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9.6.3. Expressiveness and genetic penetrance

Genetic expressiveness

Genetic expressiveness is the strength with which a gene manifests itself.

For example, Drosophila wings can be long or short, and are determined by a gene. But it depends on the temperature at which they are developed which are longer or shorter.

Another example. People who have more than five fingers on each hand or foot have polydactyly. But they may have five fingers and one abnormality only visible on X-ray, five and a stump, or more than five fingers.

Expressiveness depends on the environment. Each genotype responds to the environment in a certain way to adapt (reaction norm). Sometimes phenotypes can occur that coincide with those due to other genotypes (phenocopy). For example, Drosophila with a normal genotype, if it is in a medium with silver salts, produces a yellow phenotype. This is phenocopy of genotypic yellow, although the origin is different.

Genetic penetration

The penetrance is the proportion of individuals (percent) of a population expressing the pathological phenotype, among all carrier having a genotype of an allele mutated.

The genetic penetrance is the ability of a genotype to manifest a phenotype, and expressiveness is the ability that this phenotype is constant.

The examples we've seen so far have full insight and expressiveness. For example, the allele that determines the fatal disease Huntington's chorea is dominant, but people who carry that allele can:

  • Die from illness, but can:
    • Dying before reproductive age (lethal). Variable expressiveness.
    • Dying in mid-reproductive age (sublethal).
    • Dying after reproductive age (not lethal).
  • Dying from other causes (old age). It would be a case of incomplete penetration.

The term penetrance does not refer to the degree of expression of the phenotype, in which case we would speak of expressiveness, but only to the presence or absence of a specific phenotype.


Called phenocopy to the individual or group of individuals in a population which lack a given genotype has the same phenotype as that which does have this genotype. That is, it expresses a character that is not genetically determined due to the interference of an environmental factor and that said expression is shared by other types of individuals in which the origin is endogenous.

A clear example is achondroplasia (a cause of dwarfism).

Another example is thalidomine (a drug that was prescribed against nausea in pregnancy), which when administered to pregnant women, caused the child to be born with abnormalities such as lack of hands. However, these children were genotypically normal, and their deficiency was due to this drug.


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