genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wor ...

, expressivity is the degree to which a

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological pr ...

is expressed by individuals having a particular genotype. (Alternately, it may refer to the

expression Expression may refer to: Linguistics * Expression (linguistics), a word, phrase, or sentence * Fixed expression, a form of words with a specific meaning * Idiom, a type of fixed expression * Metaphorical expression, a particular word, phrase, o ...

of particular

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

by individuals having a certain phenotype.) Expressivity is related to the ''intensity'' of a given phenotype; it differs from

penetrance Penetrance in genetics is the proportion of individuals carrying a particular variant (or allele) of a gene (the genotype) that also express an associated trait (the phenotype). In medical genetics, the penetrance of a disease-causing mutation is t ...

, which refers to the ''proportion'' of individuals with a particular genotype that actually express the phenotype.

Variable expressivity

Variable expressivity refers to the degree in which a genotype is phenotypically expressed. For example, multiple people with the same disease can have the same genotype but one may express more severe symptoms, while another carrier may appear normal. This variation in expression can be affected by modifier genes, epigenetic factors or the environment. Modifier genes can alter the expression of other genes in either an additive or multiplicative way. Meaning the phenotype that is observed can be a result of two different

alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chrom ...

being summed or multiplied. However, a reduction in expression may also occur in which the primary

locus Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * ''Locus'' (magazine), science fiction and fantasy magazine ** ''Locus Award' ...

, where the phenotype is expressed, is affected. Epigenetic factors, such as

cis-regulatory elements ''Cis''-regulatory elements (CREs) or ''Cis''-regulatory modules (CRMs) are regions of non-coding DNA which regulate the transcription of neighboring genes. CREs are vital components of genetic regulatory networks, which in turn control morphogen ...

, can also cause variability in expression by inducing variation in transcript abundance.

Examples

Three common syndromes that involved phenotypic variability due to expressivity include: Marfan syndrome,

Van der Woude Syndrome Van der Woude syndrome (VDWS) is a genetic disorder characterized by the combination of lower lip pits, cleft lip with or without cleft palate (CL/P), and cleft palate only (CPO). The frequency of orofacial clefts ranges from 1:1000 to 1:500 birth ...

, and neurofibromatosis. The characteristics of Marfan syndrome widely vary among individuals. The syndrome affects connective tissue in the body and has a spectrum of symptoms ranging from mild bone and joint involvement to severe neonatal forms and cardiovascular disease. This diversity in symptoms is a result of variable expressivity of the FBN1 gene found on chromosome 15 (see figure 1). The gene product is involved in the proper assembly of microfibrils. Van der Woude syndrome is a condition that affects the development of the face, specifically a cleft lip (see figure 2), cleft palate or both . Carriers of the

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...

can also have pits near the centre of the lower lip which may appear to be wet due to the presence of

salivary glands The salivary glands in mammals are exocrine glands that produce saliva through a system of ducts. Humans have three paired major salivary glands ( parotid, submandibular, and sublingual), as well as hundreds of minor salivary glands. Salivary gl ...

. The resulting phenotypes expressed varies significantly among individuals. This variation can range so broadly that a study published by the Department of Orthodontics at the University of Athens showed that some individuals were unaware that they possessed the genotype for this condition until they were tested. Neurofibromatosis (NF1), also known as Von Recklinghausen disease, is a

genetic disorder A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...

that is caused by a mutation in the neurofibromin gene NF1 on chromosome 17. A

loss of function In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitos ...

mutation in the

tumor suppressor gene A tumor suppressor gene (TSG), or anti-oncogene, is a gene that regulates a cell during cell division and replication. If the cell grows uncontrollably, it will result in cancer. When a tumor suppressor gene is mutated, it results in a loss or re ...

can cause tumors on the nerves called neurofibromas. These appear as small bumps under the skin. It is stipulated that the phenotypic variation is a result of unlinked genetic modifiers.

Variable expressivity

Examples

See also

References

Further reading