4  Classical Genetics

4.1 The Father of Genetics: Gregor Mendel

4.1.1 A Monk Who Loved Pea Plants 🌱

Once upon a time, in the 1860s, there was a monk named Gregor Mendel who lived in a monastery in what is now the Czech Republic. He loved gardening and was very curious about why living things looked the way they did.

Mendel noticed something interesting: Sometimes baby plants looked like their parents, but sometimes they didn’t! He wanted to figure out the rules behind this.

4.1.2 Why Pea Plants?

Mendel chose to study pea plants because they were perfect for his experiments:

Pea plants are great because:

  • 🌿 They grow quickly (fast results!)

  • 🎨 They have easy-to-see differences (tall vs. short, green vs. yellow, smooth vs. wrinkled)

  • 👶 They can make babies easily

  • 🎨 He could control which plants made babies with which other plants

Think of it like choosing the perfect pet for a science project—you’d want one that’s easy to take care of and shows clear results!

4.1.3 What Did Mendel Study?

Mendel focused on seven traits that were easy to see:

  1. Seed shape - Round or wrinkled

  2. Seed color - Yellow or green

  3. Flower color - Purple or white

  4. Pod shape - Smooth or bumpy

  5. Pod color - Green or yellow

  6. Flower position - On the side or on top

  7. Plant height - Tall or short

Each trait had only two options, making them easy to track!

4.2 Mendel’s Clever Experiments

4.2.1 The Setup

Mendel started with pure plants—plants that always produced offspring that looked exactly like them:

  • Tall plants that always made tall babies

  • Short plants that always made short babies

  • Purple-flowered plants that always made purple-flowered babies

4.2.2 The First Experiment: Crossing Different Plants

Then Mendel did something interesting. He crossed a tall plant with a short plant.

What do you think happened?

You might guess the babies would be medium-sized, right? Like mixing white paint and black paint to get gray?

But that’s NOT what happened!

Instead, ALL the baby plants were TALL! The “short” trait seemed to disappear completely!

Mendel called the trait that showed up (tall) the dominant trait.

The trait that seemed to hide (short) was called the recessive trait.

4.2.3 The Second Experiment: A Surprise Return!

Mendel didn’t stop there. He let those tall baby plants make babies with each other.

Guess what happened?

The short plants came back! But only some of them:

  • About 3 out of every 4 plants were tall

  • About 1 out of every 4 plants were short

The ratio was about 3:1 (three tall for every one short).

The “short” trait hadn’t disappeared—it was just hiding!

4.3 Mendel’s Big Discoveries

4.3.1 Discovery 1: Genes Come in Pairs

Mendel figured out that traits are controlled by what we now call genes, and every plant has TWO copies of each gene:

  • One copy from the mother plant

  • One copy from the father plant

Think of it like having two shoes—you need a pair!

4.3.2 Discovery 2: Dominant and Recessive

Some versions of genes are stronger than others:

Dominant traits (shown with a capital letter, like T for tall):

  • If you have even ONE copy, the dominant trait shows

  • They’re like a loud voice that drowns out a quiet one

Recessive traits (shown with a lowercase letter, like t for short):

  • You need TWO copies for the recessive trait to show

  • They’re like a quiet voice that can only be heard when there’s no loud voice

4.3.3 Discovery 3: The Law of Segregation

When plants make seeds, the two gene copies separate (segregate means to separate):

  • Each seed gets only ONE copy from each parent

  • It’s random which copy it gets

Think of it like:

  • Mom has two cards (one red, one blue)

  • Dad has two cards (one red, one blue)

  • Each baby gets one random card from mom and one random card from dad

4.4 Understanding with Punnett Squares

4.4.1 What’s a Punnett Square?

Scientists use a tool called a Punnett Square to predict what babies might look like. It’s like a tic-tac-toe grid that helps us see all the possible combinations!

4.4.2 Example: Tall vs. Short Pea Plants

Let’s say:

  • T = tall (dominant)

  • t = short (recessive)

Parent 1: Has Tt (tall, but carrying the short gene) Parent 2: Has Tt (tall, but carrying the short gene)

The Punnett Square shows us four possible babies:

     T    t
   +----+----+
T  | TT | Tt |
   +----+----+
t  | Tt | tt |
   +----+----+

```

Results:

  • TT = Tall (1 out of 4) = 25%

  • Tt = Tall (2 out of 4) = 50%

  • tt = Short (1 out of 4) = 25%

So 3 out of 4 are tall, and 1 out of 4 is short—just like Mendel observed!

4.5 Mendelian vs. Non-Mendelian Inheritance

4.5.1 Mendelian Inheritance (The Simple Rules)

Mendel’s rules work perfectly when:

  • One gene controls one trait

  • There are only two versions (dominant and recessive)

  • Genes don’t influence each other

Examples:

  • Widow’s peak hairline

  • Attached or detached earlobes

  • Whether you can roll your tongue

4.5.2 Non-Mendelian Inheritance (The Complicated Cases)

But sometimes genetics is more complicated! Non-Mendelian inheritance happens when:

4.5.2.1 1. Incomplete Dominance

Neither gene is fully dominant. It’s like mixing paint!

Example: Red flowers + White flowers = Pink flowers

4.5.2.2 2. Codominance

BOTH genes show up at the same time!

Example: Blood type AB—both A and B show up together

4.5.2.3 3. Multiple Genes

Many genes work together to control one trait.

Examples:

  • Height (controlled by many genes)

  • Skin color (controlled by many genes)

  • Eye color (more complex than Mendel’s simple traits)

4.5.2.4 4. Genes on Sex Chromosomes

Some genes are on the X or Y chromosomes.

Example: Color blindness (much more common in boys than girls)

4.5.2.5 5. Environmental Factors

Sometimes the environment affects how genes are expressed.

Example: Himalayan rabbits are born white, but their fur turns black in cold areas (like ears and feet)!

4.6 Why Mendel’s Work Was Amazing

4.6.1 He Did It Without Modern Tools!

Remember, Mendel did all this:

  • In the 1860s

  • Without knowing about DNA

  • Without microscopes good enough to see chromosomes

  • Just by carefully counting thousands of pea plants!

It’s like figuring out how a computer works just by watching what appears on the screen!

4.6.2 He Discovered the Rules of Inheritance

Mendel’s work showed:

  • ✅ Traits are passed from parents to children

  • ✅ Traits are controlled by “factors” (we now call them genes)

  • ✅ These factors come in pairs

  • ✅ Some factors are dominant over others

  • ✅ The factors separate when making seeds

4.6.3 Nobody Believed Him!

Sadly, during Mendel’s lifetime (he died in 1884), most scientists didn’t understand or appreciate his work. It was “rediscovered” in 1900, and then everyone realized how brilliant he was!

4.7 Mendel’s Legacy Today

4.7.1 Modern Genetics Built on Mendel’s Foundation

Everything we know about genetics started with Mendel’s pea plants:

  • Understanding genetic diseases

  • Breeding better crops and animals

  • DNA testing

  • Personalized medicine

  • Genetic engineering

4.7.2 Real-World Applications

In Agriculture:

  • Farmers use Mendel’s principles to breed crops with desired traits

  • Creating disease-resistant plants

  • Developing more nutritious food

In Medicine:

  • Predicting genetic diseases

  • Understanding how diseases are inherited

  • Developing treatments targeted to specific genes

In Everyday Life:

  • Ancestry DNA tests

  • Understanding why you look like your parents

  • Predicting traits in children

4.8 Fun Facts About Mendel! 🎉

  • Mendel studied about 28,000 pea plants over 8 years!

  • He was also a beekeeper and studied meteorology (weather)

  • He failed his teaching exam twice but kept trying

  • His work was published in 1866 but ignored for 34 years

  • The monastery where he worked is now a museum in his honor

  • We celebrate “Mendel Day” on March 8th (though it’s not an official holiday… yet!)

4.9 Key Takeaways

  • Gregor Mendel discovered the basic rules of inheritance by studying pea plants in the 1860s

  • Traits are controlled by genes that come in pairs (one from each parent)

  • Dominant traits need only one copy to show; recessive traits need two copies

  • Punnett squares help predict what offspring will look like

  • Mendelian inheritance follows simple rules (one gene, two versions)

  • Non-Mendelian inheritance is more complex (incomplete dominance, codominance, multiple genes)

  • Mendel’s discoveries form the foundation of modern genetics

  • His work helps us breed better crops, understand diseases, and predict traits

Sources: Information adapted from Khan Academy (Mendel and His Peas), Nature Scitable (Gregor Mendel and the Principles of Inheritance), Britannica Kids (Gregor Mendel), and university genetics course materials.

Results:

  • TT = Tall (1 out of 4) = 25%

  • Tt = Tall (2 out of 4) = 50%

  • tt = Short (1 out of 4) = 25%

So 3 out of 4 are tall, and 1 out of 4 is short—just like Mendel observed!

4.10 Mendelian vs. Non-Mendelian Inheritance

4.10.1 Mendelian Inheritance (The Simple Rules)

Mendel’s rules work perfectly when:

  • One gene controls one trait

  • There are only two versions (dominant and recessive)

  • Genes don’t influence each other

Examples:

  • Widow’s peak hairline

  • Attached or detached earlobes

  • Whether you can roll your tongue

4.10.2 Non-Mendelian Inheritance (The Complicated Cases)

But sometimes genetics is more complicated! Non-Mendelian inheritance happens when:

4.10.2.1 1. Incomplete Dominance

Neither gene is fully dominant. It’s like mixing paint!

Example: Red flowers + White flowers = Pink flowers

4.10.2.2 2. Codominance

BOTH genes show up at the same time!

Example: Blood type AB—both A and B show up together

4.10.2.3 3. Multiple Genes

Many genes work together to control one trait.

Examples:

  • Height (controlled by many genes)

  • Skin color (controlled by many genes)

  • Eye color (more complex than Mendel’s simple traits)

4.10.2.4 4. Genes on Sex Chromosomes

Some genes are on the X or Y chromosomes.

Example: Color blindness (much more common in boys than girls)

4.10.2.5 5. Environmental Factors

Sometimes the environment affects how genes are expressed.

Example: Himalayan rabbits are born white, but their fur turns black in cold areas (like ears and feet)!

4.11 Why Mendel’s Work Was Amazing

4.11.1 He Did It Without Modern Tools!

Remember, Mendel did all this:

  • In the 1860s

  • Without knowing about DNA

  • Without microscopes good enough to see chromosomes

  • Just by carefully counting thousands of pea plants!

It’s like figuring out how a computer works just by watching what appears on the screen!

4.11.2 He Discovered the Rules of Inheritance

Mendel’s work showed:

  • ✅ Traits are passed from parents to children

  • ✅ Traits are controlled by “factors” (we now call them genes)

  • ✅ These factors come in pairs

  • ✅ Some factors are dominant over others

  • ✅ The factors separate when making seeds

4.11.3 Nobody Believed Him!

Sadly, during Mendel’s lifetime (he died in 1884), most scientists didn’t understand or appreciate his work. It was “rediscovered” in 1900, and then everyone realized how brilliant he was!

4.12 Mendel’s Legacy Today

4.12.1 Modern Genetics Built on Mendel’s Foundation

Everything we know about genetics started with Mendel’s pea plants:

  • Understanding genetic diseases

  • Breeding better crops and animals

  • DNA testing

  • Personalized medicine

  • Genetic engineering

4.12.2 Real-World Applications

In Agriculture:

  • Farmers use Mendel’s principles to breed crops with desired traits

  • Creating disease-resistant plants

  • Developing more nutritious food

In Medicine:

  • Predicting genetic diseases

  • Understanding how diseases are inherited

  • Developing treatments targeted to specific genes

In Everyday Life:

  • Ancestry DNA tests

  • Understanding why you look like your parents

  • Predicting traits in children

4.13 Fun Facts About Mendel! 🎉

  • Mendel studied about 28,000 pea plants over 8 years!

  • He was also a beekeeper and studied meteorology (weather)

  • He failed his teaching exam twice but kept trying

  • His work was published in 1866 but ignored for 34 years

  • The monastery where he worked is now a museum in his honor

  • We celebrate “Mendel Day” on March 8th (though it’s not an official holiday… yet!)

4.14 Key Takeaways

  • Gregor Mendel discovered the basic rules of inheritance by studying pea plants in the 1860s

  • Traits are controlled by genes that come in pairs (one from each parent)

  • Dominant traits need only one copy to show; recessive traits need two copies

  • Punnett squares help predict what offspring will look like

  • Mendelian inheritance follows simple rules (one gene, two versions)

  • Non-Mendelian inheritance is more complex (incomplete dominance, codominance, multiple genes)

  • Mendel’s discoveries form the foundation of modern genetics

  • His work helps us breed better crops, understand diseases, and predict traits

Sources: Information adapted from Khan Academy (Mendel and His Peas), Nature Scitable (Gregor Mendel and the Principles of Inheritance), Britannica Kids (Gregor Mendel), and university genetics course materials.