explain, with examples, that phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and environmental factors
Explain, with examples, that phenotypic variation is due to genetic factors, environmental factors, or a combination of genetic and environmental factors.
1. What is Phenotypic \cdot ariation?
Phenotypic variation refers to the observable differences in physical traits (morphology, physiology, behaviour) among individuals of the same species. These differences arise from differences in the underlying genotype, the environment in which the organism develops, or an interaction between the two.
2. Genetic Factors
When variation is caused solely by differences in DNA sequence, it is termed genetic variation. The genotype determines the potential range of phenotypes, and the trait is inherited according to Mendelian or non‑Mendelian patterns.
Example – Flower colour in Petunia: A single gene with two alleles (R = red, r = white) produces red, pink or white flowers depending on the genotype (RR, Rr, rr).
Example – Human blood groups: The ABO system is controlled by three alleles (IA, IB, i). The combination of alleles determines the phenotype (A, B, AB, O).
Example – Sickle‑cell trait: A point mutation in the β‑globin gene (HbS) causes the sickle‑cell phenotype in homozygotes (HbS/HbS) and provides malaria resistance in heterozygotes (HbA/HbS).
3. Environmental Factors
When variation arises because individuals experience different external conditions, it is called environmental variation. The genotype provides the capacity to respond, but the phenotype expressed depends on the environment.
Example – Temperature‑dependent sex determination in reptiles: In many turtles, incubation temperature of the eggs determines the sex of the offspring, despite identical genotypes.
Example – Plant height: The same species of wheat grown in nutrient‑rich soil may reach 1 m, whereas the same genotype in poor soil may only grow to 0.5 m.
Example – Human skin colour tanning: Exposure to ultraviolet radiation stimulates melanin production, darkening the skin without any change in DNA sequence.
4. Gene‑Environment Interactions
Most traits result from a combination of genetic potential and environmental influence. The phenotype is the product of both factors acting together.
Example – Human height: Height is polygenic (many genes each contribute a small effect) but is also strongly affected by nutrition, health during childhood, and physical activity.
Example – Coat colour in the mouse (Mus musculus): The Agouti gene determines a yellow‑brown coat, but mice raised in colder environments may develop a thicker, darker fur as an adaptive response.
Example – Phenotypic plasticity in Daphnia: Presence of fish predators induces the development of protective helmets and spines, a response that is triggered by chemical cues in the water.
5. Summary Table
Factor
Mechanism
Typical Example
Resulting Phenotype
Genetic only
Differences in DNA sequence (alleles, mutations)
ABO blood group
Distinct blood types (A, B, AB, O)
Environmental only
Variation in external conditions (temperature, nutrients, light)
Temperature‑dependent sex in turtles
Male or female depending on incubation temperature