GENETICS 101
It all started with a single question I wrote in my notebook:
> "Why do I look like my dad but have my mom's eyes?"
I thought about my little cousin — she had curly hair, just like her grandma.
And my friend Michael? He had six fingers on one hand — his dad did too.
Coincidence? No way.
So I began digging, reading, asking, and slowly, I started to understand:
It's not magic. It's biology.
Something passed down from parents to child.
Like invisible threads — tying generations together.
That something… is called inheritance.
And the science that studies it? That's called genetics.
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📘 Chapter 1: What Is Genetics?
Let's begin at the beginning.
Genetics is the study of how traits are passed from one generation to the next.
Traits mean:
Eye color
Height
Hair type
Skin shade
Dimples
Freckles
Even diseases!
Anything that can be passed from parent to child.
These traits are carried in something even smaller, something inside every living thing:
> Genes.
And genes live in something called chromosomes.
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🔬 Chapter 2: What Are Chromosomes?
Let's zoom into our body.
Our body is made up of tiny building blocks called cells.
Inside each cell, there's a nucleus — like a control center.
And inside the nucleus?
There are thread-like structures.
Chromosomes.
We humans have 46 chromosomes in each cell — that's 23 pairs.
We get 23 from our mom, and 23 from our dad.
So when a baby is made, it gets half from each parent.
That's how traits are passed.
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🧬 Chapter 3: What Are Genes?
Now, chromosomes are made of a chemical called DNA — and DNA has sections.
Each section is called a gene.
A gene is like a tiny instruction manual.
One gene tells your body:
"Make the eyes brown."
Another says:
"Grow curly hair."
Another might say:
"Have a tall height."
So your body is basically running on genetic instructions.
And you got those instructions from your parents.
> "Wow… I'm a walking storybook. Half from mom, half from dad."
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👨🌾 Chapter 4: Who Was Mendel?
The person who figured this out — long before we even saw DNA — was a monk named Gregor Mendel.
He didn't use humans.
He used… peas. Yeah. The green ones.
Why peas?
Because they grow fast and show clear differences:
Some were tall, some short
Some had smooth seeds, some wrinkled
Some were green, others yellow
And Mendel asked:
> "What happens when I cross a tall pea with a short one?"
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🌿 Chapter 5: Mendel's Experiments
Let me show you what he did — slowly.
He took two pure plants:
One tall
One short
He crossed them (that means he made them breed).
And what happened?
All the kids — called the first generation (F1) — were tall.
The short trait disappeared!
But when he let the F1 plants breed again…
Boom — in the second generation (F2) — some short plants came back.
The pattern?
3 tall
1 short
That's a 3:1 ratio.
Mendel was shocked.
But he realized: the short trait didn't vanish — it just stayed hidden in the first generation.
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🧠 Chapter 6: What Did Mendel Discover?
He made three big rules — called Mendel's Laws of Inheritance.
Let me explain each.
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⚖️ Law 1: Law of Dominance
Some traits are dominant, others are recessive.
Dominant means: it shows up, even if there's only one copy.
Recessive means: it only shows up if both copies are recessive.
So in the tall-short pea example:
Tall = dominant
Short = recessive
If a plant gets one tall and one short gene — it looks tall.
But it still carries the short gene inside.
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🧪 Law 2: Law of Segregation
Each parent has two copies of a gene (one from each of their parents).
But when they pass genes to their child, they give only one copy.
So the baby ends up with one gene from mom, one from dad.
This explains why traits mix and how traits can skip generations.
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🔄 Law 3: Law of Independent Assortment
This one's a bit tricky — but stay with me.
It means:
> "The inheritance of one trait doesn't affect another."
For example:
Just because you have brown eyes from your mom or dad doesn't mean you'll have curly hair.
Genes for eye color and hair type are inherited separately.
So traits can shuffle around freely.
> "It's like dealing cards. Every child gets a new combo."
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🧬 Chapter 7: Words You'll See in Genetics
Let me slow down and explain some tricky words.
1. Alleles
Genes have different versions — these versions are called alleles.One from dad and one from mom
Like:
A gene for height might have one tall allele and one short allele.
2. Homozygous
If both alleles are the same:
TT (both tall)
or tt (both short)
That's homozygous.
"Homo" = same
3. Heterozygous
If the alleles are different:
Tt (one tall, one short)
That's heterozygous.
"Hetero" = different
In a heterozygous pair, usually the dominant one wins.
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🧬 Chapter 8: Genotype vs Phenotype
Two more important words.
Genotype = The genes inside (TT, Tt, tt) (kind of like your potential)
It's invisible — written in your DNA.
Phenotype = What shows outside (Tall or Short)
It's visible — what you look like.
So two tall plants might look the same (same phenotype),
but one might be TT and the other Tt (different genotype).
> "Just because two people look alike doesn't mean they have the same genes."
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🧪 Chapter 9: How Do We Use Genetics Today?
Now we understand genes, we can:
Predict diseases before they happen
Choose healthy crops in farming
Help couples understand inherited risks
Breed stronger animals
Even create medicine based on your DNA
We can also trace family roots and solve crimes using DNA tests.
> "It's not science fiction. It's real. It's happening."
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🌍 Chapter 10: What Is Variation?
No two people are the same.
Even twins have tiny differences.
That's called variation — the differences between individuals.
Variation happens because:
Different combinations of genes
Mutations (small changes in DNA)
Environment (like food, lifestyle, weather)
Variation makes life interesting.
It's also important for evolution — because it gives nature choices to pick from.
> "Without variation, we'd all be the same. That's scary."
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🧑🏫 Chapter 11: Artic's Summary
Okay. Let's simplify everything.
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What is genetics?
It's the science of how parents pass traits to children.
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What are chromosomes?
Tiny X-shaped threads inside your cells.
You get 23 pairs = 46 total.
Half from mom, half from dad.
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What are genes?
Tiny instruction manuals in the chromosome.
They decide how you look and grow.
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What did Mendel discover?
1. Some traits are dominant, some recessive
2. Parents pass only one of their two genes
3. Traits are passed independently
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What is homozygous?
Both genes same — TT or tt.
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What is heterozygous?
One dominant, one recessive — Tt.
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What is phenotype?
What you see (tall, short, curly hair)
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What is genotype?
What is inside (TT, Tt, or tt)
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What is variation?
The tiny differences in people.
Makes us unique and helps us evolve.
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🌱 Artic's Last Thought
The more I learn about genes, the more I see myself clearly.
.....
Last time, I learned that some traits are dominant — they win.
Others are recessive — they hide.
But then I looked in the mirror again.
I don't have pure brown eyes like Dad.
Or bright green like Mom.
Mine are hazel — something… in-between.
Not fully brown. Not fully green.
> "So which gene won?"
That question didn't go away.
And that's when I found out:
Not everything follows Mendel's rules.
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🎨 Chapter 1: Incomplete Dominance — When No One Wins
Let's say:
Red is dominant (RR)
White is recessive (rr)
According to Mendel, if we mix them, the child should look red (Rr).
But in incomplete dominance, something different happens.
The child isn't red.
It's pink.
A blend.
> "Wait… how?"
In incomplete dominance, neither allele is strong enough to dominate completely.
So they mix — and you get a new trait, halfway between both.
Like:
Genotype Trait
RR Red flower
Rr Pink flower
rr White flower
This happens in snapdragon flowers. Real ones.
> "So… Rr = pink. That's not hiding. That's sharing space."
It's like two painters each giving up a little and mixing their colors.
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🧃 Chapter 2: Co-dominance — When Both Win
Okay, now imagine two genes — but instead of mixing, they both show up.
Fully.
Side-by-side.
That's co-dominance.
The best example? Blood types.
There are A, B, and O alleles.
If a person gets:
AA = blood group A
BB = blood group B
OO = blood group O
AO = still blood group A (A is dominant over O)
BO = still blood group B
But what if the person gets AB?
Then…
> Both A and B are expressed.
Blood group becomes AB — not A, not B, but both.
That's co-dominance.
Like two leaders who share power equally.
> "It's not mixing colors. It's putting stripes side by side."
Another example?
Cows.
A red cow + a white cow = a roan cow (red and white patches)
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🧠 Chapter 3: Pleiotropy — One Gene, Many Effects
Now, I thought one gene = one job.
Like:
One gene for height
One for eye color
One for blood type
But that's too neat.
Sometimes, a single gene can affect many traits at once.
That's called pleiotropy.
Imagine one switch that controls:
Your skin color
Your vision
Your bones
Your hair texture
All from one gene.
> "One gene. Many consequences."
Example?
There's a disease called sickle cell anemia.
One tiny change in a gene leads to:
Abnormally shaped red blood cells
Tiredness
Pain
Resistance to malaria (yes, weirdly it also protects against malaria)
One gene → many effects.
That's pleiotropy.
It reminds me that genes aren't always clean and simple. They can echo through the body.
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🎯 Chapter 4: Polygenic Inheritance — Many Genes, One Trait
Here's the opposite of pleiotropy.
Now, many genes work together to create one trait.
This is called polygenic inheritance.
> "Poly" = many
"Genic" = genes
So when you look at someone's:
Height
Skin color
Body shape
Eye color (sometimes)
…it's not just one gene.
It's a team of genes, all pushing in different directions.
Let's say 5 genes control height (ABCDE) tall (AbCdE) medium(abcde) short
If you get all the "tall" versions — you'll be very tall.
Some "tall," some "short" versions — medium height.
All "short"? Then you'll be shorter.
The result?
Continuous variation. A smooth scale, not black and white.
So you can be different from your parents not always the same
(You inherit your genes from your parents -one set from dad and one from mom but their parents inherited their genes from their parents - your grandparents and they share their genes with their siblings-your aunt your uncles.So even though your uncle isn't a direct parent , he is closely related and may share a lot of genetic material with your parents)
Conclusion: Suppose your parents height is short but your direct uncles and Aunt have tall height then you can get tall too even though, your parents are short
It's possible, but not guaranteed.