Differences in individuals of the same species or between different species is known as Variation. There may be variation in height, weight, eye colour etc.

We inherit some of our features from our parents eg. hair colour, eye colour, height etc.

Other features are caused by our environment eg. scars, hair length etc.

Most cells in our body have a Nucleus which contain thread-like structures called

Chromosomes. Every human body cell contains 46 chromosomes (23 pairs).

However, sex cells such as sperm and egg cells contain only 23 chromosomes.

DNA, Genes, Chromosomes, Nucleus and the Cell

Genes are found on Chromosomes and these contain instructions for inheritance such as eye-colour, hair colour, height etc. Each Gene contains strands of smaller DNA in a spiral Double Helix shape. Chromosomes are packed into the Nucleus of the Cell.

Fertilisation and Pregnancy

To make a baby, the male and female sex cells must join together. This is called fertilisation. At fertilisation the sperm penetrates the egg and its nucleus joins with the egg nucleus, forming a Zygote Cell. Fertilisation occurs in the oviduct. If the egg is fertilised it passes down the egg tube (oviduct) and settles in the Uterus wall to develop during pregnancy, growing and dividing by Mitosis.

In a pregnant woman the baby develops in the Uterus for 9 months. The Cervix widens and then the muscles contract during birth of the baby, to push the baby out of the uterus.

The Placenta

A plate-shaped organ called a Placenta, forms in the Uterus. Inside the placenta the blood of the foetus and the mother comes close together.

The placenta acts as a barrier to infections and harmful substances. The umbilical cord carries blood with oxygen and nutrients to the foetus from the placenta. The fluid sac acts as a shock absorber to protect the foetus.