DNA, Genetics, and the Code of Life

Warm-Up: You Are Part Banana

Take a minute to think about it, then reveal the answer.

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Core Lesson: The Code of Life

What is DNA?

DNA stands for deoxyribonucleic acid. It is a molecule that carries the instructions for building and running every living thing on Earth — from bacteria to blue whales. If you uncoiled all the DNA in a single human cell, it would stretch about 2 meters long. Your entire body contains enough DNA to stretch from the Earth to the Sun and back 600 times.

The Double Helix

DNA has a shape called a double helix — imagine a twisted ladder. The "rails" of the ladder are made of sugar and phosphate molecules. The "rungs" are made of pairs of molecules called bases.

There are four bases:

| Base | Abbreviation | Always pairs with | |------|-------------|-------------------| | Adenine | A | T (Thymine) | | Thymine | T | A (Adenine) | | Cytosine | C | G (Guanine) | | Guanine | G | C (Cytosine) |

So if one strand reads: A T C G G T A C

The other strand must be: T A G C C A T G

From DNA to Traits: Genes and Chromosomes

• A gene is a segment of DNA that contains the instructions for building one specific protein (or doing one specific job). Humans have roughly 20,000–25,000 genes.
• Chromosomes are tightly coiled bundles of DNA. Humans have 46 chromosomes (23 pairs). You get 23 from your mother and 23 from your father.
• Your complete set of DNA is called your genome. The human genome contains about 3.2 billion base pairs.

Dominant and Recessive Alleles

Different versions of the same gene are called alleles. For many traits, you carry two alleles — one from each parent.

• A dominant allele (written with a capital letter, like B) shows its effect even if you only have one copy.
• A recessive allele (written lowercase, like b) only shows its effect if you have two copies.

| Genotype | What it means | Phenotype (what you see) | |----------|---------------|--------------------------| | BB | Two dominant alleles (homozygous dominant) | Brown eyes | | Bb | One dominant, one recessive (heterozygous) | Brown eyes (dominant wins!) | | bb | Two recessive alleles (homozygous recessive) | Blue eyes |

Punnett Squares

A Punnett square is a grid that helps predict the probability of offspring inheriting particular combinations of alleles. Let's cross two heterozygous brown-eyed parents (Bb x Bb):

| | B | b | |---|-------|-------| | B | BB | Bb | | b | Bb | bb |

Results: 1 BB : 2 Bb : 1 bb

• 75% chance of brown eyes (BB or Bb)
• 25% chance of blue eyes (bb)

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Hands-On Lab Part 1: Kitchen DNA Extraction

We are going to extract real DNA from a strawberry using simple household materials. Yes — you will be able to see DNA with your naked eye.

Why strawberries?

Strawberries are perfect for DNA extraction because they are octoploid — they have 8 copies of each chromosome (most organisms have only 2). That means 8 times as much DNA per cell, making it much easier to see.

Steps

1. Mash the strawberry. Place 2–3 strawberries in a zip-lock bag and seal it. Squish and mash for 2 minutes until it is a smooth pulp. This breaks open the cells.

2. Make the extraction solution. In a cup, mix:

   • 2 teaspoons of dish soap
   • 1 teaspoon of salt
   • 1/2 cup of warm water

   Stir gently (avoid making bubbles).

3. Add the solution. Pour the extraction solution into the bag with the mashed strawberry. Seal and gently mix for 1 minute. The soap dissolves the cell membranes (which are made of fats). The salt causes the DNA to clump together.

4. Filter. Place a coffee filter over a clean cup and pour the mixture through. Let it drip for a few minutes. The liquid that passes through is your cell lysate — it contains dissolved DNA.

5. Add cold alcohol. Tilt the cup and slowly pour chilled rubbing alcohol down the side so it forms a layer on top. Use about the same volume as the filtered liquid. Do not stir.

6. Watch the magic. Within 1–2 minutes, you will see white, stringy, cloudy material forming at the boundary between the two layers. That is DNA. Use a wooden skewer to gently spool it up.

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Hands-On Lab Part 2: Punnett Squares in Python

Now let's move to the laptop and simulate genetics with code.

Program 1: Punnett Square Calculator

# Punnett Square Calculator
# Cross two heterozygous parents (Bb x Bb)

parent1 = "Bb"
parent2 = "Bb"

print(f"Crossing {parent1} x {parent2}")
print("-" * 30)

offspring = []
for allele1 in parent1:
    for allele2 in parent2:
        # Always write the dominant allele first
        if allele1.isupper() or (allele1.islower() and allele2.islower()):
            genotype = allele1 + allele2
        else:
            genotype = allele2 + allele1
        offspring.append(genotype)

print("Possible offspring genotypes:")
for i, child in enumerate(offspring, 1):
    print(f"  {i}. {child}")

# Count each genotype
from collections import Counter
counts = Counter(offspring)
print(f"\nGenotype ratios:")
for genotype, count in sorted(counts.items()):
    percentage = count / len(offspring) * 100
    print(f"  {genotype}: {count}/4 ({percentage:.0f}%)")

Program 2: Simulate 1,000 Offspring

import random

def simulate_inheritance(parent1, parent2, num_offspring):
    """Simulate genetic inheritance for many offspring."""
    results = []

    for _ in range(num_offspring):
        # Each parent randomly passes one allele
        allele1 = random.choice(parent1)
        allele2 = random.choice(parent2)

        # Sort so dominant allele comes first
        genotype = "".join(sorted([allele1, allele2], key=lambda x: (x.lower(), x.islower())))
        results.append(genotype)

    return results

# Simulate!
parent1 = "Bb"
parent2 = "Bb"
num_trials = 1000

offspring = simulate_inheritance(parent1, parent2, num_trials)

# Count results
from collections import Counter
counts = Counter(offspring)

print(f"Simulation: {parent1} x {parent2} — {num_trials} offspring")
print("=" * 45)

for genotype in sorted(counts.keys()):
    count = counts[genotype]
    percentage = count / num_trials * 100
    bar = "#" * int(percentage / 2)
    print(f"  {genotype}: {count:4d} ({percentage:5.1f}%) {bar}")

# Determine phenotypes
dominant_trait = sum(counts[g] for g in counts if g[0].isupper())
recessive_trait = sum(counts[g] for g in counts if g[0].islower())

print(f"\nPhenotype results:")
print(f"  Brown eyes (B_): {dominant_trait} ({dominant_trait/num_trials*100:.1f}%)")
print(f"  Blue eyes  (bb): {recessive_trait} ({recessive_trait/num_trials*100:.1f}%)")
print(f"\nExpected ratio: 75% / 25%")
print(f"Actual ratio:   {dominant_trait/num_trials*100:.1f}% / {recessive_trait/num_trials*100:.1f}%")

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Challenge: The Blue-Eyed Child

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Resources

• DNA from the Beginning — interactive animated guide to genetics from Cold Spring Harbor Laboratory
• Amoeba Sisters: Heredity — excellent overview of heredity and Punnett squares
• Learn Genetics — University of Utah — interactive tools for learning about DNA, heredity, and more
• Python random module documentation — reference for random number generation in Python
• NHGRI: Talking Glossary of Genetic Terms — definitions of every genetics term you might encounter