Heredity and variation - Science - Junior Secondary School

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Study Notes: Heredity and Variation

Hey everyone! Ever wondered why you have your mum's eyes but your dad's nose? Or why you and your friends, even if you're the same age, are all unique? Welcome to the amazing world of Heredity and Variation! In these notes, we'll uncover the secrets hidden inside your cells that make you, YOU. It's like being a detective for your own body's blueprint! Don't worry if it sounds complicated, we'll break it down step-by-step.

1. The Basics of Heredity: Your Family's Blueprint

What is Heredity?

Heredity is the passing of features, or traits, from parents to their children. Think of it like a secret family recipe book being passed down from one generation to the next. These traits are the characteristics that make you who you are.

Examples of traits include:

- Eye colour
- Hair colour
- Height
- The shape of your nose
- Whether your earlobes are attached or detached

Where is the "Recipe Book" Stored?

Your body is made of trillions of tiny building blocks called cells. Inside almost every cell, there's a control centre called the nucleus. And inside the nucleus, we find the "recipe book" organised into chapters. These "chapters" are thread-like structures called chromosomes.

Analogy Time: Imagine your body is a huge library (the body). Each book is a cell (the cell). In the main office of each book is the nucleus (the nucleus). The precious, super-important recipe books are the chromosomes (the chromosomes)!

Humans have 46 chromosomes in most of their body cells. They come in 23 pairs – you get one of each pair from your mum, and the other from your dad.

What are Chromosomes Made Of?

Chromosomes are made of a very long, tightly coiled molecule called DNA (which stands for Deoxyribonucleic Acid, but you only need to know DNA!). DNA is the master instruction manual for your entire body. It carries all the genetic information that tells your body how to grow and work.

So, the order is: Cell ⮕ Nucleus ⮕ Chromosomes ⮕ DNA

Did you know? If you could uncoil all the DNA from all the cells in your body and lay it end-to-end, it would stretch to the Sun and back several times!

Key Takeaway for Section 1

Heredity is the passing of traits from parents to children through genetic information. This information is stored on DNA, which makes up our chromosomes found inside the nucleus of our cells.

2. Determining Sex: The X and Y Story

The Sex Chromosomes

Out of the 23 pairs of chromosomes, one special pair determines whether a person is born male or female. These are called the sex chromosomes.

There are two types: the X chromosome and the Y chromosome.

Females have two X chromosomes. Their combination is XX.
Males have one X chromosome and one Y chromosome. Their combination is XY.

Since a mother has only X chromosomes to pass on in her egg cells, it's the father's sperm (which can carry either an X or a Y) that determines the sex of the baby. It's always a 50/50 chance!

Key Takeaway for Section 2

The 23rd pair of chromosomes are the sex chromosomes. XX makes a female, and XY makes a male.

3. Variation: Why We're All Unique

What is Variation?

Variation refers to the differences among individuals of the same species. It's why no two people (except identical twins) are exactly alike! Variation is caused by two main things:

1. Heredity: You get a unique mix of DNA from your parents.
2. The Environment: Things like your diet, lifestyle, and where you live can also affect your traits.

Analogy Time: Heredity gives you the recipe for a cake (your DNA). But the environment – like the type of oven you use, how long you bake it, and the decorations you add – will affect how the final cake turns out!

Two Types of Variation

Don't worry, this is easier than it sounds! We can sort variations into two groups.

1. Discontinuous Variation

This is for traits that fall into a few clear-cut, distinct categories. There are no "in-betweens". You either have the trait, or you don't.

Examples:

- Blood Group: You can only be A, B, AB, or O.
- Tongue Rolling: You can either roll your tongue into a 'U' shape, or you can't.
- Earlobe Attachment: Your earlobes are either attached to the side of your head or they are detached (dangling).

We use a bar chart to show discontinuous variation because the categories are separate.

2. Continuous Variation

This is for traits that show a whole range of measurements from one extreme to another. There are lots of "in-between" values.

Examples:

- Height: People aren't just "tall" or "short". There's a whole range of heights.
- Weight: There is a wide range of possible weights.
- Foot size: Think of all the different shoe sizes available!

We use a histogram (which looks like a bar chart with the bars touching) to show continuous variation because the data flows from one value to the next.

Key Takeaway for Section 3

Variation means the differences between us. Discontinuous variation has distinct categories (like blood type) and is shown on a bar chart. Continuous variation has a full range of values (like height) and is shown on a histogram.

4. The Amazing Case of Twins

Identical vs. Non-Identical Twins

Twins are a perfect real-world example of heredity and environment at work!

Non-Identical (Fraternal) Twins

These twins develop when two separate egg cells are fertilised by two separate sperm cells at the same time. They are just like regular siblings who happen to share a birthday! They don't have the same DNA. One can be a boy and the other a girl.

Identical Twins

These twins develop when a single fertilised egg splits into two very early on. This means they have the exact same DNA! They are always the same sex.

So, if they have the same DNA "recipe book", why are there still small differences between them? The answer is the environment! One twin might like different foods, play a different sport, or get more sun. These environmental factors cause small variations between them over their lifetime. This is proof that both heredity and the environment make us who we are!

Key Takeaway for Section 4

Identical twins have the same DNA, so they are a great way to study how the environment causes variation in individuals. Differences between them are due to their unique experiences and lifestyles.

5. A Deeper Look: The DNA Code (Extension Topic)

The Double Helix

DNA has a special shape called a double helix. The easiest way to picture this is a twisted ladder. The two long backbones are the sides of the ladder, and the "rungs" in the middle connect them.

The Letters of the DNA Code

The rungs of the DNA ladder are made from pairs of chemicals called bases. There are only four types:

- Adenine
- Thymine
- Cytosine
- Guanine

These bases have a strict pairing rule: A always pairs with T, and C always pairs with G. This is called the base pairing rule.

Memory Aid: A simple way to remember the pairs is...
Apple in the Tree
Car in the Garage

The instructions in your DNA are determined by the sequence (the order) of these bases. A sequence like ...A-T-T-G-C-A... might be the code for brown eyes, while a different sequence codes for blue eyes. It's like how the letters C-A-T spell "cat", but rearranging them to A-C-T spells "act" – the order matters!

Key Takeaway for Section 5

DNA is a double helix shape. The code is made of four bases (A, T, C, G) that pair up (A with T, C with G). The sequence of these bases provides the instructions for our traits.