 25,000 genes.

Genetic Scan

Analysis of the genome by typing it for multiple sequence variations. The deCODEme Genetic Scan measures over one million genomic locations with single nucleotide variations (SNPs).

Genetic Marker

A segment of DNA with a known and identifiable location on a chromosome and whose inheritance can be followed. A marker can be any section of DNA (up to a whole gene) with known or unknown function. Because DNA segments that are close to each other on a chromosome tend to be inherited together, markers are often used to track patterns of inheritance of genes that have not yet been identified, but whose approximate locations are known.

Genetic risk relative to the population (RR)

is the risk identified to be conveyed by a genetic marker, expressed as relative risk (RR) compared to the general risk of the population. If a genetic marker conveys neither increased or decreased risk its relative risk is one (1.0). RR of 1.5 means that the marker conveys increased risk that is 1.5 times the general population risk or a 50 % increase. RR of 0.5 means that the genetic risk is half the general population risk or a 50% decrease.
Related terms: Absolute risk, Adjusted life-time risk, Allelic odds ratio, Relative risk, Genotype relative risk, Life-time risk, Multiplicative risk model, Overall genetic risk

Genetic Variant or Variation

An alteration or change in the genetic sequence. Only well established risk variants are used in deCODEme.

Genome

All the DNA contained in an organism or a cell, which includes both the chromosomes within the nucleus and the DNA in mitochondria. The human genome has approximately 20,000 – 25,000 genes.

Genome Scan

Analysis of the genome by typing it for multiple sequence variations. The deCODEme Complete Scan measures over one million genomic locations with single nucleotide variations (SNPs ).

Genome-wide association study

involves rapidly scanning genetic markers across complete sets of DNA of a large number of individuals to find genetic variations associated with the particular disease under study. Such studies are particularly useful in finding genetic variations that contribute to common, complex diseases, such as asthma, cancer, diabetes, heart disease and mental illnesses. The results provided in deCODEme are based on these types of studies.

Genotype

is the combination of an individual’s maternal and paternal alleles for a particular SNP or chromosome segment. In general the term genotype refers to the genetic makeup of an individual person.

Genotype relative risk

(r): is defined as the risk associated with a genotype relative to the genotype that does not have the risk variant (non-carrier). Hence the relative risk of the genotype that does not have the risk variant (the lowest risk) is equal to one by definition.
Related terms: Absolute risk, Adjusted life-time risk, Allelic odds ratio, Relative risk, Life-time risk, Multiplicative risk model, Overall genetic risk

Genotyping

The process of determining the genotype of an individual. Single nucleotide polymorphisms (SNP) are the most common type of genetic variation.

Germ line

the DNA passed from parents to offspring through their eggs or sperm.

Guanine

One of the four nucleotides in DNA that make up the letters ATGC, guanine is the “G”. The others are adenine, thymine, and cytosine. Guanine always pairs with cytosine in the rungs of the ladder within the DNA double helix.
Related terms: Adenine, Cytosine, DNA, Guanine, Nucleotides, Thymine

H

Haploid

the number of chromosomes in a sperm or egg cell (23), or half the diploid number of 46 chromosomes that are in all other cells of the body.

Haplogroup

is a group of similar haplotypes that share a common ancestor with a single nucleotide polymorphism (SNP) mutation. Haplogroups can be used to define genetic populations and are often geographically oriented.

Haplotype

a combination of alleles at various different genetic sites (at multiple loci) that are transmitted together on the same chromosome. The term haplotype may refer to as few as two loci or to an entire chromosome.

Hemizygous

an individual who has only one member of a chromosome pair or chromosome segment rather than the usual two; refers in particular to X-linked genes in males who under usual circumstances have only one X chromosome. Thus all genes on the single X chromosome in the male are in a hemizygous state.

Heritability

in genetics, is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals.

Heterozygous

When an individual has two different forms (alleles) of a particular genetic marker, one inherited from each parent. These are often referred to as heterozygotes.

Homozygous

When an individual has two identical forms (alleles) of a particular genetic marker, one inherited from each parent. These are often referred to as homozygotes.

I

Identical twins

or Monozygotic Twins, occur when one fertilized egg splits and develops into two (or occasionally more) fetuses. Identical twins have the same chromosomes, so they generally look alike and are the same sex. Twins derived from separate eggs independently fertilized by two different sperm cells are called dizygotic or fraternal twins.

Incidence

The number of new cases of a disease that develop within a specified population over a specified period of time.

Incomplete penetrance

when the effect of a gene or a genetic variant known to cause a disease or condition does not show any effect in a proportion of those carrying the gene. Penetrance is expressed as the percentage of those who carry the variant who show any effect of it (small or large).

Insertion

a type of chromosomal abnormality in which a DNA sequence is inserted into a gene, disrupting the normal structure and function of that gene. Insertions can lead to diseases or disorders.

Intron

the parts of a gene sequence that does not code for protein. In humans, a gene’s protein coding exons are found in between long regions of introns with no apparent function.

J

“Junk DNA”

a sequence of DNA (intron) that does not code for any protein and is apparently without function.

K

Karyotype

is the chromosome combination of an individual written as 46XY for a normal male and 46XX for a normal female. It is deferred from a picture of an individual’s chromosomes. In order to create this picture chromosomes are isolated from a cell (most often a white blood cells) stained and the microscopic photographs taken at the stage of cell division when the chromosomes are most prominent.

L

Life-time risk

This is the probability at the day of birth, that an individual will get a given disease sometime during his life time. Estimates of life-time risk include not only genetic factors but also environmental and life-style factors. Thus, life-time risk estimates may vary largely between ethnic groups and geographic regions.
Related terms: Absolute risk, Allele odds ratio, Relative risk, Genotype relative risk, Multiplicative risk model, Overall genetic risk

Linkage

The association of genes and/or markers that lie near each other on a chromosome. Linked genes and markers tend to be inherited together.

Locus, (pleural: loci)

A position on a chromosome where a gene, sequence variation, or any identifiable sequence, is located.

Lod score

a statistical estimate of whether two loci are likely to lie near each other on a chromosome and are therefore likely to be inherited together. A LOD score of three or more generally indicates that the two loci are close.

M

Male line testing

if you are a male, a Y-chromosome analysis can determine where your ancestors in the direct male or paternal line came from. This analysis can also help identify your relatives through the direct male line. This is possible because the Y chromosome is passed from father to son relatively unchanged through many generations. Ultimately, all men can trace their Y chromosome through the direct male line to a single male ancestor, playfully named “Y Chromosome Adam”.

Marker (Genetic Marker)

A segment of DNA with a known and identifiable location on a chromosome and whose inheritance can be followed. A marker can be any section of DNA (up to a whole gene) with known or unknown function. Because DNA segments that are close to each other on a chromosome tend to be inherited together, markers are often used to track patterns of inheritance of genes that have not yet been identified, but whose approximate locations are known.

Mendel, Gregor Johann

Austrian priest and scientist (1822-1884), also known as the father of modern genetics. His controlled experiments with breeding peas in the monastery garden showed that the inheritance of traits follows certain predictable patterns, determined by heritable units we now call genes.

Mendelian Trait

A characteristic or condition which follows the simple patterns of inheritance proposed by Gregor J. Mendel or a trait produced by a single gene.

Metastasis

When cancer cells break away from a primary tumor and enter the bloodstream or lymphatic system (the system that produces, stores, and carries the cells that fight infections), they can spread to other parts of the body and start multiplying there. Through this process called metastasis, cancer cells can spread to other parts of the body.

Mitochondria

are small organisms found in most of the body’s cells. They generate the energy necessary for the cell. Mitochondria contain their own DNA, separate from the DNA in the nucleus, which is known as the mitochondrial genome.

Mitochondrial DNA (mtDNA)

The vast majority of our 25,000 genes are found within the nucleus of a human cell, but some 37 of them are found in the cytoplasm that surrounds the nucleus. This type of DNA is called mtDNA or mitochondrial DNA. Like the Y chromosome passed down from father to son, mitochondrial DNA is always passed down almost unchanged from a mother to all her children and is an important factor in tracing our ancestry.

Mitochondrial Inheritance

is the transmission of an allele via the mitochondrial, rather than the nuclear, DNA. Because mitochondrial conditions are inherited exclusively from the mother, the inheritance pattern is maternal or from mother-to-child.

MicroRNA

is a short piece of single-stranded RNA that does not encode a protein and controls the expression of genes.

Mitogroup

a human mitochondrial DNA haplogroup. Mitogroups are divided into 3 main groups, which are designated by the 3 sequential letters L, M, N.

Modeling genetic risks

a statistical method that usually assumes that the relative risks associated with different genetic variants multiply. This is a parsimonious model that usually fits the data of complex traits reasonably well. When judged to be well supported by data, more complex models will be implemented by deCODEme.
Related terms: Absolute risk, Adjusted life-time risk, Allelic odds ratio, Relative risk, Genotype relative risk, Life-time risk, Multiplicative risk model, Overall genetic risk

Monozygotic Twins

or identical twins, occur when one fertilized egg splits and develops into two (or occasionally more) fetuses. Identical twins have the same chromosomes, so they generally look alike and are the same sex. Twins derived from separate eggs independently fertilized by two different sperm cells are called dizygotic or fraternal twins.

Multiplicative model for a marker

In general, an individual carries two copies (or alleles) of a marker, one inherited paternally and one inherited maternally, that are jointly represented by the marker genotype. The multiplicative model assumes that we can associate a risk with each of the two alleles, so that the risk for the individual with a certain genotype can be expressed as the product of the two risk values.
Related terms: Absolute risk, Adjusted life-time risk, Allelic odds ratio, Relative risk, Genotype relative risk, Life-time risk, Multiplicative risk model, Overall genetic risk

Mutations

Changes to the base pair sequence of the genetic material. Gene mutations have varying effects on health depending on where they occur and whether they alter the function of the genes and their products, the essential proteins.

N

Natural Selection

is the process where heritable traits, that make it more or less likely for an organism to survive and successfully reproduce, become more or less common over successive generations of a population. Natural selection is therefore a key mechanism of evolution. The term “Natural Selection” was introduced by Charles Darwin in his groundbreaking 1859 book The Origin of Species.

Nuclear Family

A family consisting of parents (a biological father and mother) and their children.

Nucleotides

are the ‘rungs” in the twisting ladder of DNA molecules. The long chains of DNA form the ‘words’ and ‘sentences’ of your genetic code, in which nucleotides are the ‘letters’. Nucleotides come in four different versions, adenine, thymine, cytosine and guanine, usually represented by the letters A, T, C and G, respectively. The human genetic code consists of about 3 billion of these four kinds of nucleotides.

Non-Carrier

an individual who does not have a genetic variant for a recessive gene (a gene that causes disease only when two copies are present), and is therefore unaffected himself and will not have children with the variant.

O

Oncogenes

are genes involved in cell cycle control (growth factors, growth factor regulator genes, etc), a mutation in which can lead to tumor growth.

Overall genetic risk relative to the population

This is the genetic risk of an individual relative to the population calculated based on the genotypes of more than one marker. When a multiplicative model is assumed for the risk contributions from the multiple markers, this can be estimated by the product of the risks relative to the population computed for the markers individually. Note that this represents genetic risk derived from the markers used and does not include other risk factors that are not being measured by the genetic test such as immediate family history, other genetic markers (known or unknown), and environmental risk factors.
Related terms: Absolute risk, Adjusted life-time risk, Allelic odds ratio, Relative risk, Genotype relative risk, Life-time risk, Multiplicative risk model.

P

Penetrance

refers to the expressed effect of a genotype. If the phenotype is always expressed in the presence of the genotype, the genotype is completely penetrant. If it is not always expressed, it is incompletely penetrant.

Phenotype

The observable traits or characteristics of an individual, for example hair color, weight, or the presence or absence of a disease.

Polymorphism

Frequently occurring variation in a nucleotide (ATCG) sequence. Polymorphisms in genes result in protein polymorphisms. A polymorphism is said to occur when the most common allele has a frequency of no greater than 99%. Some forms of some polymorphisms are associated with increased risk of disease.

Prevalence

A proportion of a population that has the disease or condition at a specific point in time.

Probability

The probability of an event equals the ratio of its favorable outcomes to the total number of outcomes, provided that all outcomes are equally likely. Alternatively defined, it is the long term frequency of a favorable event relative to all possible events. Probability is a number between 0 and 1 and is usually expressed as a decimal fraction or as a percentage.
Related terms: Bayes’ rule, Bayes’ theorem,

Protein

a string of amino acids linked by peptide bonds. Some proteins have more than one polypeptide chain. Each chain is encoded by a different gene.

Q

Quantitative Trait

is a trait which can be measured on a quantitative (linear) scale. Such traits are typically affected by more than one gene, and also by the environment. Examples of quantitative traits height, weight, and blood pressure.

Quantitative Trait Loci (QTL)

The location of a gene that affects a trait measured on a quantitative (linear) scale. These traits are typically affected by more than one gene, and also by the environment.

R

Recombination

takes place when germ cells (egg or sperm) are produced and large segments of DNA are exchanged between each pair of chromosomes. This kind of genetic shuffling means that any chromosome you inherited from your mother is in fact a mosaic of chromosomes she inherited from her parents, and so on. This reshuffling increases the possible number of combinations of genetic variants, which in turn ensures greater variability of characteristics among individuals.

Recessive

A condition that is expressed in homozygotes or in compound heterozygotes (i.e., those who have inherited a different disease-related allele (at the same or different gene locus) from each parent).

Relative risk

Relative risk is used to compare the disease risk in two differently defined groups. For example, it can be used to compare a group of people with a certain genotype or a certain cholesterol level with another group having a different genotype or different cholesterol level. For a disease, a relative risk of 1 means that the groups have equal risk, a relative risk of 2 means that one group has twice the chance of developing a disease as the other group.

Risk

risk is measured by looking at very large numbers of people, rather than at a particular individual. Risk is often presented in terms of Absolute Risk and Relative Risk.

RNA or ribonucleic acid

While DNA is the genetic material that contains all instructions for the structure and function of an individual, RNA is the nucleic acid that carries out the genetic instructions and ultimately translates the genetic blueprint into the protein molecules. There are 3 types of RNA that copy information from DNA:

1. Ribosomal RNA (rRNA) or the protein factories of the cells,
2. Messenger RNA (mRNA) or the genetic blueprint transcribed from the cell’s DNA, and
3. Transfer RNA (tRNA) that transfers amino acids coded for by mRNA, and joins together to form proteins.

S

Second-degree relative

a relative with whom you share about one-quarter (25%) of your genes. Your second-degree relatives include your grandparents, grandchildren, aunts, uncles, nephews, nieces, and half-siblings.

Single gene disorder

a disorder that originates either in the presence of a single allele (dominant disorders in males or females, X-linked disorders in males), or a pair of identical alleles at the same locus (recessive disorders). The inheritance of these disorders follow the ratios first described by Gregor Mendel, hence they are also called Mendelian disorders.

Single Nucleotide Polymorphism (SNP)

Pronounced ‘snip’. SNPs are the result of alterations to DNA, usually called mutations. A SNP is a genetic marker resulting from variation in sequence at a particular position within a DNA sequence. These variations happen when a single nucleotide or base (A, T, G or C) in the genome sequence is changed. A person’s unique DNA profile is determined by the set of SNPs the person has.

Sequencing

is determining the exact order of the base pairs in a segment of DNA. Sequencing the human genome by the Human Genome Project may be the greatest accomplishment of all time in the biological sciences.

Sex chromosomes

One of the two chromosomes that specify an organism’s genetic sex. Humans have two kinds of sex chromosomes, one called X and the other Y. Normal females have two X and normal males one X and one Y.

Sex-linked inheritance

In order to understand sex-linked traits and disorders, we must first understand the genetic determination of sex. Every individual has a number of paired chromosomes (autosomes), that are similar in appearance and line up with each other during the production of reproductive cells; sperm and eggs. In addition, each individual has two chromosomes that determine sex, the X and Y chromosomes. Normal pairing of chromosomes during the production of sperm and eggs will place either and X or a Y in each sperm (since males have both an X and a Y chromosome and an X in every egg (since females have two X chromosomes). Sex-linked inheritance involves genes located on either the X or the Y chromosome. Females can be homozygous or heterozygous for genes carried on the X chromosome; males can only be hemizygous.

Surrogate marker

refers to the correlation (r²) between two markers. If two markers are highly correlated or equivalent (r² = 1), it is possible to predict the genotypes of either marker using the genotypes of the other.

T

Third degree relative

a relative with whom you share one-eighth of your genes, such as your first cousins.

Thymine

One of the four nucleotides in DNA that make up the letters ATGC, thymine the “T”. The others are adenine, guanine, and cytosine. Thymine always pairs with adenine in the rungs of the ladder within the DNA double helix.
Related terms: Adenine, Cytosine, DNA, Guanine, Nucleotides.

Trait

is a physical characteristic of looks or behavior. Traits that are genetic can be brought about by the expression of a gene or many genes. Examples of traits are height, eye color, and the ability to roll your tongue. Variations in these characteristics are dependent upon the particular alleles an individual has for the genes determining the trait. These are passed down through the genes from parents to offspring and are often referred to also as phenotypes.

Tumor suppressor gene

a protective gene that normally limits the growth of tumors. When this type of gene is mutated, it may fail to keep a cancer from growing. BRCA1 and BRCA2, the breast cancer genes are examples of tumor suppressor genes.

Twin Studies

Studies that assess identical (monozygotic) twins for medical, genetic, psychological, and/or environmental characteristics to try to isolate the influence of genes versus environment on the disease or characteristic of interest.

U

Uracil

One of the four bases in RNA. The others are the same as those found in DNA; adenine, guanine, and cytosine. Uracil replaces thymine, the fourth base in DNA and like thymine, uracil always pairs with adenine.
Related terms: Adenine, Cytosine, DNA, Guanine, Nucleotides, Thymine.

V

Variation

An alteration or change in the genetic sequence, resulting in a variant of the normal gene. Only well established risk variants are used in deCODEme.

W

Wild type

a term used for the normal genetic status, either with regard to a whole organism (e.g. a wild-type strain of mice), or with reference to a particular mutation (wild-type at that locus or site).

X

X crhomosome

One of the two chromosomes that specify an individual’s sex. Humans have two kinds of sex chromosomes, one called X and the other Y. Normal females have two X chromosomes and normal males one X and one Y.

X-linked recessive

is a pattern of inheritance in which the transmission of a trait or disorder on the X chromosome results in a carrier state in a heterozygous female. Males with the trait are affected, because they have only one X chromosome. The risk to sons of a carrier mother is 50%. Affected males do not have affected children, but all of their daughters are carriers.

Y

Y chromosome

One of the two chromosomes that specify an individual’s sex. Humans have two kinds of sex chromosomes, one called X and the other Y. Normal females have two X chromosomes and normal males one X and one Y.

Y-linked inheritance

is a pattern of inheritance linked to genes on the Y chromosome. Since only males normally have a Y chromosome Y-linked genes can only be transmitted paternally, or from father to son.

Z

Zygosity

refers to the genetic condition of a zygote at a specific genetic locus; homozygous describes two identical alleles or DNA sequences at one locus, heterozygous describes two different alleles at one locus, and hemizygous describes the presence of only a single copy of the gene in an otherwise diploid organism.

Zygote

is a cell that is the result of fertilization, a fertilized egg. Two haploid cells (with 23 or a haploid (half) number of chromosomes), an egg cell from a female and a sperm cell from a male, merge into a single diploid cell (with the full diploid number of 46 chromosomes) called the zygote.

For more glossaries see for example:

• Genetics Home Reference from the National Library of Medicine
• Glossary of Genetic Terms from the National Human Genome Research Institute
• Medical Dictionary from the National Library of Medicine

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