Each body cell nucleus contains 46 chromosomes which make up 23 pairs. One of these pairs determines whether an individual will be male or female.

• In males the two sex chromosomes are XY.
• In females the two sex chromosomes are XX.

Finger printing

Due to the fact everybody (apart from identical twins) contains DNA unique to themselves, DNA finger printing is a full proof way of identifying people.

Techniques are used to create an image much like a barcode. The small differences between people’s DNA can then be clearly seen. This is particularly useful at crime scenes for matching suspects with samples found.

NOTE: You don’t need to be aware of genetic finger printing techniques.

Genetic diagrams

Gregor Mendel was interested in how pea plants inherited different characteristics. He bred red and white flowered plants with each and noticed interesting results.

• When he bred white and red flowered plants the offspring was red.
• However, when he bred this red flowered offspring with each other most of the offspring was red but some were white.

In Mendel’s era people were unaware of the idea of genetics. Today we’re aware of how genetics works and so can tell from this experiment that the red flower allele must be dominant while the white flower allele is recessive.

A genetic diagram is a visual way of interpreting this. It can show all the possible allele variations for a specific characteristic. Each parent has two alleles. Dominant alleles are written with a capital letter and recessive with a lower case letter. Each allele can then be paired up with each other by drawing lines. The outcome is four possible combinations.

For example, a diagram of Mendel’s first cross would look like this in which all the plants have red flowers.

His second cross would appear like this in which three quarters of the plants have red flowers and a quarter has white flowers.