Genes, Mutations, and Heredity: The Raw Material of Evolution

1. From Evolution to DNA: Connecting the Big Ideas

In earlier modules you learned:

• Evolution is change in populations over time.
• Natural selection needs variation in traits, and some variants help organisms survive and reproduce better.

Now we connect this to genetics.

Key idea for this module:

> DNA → genes → traits → differences in survival and reproduction.

Natural selection can only work on traits that are:

1. Variable (not all individuals are the same), and
2. Heritable (passed from parents to offspring).

In this module you will see how:

• DNA stores hereditary information.
• Genes are sections of DNA that influence traits.
• Mutations and recombination create new gene versions.
• These genetic differences feed into natural selection and adaptation.

Keep in mind: evolution happens in populations, but it is built from changes in genes carried by individuals.

2. DNA and Genes in Plain Language

Think of a living cell as a tiny factory. It needs instructions to build and run the factory. Those instructions are stored in DNA.

DNA (Deoxyribonucleic Acid)

• A long, double-stranded molecule shaped like a twisted ladder (double helix).
• The “rungs” of the ladder are made of four bases: A, T, C, G.
• The order (sequence) of these bases is like letters in a sentence.

Genes

• A gene is a specific stretch of DNA that contains instructions for making a protein (or sometimes an RNA molecule).
• Proteins help build and run your body: they form structures (like hair, muscle) and control processes (like digestion, immunity).

So:

• DNA = the whole instruction manual.
• Chromosomes = chapters in the manual.
• Genes = individual recipes or paragraphs.

Visual description:

• Imagine a very long string of beads. Each bead is one base (A, T, C, or G).
• A gene is a particular section of that string with a meaningful pattern of beads.

Even small changes in the sequence can change how a protein works, and therefore how a trait appears.

3. Genes and Traits: A Concrete Example

Let’s connect genes to a visible trait.

Example: Human eye color (simplified)

Real human eye color is influenced by many genes, but we’ll use a simplified version to see the logic.

• There is a gene that helps control how much brown pigment is produced in the iris.
• Different versions of this gene (called alleles) can lead to:
• A lot of pigment → brown eyes
• Less pigment → blue or lighter eyes

In this simplified model:

• The gene is the section of DNA related to pigment production.
• Different alleles are slightly different DNA sequences of that gene.
• These differences change how much pigment is made, which changes the trait (eye color).

The important point:

> Different alleles of a gene can produce different forms of a trait.

This is the variation that natural selection can act on, when the trait affects survival or reproduction.

4. Mutation: The Ultimate Source of New Alleles

Where do new alleles come from? Mostly from mutations.

What is a mutation?

A mutation is a change in the DNA sequence.

Types of mutations (simplified):

• Point mutation: one base is changed (e.g., A → G).
• Insertion: one or more extra bases are added.
• Deletion: one or more bases are removed.

Causes of mutations:

• Random errors when DNA is copied during cell division.
• Physical or chemical agents (mutagens), like some types of radiation or chemicals.

Cells have repair systems that fix many errors, but not all. The remaining changes become mutations.

Most mutations:

• Have no noticeable effect (neutral).
• Some are harmful (reduce survival or reproduction).
• A few are beneficial in a given environment.

Those rare beneficial mutations can spread through a population by natural selection, contributing to adaptation.

5. Thought Exercise: Mutation and Trait Change

Imagine a simple gene that helps produce a pigment in a butterfly’s wings. The original DNA segment (hugely simplified) is:

`A A A C C C G G G`

This sequence leads to a bright orange wing color, which helps butterflies attract mates.

Now consider three possible mutations:

1. Point mutation: `A A A C C T G G G`
2. Insertion: `A A A C C C T G G G`
3. Deletion: `A A A C G G G`

Think through these questions (no single correct answer, focus on reasoning):

1. Which mutation is most likely to change the pigment protein a lot, and why?
2. Which mutation might have no visible effect on wing color, and why?
3. In what situation could a change from bright orange to duller brown actually be beneficial?

Write down short answers in your own words:

• For Q1, consider how adding/removing bases might shift the “reading frame” of the gene.
• For Q3, think about predators, camouflage, and the butterfly’s environment.

After you think it through, connect back to evolution:

> How could a rare mutation that changes wing color spread through the population over many generations?

6. Recombination and Sex: Shuffling the Genetic Deck

Mutations create new alleles. Recombination and sexual reproduction rearrange them.

Sexual reproduction

In many organisms (including humans):

• Offspring receive half their chromosomes from each parent.
• During the formation of eggs and sperm, chromosomes undergo crossing over (recombination): they swap matching segments.

Result:

• Each egg or sperm cell has a unique combination of alleles.
• Siblings (except identical twins) are genetically different, even with the same parents.

Visual description:

• Imagine two decks of cards (one from each parent).
• Before dealing a hand to the offspring, you shuffle both decks and sometimes swap groups of cards between them.
• The hand the offspring gets is a new combination, even if all the cards (alleles) already existed.

So variation in a population comes from:

1. Mutation (new alleles).
2. Recombination and independent assortment (new combinations of existing alleles).

Both are essential for giving natural selection something to work with.

7. Real-World Example: Lactose Tolerance in Humans

A well-studied example connecting genes, mutation, and natural selection is lactose tolerance in adults.

The trait

• Lactose is the sugar in milk.
• Babies produce an enzyme called lactase to digest lactose.
• In most mammals, lactase production drops after weaning, so adults are lactose intolerant.

The genetic change

• In some human populations, a mutation occurred in DNA near the LCT gene (which controls lactase).
• This mutation changed when the gene is turned on, so lactase stays active into adulthood.
• People with this allele are lactose tolerant as adults.

Natural selection

• In populations that practiced dairy farming and drank animal milk, adults who could digest lactose:
• Gained extra calories, protein, and water from milk.
• Had an advantage, especially in times of food shortage.

Over many generations:

• The lactose-tolerance allele became more common in those populations.
• Today, some populations (for example, many in Northern Europe and parts of East Africa) have a high frequency of lactose-tolerant adults, while others do not.

This example shows:

1. A mutation created a new heritable variant.
2. The environment (dairy culture) made that variant beneficial.
3. Natural selection increased its frequency in the population.

8. Quick Check: Sources of Variation

Answer this question to check your understanding of where heritable variation comes from.

9. From Genes to Populations: How Heredity Drives Evolution

Now connect individual genetics to population-level evolution.

In individuals

• Each individual has a particular combination of alleles.
• That combination influences their traits (phenotype), like body size, coloration, disease resistance.

In populations

• A population contains many different alleles at many genes.
• The gene pool is the collection of all alleles in the population.

When natural selection acts:

1. Individuals with certain genetic combinations survive and reproduce more.
2. Their alleles are overrepresented in the next generation.
3. Over many generations, the frequencies of alleles in the population change.

This is the genetic definition of evolution:

> Evolution is a change in allele frequencies in a population over generations.

So heredity links:

• Genes and alleles → traits → differences in survival and reproduction → evolution of populations.

10. Apply It: Sickle-Cell Trait and Malaria

The sickle-cell allele in the hemoglobin gene is a classic example of how environment shapes which alleles are favored.

Basic facts (simplified):

• People with two normal alleles (AA): normal red blood cells, but more vulnerable to severe malaria.
• People with two sickle alleles (SS): sickle-cell disease, often serious health problems.
• People with one normal and one sickle allele (AS): usually mild or no symptoms, and better resistance to malaria.

In many regions where malaria is common, the AS genotype has a survival advantage.

Your task:

1. Explain, in 2–3 sentences, why the sickle-cell allele can remain common in some populations even though the SS genotype is harmful.
2. Identify the roles of:

• Mutation (Where did the sickle-cell allele originally come from?), and
• Natural selection (Why is the allele maintained in malaria regions but less common where malaria is rare?).

Write your answers in your own words, using the concepts of alleles, fitness, and environmental conditions.

Reflect: this example shows that whether an allele is “good” or “bad” can depend strongly on the environment.

11. Review Key Terms

Flip the cards (mentally or with a partner) and try to recall each definition before checking the back.

12. Final Check: Connecting Genetics and Natural Selection

Test your understanding of how genes, mutations, and heredity connect to evolution.