Genetics

Welcome to the World of Genetics!

Ever wondered why you have your mother’s eyes or your father’s height? Or why some people can roll their tongues while others can't? That is exactly what Genetics is all about! Think of it as the "instruction manual" that tells your body how to build and maintain itself. Don't worry if this seems a bit "science-heavy" at first; we will break it down into small, easy-to-manage chunks together!

1. DNA: The Instruction Manual

Inside almost every cell in your body, there is a molecule called DNA (Deoxyribonucleic Acid). It carries all the information needed to make you, "you."

What does DNA look like?

DNA is a polymer (a large molecule made of repeating units). You need to know these three specific things about its structure:

• Double Helix: It consists of two strands coiled together like a twisted ladder.
• Complementary Base Pairs: The "rungs" of the ladder are made of four bases (A, T, C, and G). A always pairs with T, and C always pairs with G. These are held together by weak hydrogen bonds.
• Nucleotides: DNA is made of units called nucleotides. Each one has a sugar, a phosphate group, and one of the four bases.

Important Definitions

• Genome: This is the entire DNA of an organism. Every single instruction.
• Gene: A small section of DNA. Each gene is a code for a specific protein.

Analogy: If your DNA is a massive library (The Genome), a Gene is just one single book (The Recipe) that tells the body how to make one specific dish (The Protein).

Quick Review: Fruit DNA Extraction

You can actually see DNA at home or in class! To extract it from fruit (like a strawberry), you follow these steps:

1. Mash the fruit: Breaks the cell walls.
2. Add detergent and salt: The detergent breaks open the cell membranes and the salt makes the DNA stick together.
3. Filter the mixture: To remove the big lumps of fruit.
4. Add ice-cold ethanol: DNA is not soluble in alcohol, so it precipitates (appears as a white, stringy layer) at the top!

Key Takeaway: DNA is a double helix polymer made of nucleotides. Genes are small sections of DNA that code for proteins.

2. Making Gametes: Meiosis

Before we talk about how you get your traits, we need to look at how "sex cells" (sperm and eggs) are made. This process is called Meiosis.

Note: You don't need to learn the individual stages of meiosis for this course, just the result!

The Outcome of Meiosis

Unlike normal cell division (mitosis) which makes identical copies, Meiosis is different because it creates:

• Four daughter cells: One cell divides twice to produce four.
• Haploid cells: Each cell has only half the number of chromosomes (23 in humans).
• Genetically different: Every single cell produced is unique! This is why siblings look different even though they have the same parents.

Key Takeaway: Meiosis produces four genetically different haploid gametes (sex cells).

3. The Genetic Code: Key Terms

To understand inheritance, you need to speak the language of genetics. These terms are the most common places where students lose marks, so let's get them straight!

• Allele: Different versions of the same gene (e.g., the gene for eye colour has a "blue" allele and a "brown" allele).
• Dominant: An allele that always shows up if it's present (written as a Capital letter, like B).
• Recessive: An allele that only shows up if there is no dominant allele present (written as a lowercase letter, like b).
• Homozygous: Having two of the same alleles (e.g., BB or bb).
• Heterozygous: Having two different alleles (e.g., Bb).
• Genotype: The combination of alleles you have (the "letters," e.g., Bb).
• Phenotype: The physical characteristic you see (the "look," e.g., Brown eyes).
• Gamete: A sex cell (sperm or egg).
• Zygote: A fertilised egg.

Memory Aid: Phenotype starts with 'Ph', just like Physical look! Genotype starts with 'G', just like Genes (the letters).

Key Takeaway: Alleles are versions of genes. Dominant alleles mask recessive ones. Genotype is the code; phenotype is the result.

4. Predicting the Future: Genetic Diagrams

We use Punnett Squares to calculate the probability of a child inheriting a certain trait. This is called monohybrid inheritance.

How to Draw a Punnett Square

Imagine two parents are heterozygous for brown eyes (Bb). Brown is dominant (B) and blue is recessive (b).

1. Put one parent’s alleles on the top and the other on the side.
2. Fill in the boxes by combining the letters.

Resulting combinations:
- BB (25% chance) - Homozygous Dominant (Brown eyes)
- Bb (50% chance) - Heterozygous (Brown eyes)
- bb (25% chance) - Homozygous Recessive (Blue eyes)

Math Skill: The ratio of Brown eyes to Blue eyes here is \( 3:1 \). The probability of blue eyes is \( 1/4 \) or \( 25\% \).

Determining Sex

Did you know that your sex is determined by just one pair of chromosomes?
- Females have XX
- Males have XY

When you do a Punnett Square for sex, there is always a 50% chance of a boy and a 50% chance of a girl at fertilisation.

Key Takeaway: Punnett squares show the probability of traits. Sex is determined by X and Y chromosomes.

5. Variation and Mutations

Not everything about you is in your genes! Differences between individuals are called variation.

Causes of Variation

1. Genetic Variation: Caused by different alleles inherited through sexual reproduction and mutations (random changes in DNA).
2. Environmental Variation: Characteristics caused by your surroundings (e.g., a scar, your language, or a plant growing bent because of the wind).

Important Note: Most phenotypic features (like height or skin colour) are the result of multiple genes working together, rather than just a single gene!

What about Mutations?

A mutation is a change in the DNA sequence.
- Most mutations have no effect on the phenotype.
- Some mutations have a small effect.
- Rarely, a single mutation can significantly affect the phenotype (like in some genetic diseases).

Quick Review: Human Genome Project (HGP)
The HGP was a massive international project to map every single gene in the human body. Because of it, we can now:
- Predict and prevent diseases.
- Develop new, personalised medicines.
- Understand human evolution better.

Key Takeaway: Variation is caused by both genes and the environment. Mutations are random DNA changes that usually have little to no effect.

Common Mistakes to Avoid

• Confusing Meiosis and Mitosis: Remember, Meiosis makes Me! It’s for making babies (gametes). Mitosis is for my "toes" (growth and repair of body cells).
• Incorrect Ratios: Always check if the question asks for the ratio of genotypes or phenotypes. They are often different!
• Capital Letters: Be very careful when writing letters like 'C' and 'c' or 'S' and 's' in Punnett squares. Make the capital one much bigger so you don't get confused.

You've reached the end of the Genetics chapter! Take a deep breath—you've covered DNA structure, cell division, and the laws of inheritance. Great job!