Expressivity and Phenocopy – Explaination & Examples

Expressivity and Phenocopy

Introduction

In the above image, we can see a cross between a homozygous dominant (purple flower) and homozygous recessive (white flower). According to the principles of Mendelian inheritance, all flowers must be purple as it is the dominant trait. However, we can observe in the F2 generation that one of the flowers, despite having the genotype for the purple flower, expresses the phenotype of the white flower. 

We have already studied penetrance in the previous lecture; it is defined as the percentage of people in a population who have a particular gene and express the associated trait. In the above example, we can say that the percentage of flowers in the F1 generation that is purple in color gives the penetrance. 

If we calculate the penetrance of the trait that leads to the purple flower color in the above example, it would come around 85%. 

Explanation

Expressivity: 

In the above image under observed results, we can see that the individuals in the F1 generation are showing a variety of phenotypes that are slightly different from one another, but all of them have the same genotype. This is called expressivity. 

Sometimes, individuals with the same phenotype can exhibit different degrees of the same phenotype. 

Expressivity is referred to as the degree to which expression of a certain phenotype varies among individuals. However, one must not get confused between penetrance and expressivity, as penetrance describes the statistical variability among a population of genotypes whereas expressivity describes individual variability. 

Example of expressivity: 

Expressivity in humans can be explained using Marfan’s syndrome. Marfan syndrome is a disorder in humans. One gene is responsible for a series of symptoms, including thinness, joint hypermobility, limb elongation, lens dislocation, and increased susceptibility to heart disease. 

The features of Marfan syndrome differ extensively; some people only have mild symptoms like being tall and having elongated, thin fingers, while others can experience life-threatening symptoms which involve complications in the heart and blood vessels. 

Even though the expressivity of the symptoms of Marfan syndrome varies in different individuals, all individuals having this disorder carry a dominant mutation in the gene FBN1, which codes for fibrillin. However, it is observed that the position of the mutation in the FBN1 is correlated to its severity. 

Phenocopy: 

Consider two plants, one with a homozygous tall (TT) genotype and the other with a homozygous dwarf genotype (tt). According to Mendelian inheritance, the plant with (TT) should grow tall, and the plant with (tt) should be dwarf. 

Under normal environmental conditions, these plants will grow as per Mendelian inheritance. But what will happen if the plant with (TT) genotype is kept in the darkroom with no sunlight and not enough source of nutrition? 

External environmental factors can change the phenotype of a particular trait. As a result, the environmental stress affects the growth of the tall plant, and it remains dwarf even after having the genotype for tallness (TT). 

This phenomenon wherein there is a variation in phenotype caused by environmental factors, in such a way that the individual’s phenotype matches a phenotype which is determined by genetic factors is called phenocopy. 

Depending upon the level to which environmental factors influence the genotype, the changes in the phenotype may be small or extremely noteworthy. 

Sometimes, the phenotypes are changed by the environment in a way that the new phenotype matches another phenotype produced by known genes. The induced phenotype is not heritable and is called a phenocopy. 

Example of phenocopy: 

Phenocopy can be observed in Himalayan rabbits. When these rabbits are raised in moderate temperatures, they are white in color with a black tail, ears, and nose. However, when these rabbits are raised in colder temperatures, they show the black coloration of their coats, thereby resembling the black rabbits. 

Hence, we can say that this black colored Himalayan rabbit is a phenocopy of the genetically black rabbit. 

Similarly, the butterfly genus Vanessa is able to change its phenotype based on local temperature. 

If they are introduced to Lapland, they mimic butterflies localized to this area; and if localized to Syria they mimic butterflies of Syria. 

Summary

• Expressivity is referred to as the degree to which expression of a certain phenotype varies among individuals. Marfan syndrome explains expressivity in humans.
• In Marfan syndrome some people only have mild symptoms while others can experience life-threatening symptoms.
• Even though the expressivity of the symptoms of Marfan syndrome varies, all individuals
  with this disorder carry a dominant mutation in the gene FBN1.
• This phenomenon wherein there is a variation in phenotype caused by environmental factors, in such a way that the individual’s phenotype matches a phenotype which is determined by genetic factors is called phenocopy.
• The environmentally induced phenotype in phenocopy is not heritable.
• Phenocopy can be observed in Himalayan rabbits. When these rabbits are raised in moderate temperatures, they show a different phenotype. However, when these rabbits are raised in colder temperatures, they show the black coloration of their coats, thereby resembling the black rabbits.
• Hence, we can say that this black colored Himalayan rabbit is a phenocopy of the genetically black rabbit.