The Bio Brief

Basic Idea

Traits (like height, colour, shape) are controlled by genes.

Genes are made of DNA.

DNA tells the cell how to make proteins, and proteins decide how a trait appears.

So the chain is:

DNA → Gene → Protein → Trait

What is a Gene?

• A gene is a small part of DNA.
• Each gene gives instructions to make one protein.
• These proteins control body features.

 That’s how genes control traits.

 Example: Height in Plants (Tall / Short)

Let’s understand this with plant height.

Plants grow using growth hormones.

Step-by-step:

1. Gene makes an enzyme (a protein)
2. Enzyme helps make growth hormone
3. Hormone controls plant height

Two Situations:

 If enzyme works well:

More hormone → Tall plant

 If enzyme works poorly (gene is altered):

Less hormone → Short plant

Simple Flow Chart:

Gene

 ↓

Enzyme

 ↓

Growth Hormone

 ↓

Plant Height

(Tall / Short)

 Conclusion

 Genes control proteins

 Proteins control traits

Why Do We Have TWO Copies of Each Gene?

In sexual reproduction:

• Mother gives one copy
• Father gives one copy

So every child gets:

2 copies of each gene

(one from each parent)

That’s why traits depend on both parents.

Example (Pea Plants):

Parent 1 → Gene copy

Parent 2 → Gene copy

Child → Two gene copies

These copies may be:

• Same (TT or tt)
• Different (Tt)

 What Happens in Germ Cells?

Normal body cells have:

Two sets of genes (2n)

But germ cells (sperm / egg) have:

Only ONE set of genes (n)

 Why is this important?

Because during fertilization:

Sperm (n) + Egg (n)

        ↓

Zygote (2n)

This restores the normal number of chromosomes.

 Role of Chromosomes

Genes are carried on chromosomes.

• Each cell has pairs of chromosomes
• One from mother
• One from father

Simple Diagram:

Body cell:

Mother chromosome + Father chromosome

        ↓

       Pair

During germ cell formation:

Chromosome pair separates

↓

Each germ cell gets ONE chromosome

 Fertilization

Male germ cell + Female germ cell

            ↓

        New individual

(with full chromosome set)

This keeps DNA stable in a species.

Independent Inheritance (Mendel’s Result)

Genes are on separate chromosomes, so:

 Different traits separate independently.

That’s why new combinations appear like:

• Tall + wrinkled
• Short + round

This supports Mendel’s experiments.

 What About Asexual Reproduction?

• Only one parent
• No mixing of genes
• Offspring almost identical
• Only small variations due to DNA copying errors

 Key Points

• Genes are parts of DNA.
• Genes make proteins.
• Proteins control traits.
• Each trait has two gene copies.
• Parents contribute equally.
• Germ cells have one gene set.
• Chromosomes carry genes.
• Independent inheritance creates variation.

One-Line Summary

Traits are expressed through proteins made by genes present on chromosomes, with each parent contributing one gene copy, ensuring inheritance and variation.