# Mixing It Up

Every living organism on Earth must reproduce to pass its genes to the next generation. But did you know there's more than one way to do this?

Some organisms create offspring that are exact genetic copies of themselves. Others combine genetic information from two parents to create unique offspring. Each strategy has advantages and disadvantages.

In this lesson, you'll discover the trade-offs between these approaches—and learn why the bananas you eat might be in serious trouble.

Essential Question: Why does genetic variation matter for survival?

# Two Paths to Reproduction

## Sexual Reproduction

Sexual reproduction involves TWO parents who combine their genetic information to create offspring. Each parent contributes half of the genetic material.

Key Characteristics: - Requires two parents (usually male and female) - Offspring receive 50% of genes from each parent - Offspring are genetically UNIQUE—different from both parents and from siblings - Requires specialized sex cells (gametes): sperm and eggs - Usually takes more time and energy - Must find a mate to reproduce

Examples: Humans, dogs, cats, birds, fish, most insects, most flowering plants

## Asexual Reproduction

Asexual reproduction involves ONE parent that creates offspring without combining genetic material from another organism. The offspring are genetically identical to the parent.

Key Characteristics: - Requires only ONE parent - Offspring are exact genetic copies (clones) of the parent - Faster and requires less energy - No need to find a mate - All offspring are genetically identical to each other and to the parent

Examples: Bacteria, yeast, some plants (strawberry runners, potato eyes), sea stars, hydra

# Types of Asexual Reproduction

## Methods of Asexual Reproduction

Binary Fission - A single-celled organism divides into two identical cells - Used by bacteria, which can double every 20 minutes under ideal conditions - One bacterium could become over 1 billion in 10 hours!

Budding - A new organism grows as an outgrowth from the parent's body - The bud eventually detaches and becomes independent - Examples: yeast, hydra, coral

Fragmentation - An organism breaks into pieces, and each piece grows into a complete new organism - Examples: sea stars (if an arm breaks off, it can regrow into a complete sea star), some worms, some plants

Vegetative Propagation - Plants create new individuals from stems, roots, or leaves—not from seeds - Examples: - Strawberries send out runners that root and become new plants - Potatoes grow from 'eyes' on tubers - Spider plants drop plantlets that take root - Grass spreads through underground stems

## Comparison Table

| Feature | Sexual Reproduction | Asexual Reproduction | |---------|--------------------|--------------------| | Parents needed | Two | One | | Genetic variation | High—offspring unique | None—offspring are clones | | Speed | Slower | Faster | | Energy required | More | Less | | Need to find mate | Yes | No | | Adaptation potential | High | Low | | Disease vulnerability | Lower (variation helps) | Higher (all identical) |

# Why Genetic Variation Matters

## The Power of Diversity

Genetic variation is the diversity of genes within a population. Sexual reproduction creates variation because: 1. Each parent contributes different alleles 2. The combination is random 3. Every offspring (except identical twins) is genetically unique

## The Survival Advantage

Imagine a forest of trees. If all trees are genetically identical (clones): - A disease that kills one can kill them ALL - A climate change that harms one affects ALL - A pest that attacks one attacks ALL

But if the trees have genetic variation: - Some might resist the disease - Some might tolerate the new climate - Some might repel the pest - The species survives because SOME individuals can adapt

This is natural selection in action—variation gives species the raw material to evolve and adapt.

# Case Study: The Cavendish Banana Crisis

## The Bananas You Eat Are Clones

The Cavendish banana—the yellow banana sold in every grocery store—is reproduced asexually. Farmers plant cuttings from existing plants, not seeds. This means:

- Every Cavendish banana plant in the world is genetically identical - They are all clones of each other - They ALL share the exact same strengths... and weaknesses

## The Problem: Panama Disease TR4

A fungal disease called Tropical Race 4 (TR4) is spreading through banana plantations worldwide. Because all Cavendish bananas are genetically identical:

- If TR4 can infect ONE plant, it can infect ALL plants - There is NO natural resistance anywhere in the population - Scientists cannot find resistant Cavendish plants to breed—they're all the same - The entire commercial banana industry is threatened

## History Repeating

This has happened before! Before the Cavendish, the world's most popular banana was the Gros Michel ("Big Mike"). It was reportedly tastier than today's bananas.

In the 1950s, a different strain of Panama disease wiped out nearly every Gros Michel plantation. The variety is now essentially extinct commercially.

The Cavendish was chosen as the replacement specifically because it was resistant to THAT strain. But TR4 is different—and the Cavendish has no defense.

Lesson: Cloning creates vulnerability. Genetic diversity provides security.

# Summary

## Key Concepts Review

Sexual Reproduction: - Two parents contribute genetic material - Offspring are genetically unique - Creates genetic variation - Slower but provides adaptability

Asexual Reproduction: - One parent produces clones - Offspring are genetically identical to parent - No genetic variation - Faster but creates vulnerability

Why Genetic Variation Matters: - Allows some individuals to survive threats (disease, climate change, predators) - Provides raw material for natural selection - Enables species to adapt over time - Acts as 'insurance' against environmental changes

The Banana Lesson: The Cavendish banana crisis demonstrates the real-world danger of relying on genetically identical organisms. Without variation, a single threat can eliminate an entire population.