Allelic expression imbalance

A phenomenon where the two alleles of a given gene are expressed at different levels, either due to epigenetic inactivation of one of the two alleles, or because of genetic variation in the regulatory regions.


Partial or complete fusion of the eyelids by webs of skin. 


A thread-like structure in our cells that contains all our genetic characteristics. One chromosome contains many genes. In humans, there are 23 pairs of chromosomes altogether, 1 pair is inherited from each parent.

Learn more about DNA, genes and chromosomes.


Inwardly curved fingers or toes.

Copy number variations (CNVs)

Deletions or duplications of a segment of DNA which may lead to structural variation in a chromosome. CNVs may be benign, pathogenic or of uncertain clinical significance.   

De novo sporadic mutation

A genetic alteration that is present for the first time in one family member as a result of a mutation in a germ cell (egg or sperm) of one of the parents, or a mutation that arises in the fertilised egg itself during early embryogenesis. 


A condition caused by harmful mutations in two different genes.

Dominant negative

A single heterozygous pathogenic variant which produces a protein that interferes with the normal protein transcribed from the wild-type (normal) allele. 


Also known as a split hand/split foot malformation. Deficiency or absence of one or more central digits giving rise to a “claw-like” appearance. 


An unborn offspring in the early stages of human development after the fertilisation of female egg cells by male sperm.


A doctor who specialises in diagnosing and treating patients with hormone-related conditions. 


Protein-coding regions of the human genome.  

Fluorescent in-situ hybridisation (FISH)

A technique used to detect and localise the presence/absence of specific DNA sequences on a chromosome with fluorescently labelled DNA probes. Learn more on Nature Education.

Free radicals

Highly reactive substances created in the body as a result of oxygen consumption in normal bodily functions such as breathing and eating. 


A doctor that specialises in diagnosis and treating disorders of the digestive system. 


The most basic functional unit of inheritance. Made up of DNA and provide instructions to cells to make proteins.

Learn more about DNA, genes and chromosomes.


A doctor who diagnoses and manages patients with genetic disorders.


A doctor who specialises in the diagnosis and management of disorders of the female reproductive organs.


A blood-filled dilated vagina due to menstrual blood in the setting of an anatomical obstruction.


A doctor that specialises in diagnosing and treating blood disorders. 

Hypomorphic allele

A variant leading to partial loss of gene activity.

Incomplete penetrance

A pathogenic variant may not necessarily result in disease. Frequently seen in autosomal dominant conditions where some heterozygous carriers are asymptomatic.

Inherited retinal dystrophies

A group of inherited conditions caused by mutations in any one of more than 300 genes that eventually leads to gradual degeneration of photoreceptor cells in the retina and subsequent visual loss. The most common IRD is retinitis pigmentosa.

Intergenic regions

A stretch of DNA sequences located between genes. 


Non-protein coding regions of the genome which are removed from messenger RNAs prior to translation. 


A chromosomal abnormality where a segment of chromosome is removed, rotated 180 degrees and re-inserted into the same location. 

Missense allele/mutation/variant

A change in one DNA nucleotide which may result in the substitution of one amino acid for another in a protein produced by a gene. 


The occurrence of two or more cell lines with a different genetic or chromosomal composition within a single individual or tissue. It may occur in somatic cells, germ cells, and/or tumour cells.


A doctor who specialises in diagnosis and treating patients with kidney problems. 

Null allele/mutation/variant

Variants that lead to minimal/absent production of functional protein.


Fewer than five fingers or toes. 


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Objects appear to move around when it is actually still.

Paediatric oncologist

A doctor who specialises in childhood cancers.


The proportion of individuals with a specific genetic mutation that is known to cause a particular disorder displaying the clinical features of that said disorder. It is most often used to described autosomal dominant conditions. 


A drug that does not have any active ingredients.


One or more extra fingers. 


Naturally occurring genetic variations (present in at least 1% of the population) in a single nucleotide, DNA sequence, gene or chromosome that have no adverse effects on the individual.


This type of inheritance is described in families in which consecutive generations are affected by recessive disorders, hence appearing to follow a dominant inheritance pattern. It is more likely to occur in families with consanguinity or in recessive disorders where there is a high carrier frequency, such as in an isolated population.

Qualitative fluorescent polymerase chain reaction (QF-PCR)

A technique used to quantify and confirm copy number variations in specific regions of DNA through polymerase chain reaction (PCR) amplification.


Enlarged dental roots.


Either a disproportion of the length of the proximal limb (usually shortening) or deformity affecting shoulder and arm or hip and thigh. 

Splicing defect/splice site mutation

A precursor messenger RNA (pre-mRNA) is produced during DNA transcription. The pre-mRNA contains both exons (segments of DNA coding for a protein) and introns (segments of DNA not coding for proteins), which needs to be processed to form mature mRNA in preparation for translation. To achieve this, introns are removed through a process called splicing so that only exons remain in the mRNA. Splice site mutations cause one or more introns to remain in the mRNA, potentially leading to the production of abnormal proteins. 

Structural variants

Large genomic alterations due to chromosomal rearrangements—deletions, duplications, insertions, inversions and translocations. Copy number variations (CNVs) is a subset of structural variants.  


Fusion of two or more digits.


A group of symptoms consistently occurring together that characterise a particular condition.


Increased medial canthal distance but interpupillary distance remains normal. 

Trans configuration

Two heterozygous variants on different alleles (usually used to describe variants within the same gene). This is in contrast to cis configuration where two variants are on the same allele (within the same gene).


A chromosomal abnormality where a segment of a chromosome/whole chromosome becomes attached or interchanged with another segment/whole chromosome. It can be further sub-divided into balanced (no net loss/gain of chromosomal materials) and unbalanced (there is resultant loss/gain of chromosomal materials) translocations. 


A doctor who specialises in disorders of the urinary tract and male reproductive organs.

Variable expressivity

A pathogenic variant associated with variable phenotypic features and severity among individuals with the same genotype.

Variant of unknown significance (VUS)

Genetic variants are classified according to the American College of Medical Genetics and Genomics (ACMG) five-tiered system: pathogenic, likely pathogenic, variant of unknown significance, likely benign and benign. A variant is termed a VUS if the criteria are not met to classify it in the other categories or if the criteria for benign and pathogenic are contradictory. A VUS may or may not be disease-causing or associated with an increased risk in developing an abnormal phenotype. Therefore, the identification of a VUS does not confirm nor exclude a diagnosis. 

Related link

X-inactivation (X-lyonisation)

A random process in females to ensure there is only one functional copy of X chromosome in each cell. This results in some cells having active maternal X chromosome while other cells have active paternal X chromosome.

Updated on November 26, 2020

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