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Chapter 24: Problem 9

List all the components that contribute to the phenotypic variance, and define each component.

Short Answer

Expert verified
Phenotypic variance is composed of genetic variance, environmental variance, and possibly genotype-environment interaction variance.

Step by step solution

01

Understanding Phenotypic Variance

Phenotypic variance, denoted as \(V_P\), refers to the variation in a phenotype observed in a population. It is a measure of how much individuals differ in observable characteristics.
02

Components of Phenotypic Variance

The components of phenotypic variance include genetic variance, environmental variance, and sometimes an interaction between genotype and environment. These components combine to explain the overall variation in phenotypes.
03

Genetic Variance

Genetic variance (\(V_G\)) is the portion of phenotypic variance that is attributable to differences in genotypes among individuals. It reflects the variation in genes that contribute to the phenotype.
04

Environmental Variance

Environmental variance (\(V_E\)) refers to the portion of phenotypic variance that arises due to differences in the environment. This component reflects how varying experiences and conditions affect the phenotype.
05

Genotype-Environment Interaction Variance

Sometimes phenotypic variance also includes a genotype-environment interaction component (\(V_{G \times E}\)). This interaction occurs when certain environments affect the expression of a genotype differently, contributing to phenotypic variance.
06

Summing Up Phenotypic Variance

Phenotypic variance is the sum of genetic variance, environmental variance, and possibly genotype-environment interaction variance, expressed as: \[ V_P = V_G + V_E + V_{G \times E} \]

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Genetic Variance
Genetic variance is a fundamental concept in genetics that refers to the diversity in genetic make-up among individuals within a population. This diversity is responsible for the differences in various traits expressed in individuals. When we talk about genetic variance, we are specifically discussing the variation in genetic information that leads to observable traits.

Genetic variance comes from several sources:
  • **Mutations:** Random changes in an individual's DNA that can introduce new genetic variations.
  • **Recombination:** During sexual reproduction, chromosomes exchange segments, leading to new combinations of genes.
  • **Migration:** Individuals moving between populations can introduce new genes to a population.
Without genetic variance, all individuals would be genetically identical, and evolutionary changes would not occur. It's this variance that allows populations to adapt to changing environments.
Environmental Variance
Environmental variance is a key component of phenotypic variance that arises from differences in the environment where individuals live and develop. This variance shows how external factors influence the expression of traits, independent of genetic makeup.

Factors that contribute to environmental variance include:
  • **Climate:** Variations in temperature, humidity, and weather conditions.
  • **Nutritional availability:** Access to food resources, which can affect growth and health.
  • **Social interactions:** Interactions with other individuals that can impact behavior and learning.
  • **Physical environment:** Features of the habitat, like terrain or perch availability for birds.
Environmental variance highlights the role that natural surroundings play in shaping the phenotype of organisms.
Genotype-Environment Interaction
Genotype-environment interaction occurs when different genotypes respond to environmental variations in distinct ways. This interaction adds another layer to the complexity of phenotypic expression by illustrating that the same environmental condition can produce different outcomes depending on the genetic makeup of individuals.

**Examples include:**
  • **Crop yield:** Certain strains of plants may thrive in specific climates, while others may not.
  • **Animal behavior:** Some animals may develop different behaviors based on the environment if they have particular genetic predispositions.
Understanding genotype-environment interactions is crucial because it explains why individuals with similar genetic backgrounds can exhibit different traits in diverse environments.
Genetic Variation
Genetic variation is the presence of different alleles, or versions of genes, within a population. This variation is the raw material for evolution and natural selection, and it fuels the genetic variance we observe. Without genetic variation, populations wouldn't have the adaptability they need to respond to environmental changes and selective pressures.

This variation can arise from processes such as:
  • **Gene flow:** Movement of genes across populations.
  • **Genetic drift:** Random changes in allele frequencies, especially in small populations.
  • **Natural selection:** Differential survival and reproduction based on genetic traits.
Genetic variation is critical because it increases a population's potential to survive in changing environments.
Environmental Factors
Environmental factors encompass a wide range of non-genetic influences that can alter an individual's phenotype. These factors shape characteristics by affecting the expression of genes. Unlike genetic factors, these influences usually arise from the surroundings and can vary over time and space.

**Some key environmental factors include:**
  • **Habitat:** Type and quality of the living environment.
  • **Diet:** Nutritional intake impacting growth and development.
  • **Weather:** Seasonal changes affecting behaviors and physiological states.
  • **Pollution:** Human-induced changes that can lead to health impacts.
Understanding these factors is fundamental in disciplines like ecology and conservation, as they often interact with genetic factors to determine the overall phenotype of organisms.

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Most popular questions from this chapter

Why do polygenic characteristics have many phenotypes?

Bipolar disorder is a psychiatric illness with a strong hereditary basis, but the exact mode of its inheritance is not known. Research has shown that siblings of patients with bipolar disorder are more likely to develop the disorder than are siblings of unaffected people. Findings from one study demonstrated that the ratio of bipolar brothers to bipolar sisters is higher when the patient is male than when the patient is female. In other words, relatively more brothers of patients with bipolar disorder also have the disease when the patient is male than when the patient is female. What does this observation suggest about the inheritance of bipolar disorder?

Seed size in a plant is a polygenic characteristic. A grower crosses two pure- breeding varieties of the plant and measures seed size in the \(\mathrm{F}_{1}\) progeny. She then backcrosses the \(\mathrm{F}_{1}\) plants to one of the parental varieties and measures seed size in the backcross progeny. The grower finds that seed size in the backcross progeny has a higher variance than does seed size in the \(\mathrm{F}_{1}\) progeny. Explain why the backcross progeny are more variable.

A rancher determines that the average amount of wool produced by a sheep in her flock is 22 kg per year. In an attempt to increase the wool production of her flock, the rancher picks the five male and five female sheep that produce the most wool; the average amount of wool produced per sheep by those selected sheep is \(30 \mathrm{~kg} .\) She interbreeds these selected sheep and finds that the average wool production among their progeny is \(28 \mathrm{~kg} .\) What is the narrow-sense heritability for wool production among the sheep in the rancher's flock?

A strawberry farmer determines that the average weight of individual strawberries produced by plants in his garden is 2 g. He selects the 10 plants that produce the largest strawberries; the average weight of strawberries produced by these selected plants is 6 g. He interbreeds these selected plants. The progeny of these selected plants produce strawberries that weigh an average of 5 g. If the farmer were to select plants that produce strawberries with an average weight of \(4 \mathrm{~g}\), what would be the predicted weight of strawberries produced by the progeny of those selected plants?
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