Reproduction and Inheritance

Reproduction is how living things produce new individuals of the same kind. In humans, this needs two special sex cells: an egg cell and a sperm cell.

The female reproductive system produces egg cells (also called ova). The main parts are:

• Ovaries — produce egg cells.
• Oviducts — tubes that carry an egg towards the uterus; this is where fertilisation usually happens.
• Uterus — where a baby grows during pregnancy.

The male reproductive system produces sperm cells. Its main parts are:

• Testes — produce sperm cells.
• Sperm ducts — tubes that carry sperm.
• Penis — passes sperm into the female reproductive system.

Each system is built for a different role, but they only create new life when they work together. The egg cell and the sperm cell each carry half of the instructions for a new person — which is why a child inherits features from both parents.

The menstrual cycle is a repeating pattern, lasting about 28 days, that prepares the female body for a possible pregnancy.

The main stages are:

• Menstruation (days 1-5) — the lining of the uterus breaks down and leaves the body. This is the "period".
• Building up (days 6-13) — the lining of the uterus thickens again, getting ready.
• Ovulation (around day 14) — an egg cell is released from an ovary into the oviduct.
• Waiting (days 15-28) — the lining stays thick, ready in case the egg is fertilised.

If the egg is not fertilised, the lining breaks down and the cycle starts again. If the egg is fertilised, the cycle pauses — a pregnancy has begun.

Fertilisation is the moment a sperm cell joins with an egg cell. The two sex cells fuse, and their instructions combine to make a single new cell — a fertilised egg.

Notice the sizes: the egg cell is large because it carries a food store for the early stages of growth. Sperm cells are tiny and have a tail, so they can swim towards the egg. Structure suited to function again.

After fertilisation, the fertilised egg travels to the uterus and settles into its thick lining. There it divides again and again, growing into a ball of cells, then an embryo, and over about nine months into a baby. This whole period of growth and development is called pregnancy.

Have you ever been told you have "your father's eyes" or "your mother's smile"? This is inheritance — the passing of features from parents to their offspring (children).

It happens because of those two sex cells. The egg cell carries instructions from the mother, and the sperm cell carries instructions from the father. When they join at fertilisation, the new individual receives a set of instructions from each parent.

This explains a lot:

• A child can have features from both parents — perhaps the mother's hair colour and the father's height.
• Brothers and sisters are not identical, because each one receives a slightly different mix of the parents' instructions.
• Some features, like eye colour, are inherited. Others, like a scar or a learned skill, are not — they are gained during life.

The instructions themselves are carried by a remarkable chemical called DNA, which we meet next.

The instructions for a living thing are stored as a chemical called DNA. To understand how it is organised, picture a set of objects fitting neatly inside one another.

Here is how the parts fit together:

• The nucleus of a cell holds thread-like structures called chromosomes.
• A chromosome is a long, coiled-up molecule of DNA.
• A short section of DNA that controls one feature is called a gene — for example, a gene for eye colour.

So when instructions pass from parent to child, what is really being passed on are genes, carried on chromosomes, inside the nucleus of the sex cells.

Look at any group of the same kind of living thing — a class of students, a litter of puppies — and you will notice they are not all exactly alike. These differences are called variation.

Variation comes from two sources working together:

• Inherited variation — the mix of genes received from both parents. This is why you differ from your brothers and sisters.
• Variation from the environment — the conditions an individual lives in. A plant given more light may grow taller than an identical one in the shade.

Understanding reproduction and inheritance now sets you up for later topics: how species change over time, how new plants are bred for farming, and how features are passed down through families. Whenever a later topic asks why living things differ, think back to genes, inheritance and variation.