Hello Biologists! Welcome to Genetic Variation

This chapter is all about differences! Have you ever noticed that you look a little like your parents, but you're not an exact copy? Or that even siblings who share the same parents still look different from each other?
That's variation in action! Understanding variation is crucial because it’s the engine that drives evolution and ensures species can survive when their environment changes. Don't worry if this seems tricky at first—we'll break down the types and causes of these differences step-by-step. Let’s dive in!

What Exactly is Variation?

Variation simply means the differences that exist between individuals within a species (or even differences between different species).

Analogy: Imagine a batch of cookies you bake using the exact same recipe. They all look like cookies, but some might be slightly bigger, some slightly darker, and some might have slightly more chocolate chips. These small differences are variation!

Section 1: Types of Variation

Biologists categorise variations into two main types based on how they can be measured and displayed: Discontinuous and Continuous variation.

1. Discontinuous Variation

This type of variation results in characteristics that fit into clear, distinct, separate categories. There are no "in-between" values.

  • Key Feature: Characteristics are usually controlled by a single gene or a small number of genes.
  • Visualisation: If you plotted this data, you would use a bar chart showing separate columns.
  • Examples:
    • Human blood groups (A, B, AB, O). You are A or B, never "A-and-a-half."
    • Eye colour (Blue, Brown, Green).
    • Ability to roll your tongue. (You either can or you can't).

2. Continuous Variation

This type of variation results in characteristics that can take any value within a range. There is a complete spectrum of measurements.

  • Key Feature: Characteristics are usually controlled by many genes (polygenic inheritance) AND are significantly affected by the environment.
  • Visualisation: If you plotted this data for a large population, it usually forms a smooth curve, often a "bell curve."
  • Examples:
    • Height (You can be 1.70m, 1.71m, 1.715m, etc.)
    • Weight and Mass
    • Foot size
    • Leaf area in plants
Quick Review: Discontinuous vs. Continuous

Think of a light switch versus a dimmer switch.
Discontinuous (Light Switch): Off or On (separate categories).
Continuous (Dimmer Switch): Any shade of brightness in between (a range).

Section 2: The Causes of Variation

Why do these differences exist? Variations can be caused by what you inherit (your genes) or what happens to you throughout your life (your environment), or often, a combination of both.

1. Genetic Variation (Inherited)

This variation comes directly from the alleles (different versions of a gene) passed down from your parents.

  • Characteristics affected: Mainly discontinuous variation (like blood type) but also contributes heavily to continuous traits (like potential height).
  • Key Point: Genetic variation is inherited and cannot be changed by lifestyle or diet.

2. Environmental Variation (Acquired)

This variation is caused by external factors that affect an organism during its lifetime. These changes are acquired and cannot be passed on to the next generation.

  • Characteristics affected: These factors mostly affect continuous variation.
  • Examples:
    • A person's natural maximum height is genetic, but poor diet (environmental) during childhood can prevent them from reaching that height.
    • A plant’s colour is affected by sunlight exposure (lack of sun = pale leaves).
    • Scars, tattoos, or dyed hair.

The Combination: Nature vs. Nurture

Most continuous characteristics, like height, intelligence, or skin colour, are determined by a mix of both genetics and the environment. This is often called the Nature vs. Nurture debate.

Did you know? The height you reach is determined by the genes (Nature) that set your potential, but the quality of food, healthcare, and lifestyle (Nurture/Environment) you have will determine whether you reach that potential!

Section 3: The Sources of Genetic Variation

If variation comes from genes, where do these different combinations and new genes come from in the first place? There are two main sources of the variation we see:

1. Sexual Reproduction (Mixing Alleles)

Sexual reproduction is the most common cause of variation in a population. It creates unique combinations of existing alleles.

How Sexual Reproduction Creates Variation:

Step 1: Making Gametes
The parent organisms produce specialised sex cells (gametes: sperm and egg in animals, pollen and ovules in plants). These gametes only contain half the amount of DNA (one set of chromosomes).

Step 2: Independent Assortment (Mixing Up Chromosomes)
When the gametes are formed, the chromosomes from the original parents are shuffled and sorted randomly. Imagine having a box of red shirts and a box of blue shirts—each gamete gets a random mix of red and blue, ensuring every gamete is unique!

Step 3: Random Fertilisation
When a sperm fertilises an egg, the combination of which specific sperm meets which specific egg is entirely random. This random fusion of two unique gametes guarantees that the resulting zygote (the first cell of the offspring) has a completely unique set of two alleles for every gene.

Key Takeaway: Sexual reproduction doesn't create *new* alleles, it just shuffles the existing ones into trillions of different combinations!

2. Mutation (Creating New Alleles)

Mutation is the ultimate source of all completely new genetic variation.

What is a Mutation?

A mutation is a random, permanent change to the base sequence in the DNA of an organism.

  • Think of your DNA as a recipe book. A mutation is like a typo in that recipe.
  • If this typo happens in a gene, it can result in a new version of that gene, which we call a new allele.
  • Mutations happen continuously and randomly.
The Impact of Mutations:

Mutations can be:

  1. Harmful: Most mutations cause a protein to malfunction or not be produced at all. This often leads to genetic disorders (e.g., Cystic Fibrosis).
  2. Neutral: The mutation changes the DNA, but the resulting protein still works fine, or it happens in a part of the DNA that doesn't code for anything. These have no effect.
  3. Beneficial: Very rarely, a mutation produces a new allele that gives the organism an advantage in its environment (e.g., resistance to a disease). This beneficial new allele can then be passed on and become more common in the population through natural selection.

Important Check: For a mutation to be passed on to offspring, it must occur in the gametes (sex cells). If a mutation happens in your skin cells, it won't be inherited by your children.

Chapter Summary: Genetic Variation Checklist

What did we learn?

  • Variation is the difference between organisms.
  • Discontinuous Variation (e.g., blood group) falls into distinct categories and is mainly genetic.
  • Continuous Variation (e.g., height) exists on a range and is influenced by both genes and the environment.
  • Variation is caused by Genetic Factors (inherited alleles) and Environmental Factors (acquired traits).
  • The main sources of Genetic Variation are Sexual Reproduction (shuffling existing alleles) and Mutation (creating new alleles).

Great work! You now understand why we are all unique. This concept is fundamental to understanding how species adapt and change over time!