5.5 - Environmental Effects on Phenotype

Introduction to Environmental Effects on Phenotype

• Phenotypic Plasticity: The ability of one genotype to produce different phenotypes depending on the environment.

• How It Works: Cells detect environmental signals (temperature, light, day length, soil chemistry, nutrition) and pass them through signaling pathways that turn genes on or off or change how much protein is made. The DNA sequence itself doesn't change.

• Genotype Sets the Range: The genotype sets the range of phenotypes that are possible. The environment decides where in that range an individual ends up.

• Why It Helps: Plasticity lets an individual adjust to its surroundings during its own lifetime, which can help it survive and reproduce when conditions change.

Examples

• Flower Color and Soil pH: Hydrangeas with the same genotype make blue flowers in acidic soil and pink flowers in basic (alkaline) soil. Acidic soil makes aluminum available to the plant, and the aluminum binds the flower's pigment and turns it blue.

• Seasonal Fur Color: Arctic foxes and snowshoe hares grow brown fur in summer and white fur in winter. Changes in day length trigger the change, and the new winter fur grows in without pigment, which camouflages the animal in the snow.

• Temperature and Coat Color: Siamese cats and Himalayan rabbits make an enzyme for dark pigment that only works at cooler temperatures. Their ears, nose, paws, and tail are cooler than the rest of the body, so only those parts turn dark.

• Sex Determination in Reptiles: In many turtles, and in crocodiles and alligators, sex isn't set by sex chromosomes. The temperature of the egg during a key stage of development decides which sex-development genes get turned on. In many turtles, warmer eggs hatch as females and cooler eggs hatch as males.

• UV Light and Melanin: More UV exposure causes skin cells called melanocytes to make more melanin, the pigment that absorbs UV light. That's why the same person's skin gets darker (tans) after more time in the sun.

• Height and Weight in Humans: Height and weight are polygenic, but nutrition, health, and activity also affect them. Identical twins have the same genotype, yet they can end up with different heights and weights if their diets or health differ.

Key Takeaways

• Phenotype Depends on Both: Phenotype comes from the genotype and the environment acting together.

• Gene Expression, Not Mutation: Environmental effects change which genes are turned on, how much protein is made, or how well a protein works. They don't change the DNA sequence.

• On the Exam: Connect the steps: an environmental signal, then a change in gene expression, then a change in the amount or activity of a protein, then a different phenotype.