17.01 Variation in Populations

1. Introduction to Genetic Variation

• Genetic variation refers to the diversity in gene frequencies within a population. This variation is crucial for natural selection, allowing populations to adapt to changing environments and ensuring the survival of species.

2. Sources of Genetic Variation

• Genetic variation in populations arises from several key mechanisms, primarily associated with sexual reproduction and DNA alterations.

a. Independent Assortment of Chromosomes during Meiosis

• Definition: During meiosis, homologous chromosome pairs are separated independently of each other.
• Mechanism: Each pair of chromosomes aligns randomly at the metaphase plate, leading to a random assortment of maternal and paternal chromosomes into gametes.
• Result: Increases genetic diversity by producing gametes with different combinations of chromosomes.
• Example: In humans, with 23 chromosome pairs, there are 2²³ (over 8 million) possible combinations of chromosomes in gametes due to independent assortment.

b. Crossing Over between Chromatids of Homologous Chromosomes

• Definition: Exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis.
• Mechanism: Homologous chromosomes pair up and form chiasmata, where segments of chromatids are swapped.
• Result: Creates new allele combinations on each chromosome, further increasing genetic variation.
• Example: A chromosome with alleles A and a can exchange segments with its homologous pair carrying B and b, resulting in new allele combinations like AB and ab.

c. Random Fertilization (Random Fusion of Gametes)

• Definition: The process by which any sperm can fertilize any egg, leading to a random combination of genetic material.
• Mechanism: Each gamete carries a unique set of alleles; the combination of two gametes during fertilization is random.
• Result: Generates a vast number of possible genetic combinations in offspring.
• Example: Combining a gamete with genotype AaBb with another gamete with genotype AaBb can produce offspring with various genotypes (AABB, AaBb, aabb, etc.).

d. Mutation (New Alleles)

• Definition: Changes in the DNA sequence that can create new alleles.
• Mechanism: Mutations can occur due to errors during DNA replication, exposure to radiation, chemicals, or viral insertions.
• Result: Introduces completely new genetic variations into a population that were not present before.
• Example: The sickle cell allele (HbS) arises from a mutation in the β-globin gene, changing the amino acid sequence of hemoglobin.

3. Types of Variation

Genetic variation can be categorized based on its origin and heritability.

a. Genetic Variation

• Source: Arises from the reshuffling of alleles during sexual reproduction (independent assortment, crossing over, random fertilization) and from mutations.
• Effect: Leads to phenotypic diversity, enabling populations to adapt to their environments.
• Example: Different flower colors in a plant population resulting from various allele combinations.

b. Environmental Variation

• Definition: Differences in phenotype among individuals due to environmental factors affecting gene expression.
• Sources: Sunlight, water availability, soil nutrients, temperature, and other external conditions.
• Example: A plant genetically predisposed to grow tall may only reach its potential height in an environment with sufficient sunlight and nutrients.
• Note: Environmental effects are non-heritable and cannot be passed on to offspring.

4. Mutation and Inheritance

• Mutations can occur in different types of cells, affecting whether the changes are heritable.

a. Mutations in Somatic (Body) Cells

• Definition: Occur in non-reproductive cells.
• Impact: Generally do not affect the organism’s offspring since they are not passed to gametes.
• Exceptions:
  • Cancer Development: Mutations that disrupt cell cycle control can lead to uncontrolled cell division and tumor formation.
  • Tissue Impact: If mutations affect essential genes in a single cell, they can impact the function of that cell or tissue.

b. Mutations in Germ Cells (Gametes)

• Definition: Occur in reproductive cells (ovaries, testes in animals; ovaries, anthers in plants).
• Impact: Can be inherited by offspring if a mutated gamete fuses during fertilization.
• Result: All cells in the resulting organism will carry the mutated allele, potentially affecting the phenotype.
• Example: A mutation in a sperm or egg cell can lead to genetic disorders like cystic fibrosis or Huntington’s disease in the offspring.

5. Key Points

• Heritable Variation:
  • Generated through independent assortment, crossing over, random fertilization, and mutations.
  • Essential for evolution and adaptation.
  • Mutation introduces new alleles, increasing phenotypic diversity.
• Non-Heritable Variation:
  • Caused by environmental factors influencing gene expression.
  • Does not contribute to genetic diversity passed to the next generation.
• Importance of Genetic Variation:
  • Enhances a population’s ability to survive changing environments.
  • Prevents genetic homogeneity, reducing vulnerability to diseases and environmental changes.

6. Figures & Diagrams

• Figure A: Independent Assortment
  • Diagram showing the random alignment of homologous chromosome pairs during metaphase I of meiosis.

• Figure B: Crossing Over
  • Illustration of homologous chromosomes pairing and exchanging segments at chiasmata during prophase I of meiosis.

• Figure C: Random Fertilization
  • Depiction of gamete fusion showcasing the random combination of maternal and paternal alleles.

• Figure D: Mutation Example
  • Representation of a normal β-globin gene versus the mutated gene causing sickle cell disease.

7. Key Terms

• Allele: Different forms of a gene that arise by mutation and are found at the same place on a chromosome.
• Heterozygous: Having two different alleles for a particular gene.
• Homozygous: Having two identical alleles for a particular gene.
• Chiasma (plural: Chiasmata): The point where two homologous non-sister chromatids exchange genetic material during crossing over.
• Gamete: A reproductive cell (sperm or egg) that contains half the genetic information of an organism.
• Germ Cells: Cells that give rise to gametes; mutations here can be inherited.
• Somatic Cells: All body cells except gametes; mutations here are not inherited.
• Phenotype: The observable characteristics of an organism resulting from the interaction of its genotype with the environment.
• Genotype: The genetic makeup of an organism; the information encoded in its DNA.

8. Discussion Questions

Equation for Mutation:

• Question: Explain how a point mutation can lead to a new allele using the sickle cell example.
• Answer: A point mutation involves a single nucleotide change in the DNA sequence. In the sickle cell example, a mutation in the β-globin gene changes the codon from GAG (glutamic acid) to GTG (valine), resulting in the production of abnormal hemoglobin (HbS). This single amino acid substitution creates a new allele that affects the protein’s structure and function.

Energy Currency:

• Question: Discuss why ATP is termed the “energy currency” of the cell, drawing parallels to genetic variation mechanisms.

• Answer: (Note: This question seems misaligned with genetic variation; it pertains to energy in cells. Possibly the user meant to adapt a previous question. Here’s an adjusted question.)

• Adjusted Question: Why is genetic variation essential for the survival and evolution of a population?

• Answer: Genetic variation provides the raw material for evolution by natural selection. It allows populations to adapt to changing environments, resist diseases, and maintain resilience against genetic disorders. Without genetic variation, populations would be more susceptible to extinction due to lack of adaptability.