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Often technical terms can get in the way of learning about twins, their biological development, and DNA testing. We've translated common terms that are often used by doctors and researchers when discussing twins. If there is a particular term not listed here for which you are seeking a definition, please contact us and we will be happy to provide an answer.


Two different forms or versions of a gene that occupy corresponding positions on homologous chromosomes. A person has two alleles of a trait--one from each parent. For example, at a locus for eye color the allele might result in blue or brown eyes.

amino acids
The basic building blocks of proteins; there are twenty different types linked together in different sequences to form different proteins.

base pairs
Two nitrogenous bases (adenine and thymine or guanine and cytosine) held together by weak bonds. Two strands of DNA are held together in the shape of a double helix by the bonds between base pairs. Adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G)

base sequence
The order of bases in DNA.

base sequence analysis
A method, sometimes automated, for determining the base sequence.

The chemical units (adenine, thymine, guanine and cytosine) in a DNA molecule that constitutes the genetic code.

The molecular building blocks of DNA: adenine (A), cytosine (C), guanine (G), thymine (T). In DNA, A attaches only to T, and C attaches only to G.

See base pair.

buccal cell sample
An oral, mouth or cheek sample obtained using a cotton or sponge swab. A gentle, non-invasive approach to sample collection which collects cells from the inside surface of the cheek for use in DNA analysis.

The basic unit of structure and function in all living things ; the smallest basic part of every living thing that can function by itself. It is made of a nucleus surrounded by cytoplasm, organelles (similar to organs in a body) and proteins. Each cell contains the entire genome.

The location of heriditary (genetic) material within the cell. Genes are found arranged in a linear sequence along chromosomes.There are 46 human chromosomes, each containing DNA for hundreds or thousands of individual genes.

chromosome map
A diagram showing the locations and relative spacing of genes along a chromosome.

See genetic code.

See genetic code.

When two eggs are independently fertilized by two sperm cells. See also: fraternal twins

When two eggs are independently fertilized by two sperm cells. See also: fraternal twins

DNA (deoxyribonucleic acid)
A large double-stranded, spiraling molecule that contains genetic instructions for growth, development and replication. It is organized into bodies called chromosomes and found in the nucleus of a cell. It is held together by weak bonds between base pairs of nucleotides.

DNA fingerprinting
The use of fragments of DNA to identify the unique genetic makeup of an individual. There is the RFLP method and the PCR method.

DNA sequence
The relative order of base pairs, whether in a fragment of DNA, a gene, a chromosome, or an entire genome. See base sequence analysis.

DNA sequencing
The process of determining the order of bases in a segment of DNA.

double helix
The shape that two linear strands of DNA assume when bonded together ; the spiraling structure of the molecule.

The female reproductive cell.

A technique for arranging biological particles across a gel medium by exposing them to an electric field.

A protein that acts as a catalyst, speeding the rate at which a biochemical reaction proceeds but not altering the direction or nature of the reaction.

fraternal twins
When two eggs are independently fertilized by two sperm cells, fraternal or dizygous twins result. Fraternal twins do not have identical DNA but will have half of their DNA in common, just like siblings of the same parents that are born at different times. Fraternal twins can either be of the same sex or different sex.

The portion of a DNA molecule that specifies a protein, and contains the basic functional unit of heredity. The collection of genes in an organism determine the characteristics of that organism.

gene expression
The way information in a gene is changed to a protein. The DNA sequence is changed into RNA and usually (not always) then changed into protein.

gene linkage map
A map of relative positions of genes on a chromosome. Genes inherited together are close to each other on the chromosome, and said to be linked.

genetic code
The sequence of nucleotides, coded in triplets (codons) along the mRNA, that determines the sequence of amino acids in protein synthesis. The DNA sequence of a gene can be used to predict the mRNA sequence, and the genetic code can in turn be used to predict the amino acid sequence.

genetic marker
A landmark that identifies the presence or location of a gene or heredity characteristic.

A scientist who studies genes.

The study of genes and heredity, or how characteristics are passed from parents to children.

half-identical twins
see polar body twins

Refers to having dissimilar alleles of one or more genes, as opposed to homozygous.

Similarities in DNA or protein sequences between individuals of the same species or among different species.

identical twins
When one egg is fertilized by a single sperm cell and then splits, two identical or monozygous twins will result. These twins are referred to as identical since the sequence of the twins' DNA is identical. Identical twins must be of the same sex. Importantly, the term "identical" often does not apply to the actual physical or behavioral similarities between identical twins. However, identical twins tend to be very similar for traits such as hair color, hair texture, eye color, height, and weight.

Determination of the original position (locus) of a gene or other marker on a chromosome.

plural form of locus

The position of a gene for a particular trait on a chromosome. The use of locus is sometimes restricted to mean regions of DNA that are expressed. See gene expression.

An identifiable physical location on a chromosome (e.g., restriction enzyme cutting site, gene) whose inheritance can be monitored. Markers can be expressed regions of DNA (genes) or some segment of DNA with no known coding function but whose pattern of inheritance can be determined. See RFLP, restriction fragment length polymorphism.

see placenta

Stretches of DNA that repeat over and over again, like AACT AACT AACT AACT

mirror image twins
Mirror image twins are identical twins that result when a fertilized egg splits later in the embryonic stage than usual, typically from day 9 to 12. Mirror image twins tend to exhibit characteristics with reversed asymmetry (e.g., one twin is left-handed and the other is right-handed). If this split happens much later than this, then the twins can be co-joined. A DNA-based zygosity test cannot determine is twins are mirror image twins.

When there is only one set of membranes which surround the twins in the womb.

When one egg is fertilized by a single sperm cell and then splits. See also: identical twins

When one egg is fertilized by a single sperm cell and then splits. See also: identical twins

A sequencing approach that uses several pooled samples simultaneously, greatly increasing sequencing speed.

The egg; the reproductive cell from the ovary; the female gamete; the sex cell that contains the woman's genetic information.

PCR (polymerase chain reaction)
A method of DNA analysis that amplifies a specific DNA (gene) region allowing rapid DNA analysis. PCR also can be used to detect the existence of the defined sequence in a DNA sample.

The embryonic tissue that implants in uterine wall and provides a mechanism for exchanging the baby's carbon dioxide and waste products for the mother's nutrients and oxygen. The baby is connected to the placenta by the umbilical cord.

polar body twins
Polar body twins are believed to occur when the mother's egg splits before fertilization and one half (referred to as the 'polar body') is also fertilized by a different sperm. This results in twins who share half their genes in common (from the mother) and the other half different (from the two sperm). They share some features of identical twins and some features of fraternal twins and are so-called half-identical twins. However, DNA-based zygosity tests are currently not available to determine if twins are polar-body twins.

The differences in DNA sequences that occur naturally. They are used as genetic markers because researchers can use them to tell apart DNA from different sources.

Short preexisting polynucleotide chain to which new deoxyribonucleotides can be added by DNA polymerase.

A molecule consisting of linked amino acids. Proteins are the products of gene expresion and are the functional and structural components of cells. Examples are hormones, enzymes, and antibodies.

RFLP (restriction fragment length polymorphism)
Considered to be the most accurate DNA test; it takes several weeks to complete, and is becoming more frequently used in forensics cases.

Determination of the order of nucleotides (base sequences) in a DNA or RNA molecule or the order of amino acids in a protein.

The male reproductive cells.

Zygosity is the genetic relationship between two twins. With respect to genetic similarities or dissimilarities, twins are classified as identical or fraternal.

The new cell formed when the male sperm unites with a female egg ; a fertilized egg which has not yet divided.

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