You will remember from your Key Stage 3 studies that the nucleus controls the activities of a cell. The instructions for how an organism develops are found in the nuclei of its cells

Chromosomes

Chromosomes are structures found in the nucleus of most cells. They consist of long strands of a substance called deoxyribonucleic acid, or DNA for short. A section of DNA that has the genetic code for making a particular protein is called a gene. The proteins can either be:

• structural proteins such as the ones found in muscles and hair
• enzymes, such as proteases and other digestive enzymes

Human body cells each contain 23 pairs of chromosomes.

Parents pass on their genes to their offspring in their sex cells.

• female sex cells are called egg cells, or ova
• male sex cells are called sperm

A pair of chromosomes carry the same genes, in the same place, on the chromosome. However, there are different versions of a gene called alleles. These alleles may be the same on each pair of chromosomes, or different, for example, blue eyes or brown eyes.

Sex cells only contain one chromosome from each pair. When an egg cell and sperm cell join together, the fertilised egg cell contains 23 pairs of chromosomes. One chromosome in each pair comes from the mother, the other from the father.

Which chromosome we get from each pair is completely random. This means different children in the same family will each get a different combination. This is why children in the same family look a little like each other and a little like each parent, but are not identical to them.

Sex chromosomes

A set of chromosomes can be separated from its cell, spread out on a microscope slide and magnified many thousands of times. When stained and photographed, they look like this:

The highlighted pair of chromosomes are called the sex chromosomes. The longer sex chromosome is called the X chromosome, the shorter one the Y chromosome.

• females are **
• males are XY

Individuals differ in all sorts of ways, even when they are offspring of the same parents. These differences are called variation.

Most characteristics, such as height, are determined by several genes working together. They are also influenced by environmental factors. These include: climate,diet ,physical accidents ,culture, lifestyle.

Identical twins

Identical twins are genetically the same. They are a good example of the interaction between inheritance and the environment. For example, an identical twin who takes regular exercise will have better muscle tone than one who does not. All of the differences that you see between identical twins, for example, in personality, tastes and aptitude, are due to differences in their experiences or environment.

The chromosomes in a pair carry the same genes in the same places. But there are different versions of the same gene.

Different versions of the same gene are called alleles, which is pronounced al-eels. For example, the gene for eye colour has an allele for blue eye colour and an allele for brown. For any gene, a person may have the same two alleles, or two different ones.

Recessive or dominant alleles

Alleles may be either recessive or dominant.

• A recessive allele only shows if the individual has two copies of it. For example, the allele for blue eyes is recessive. You need two copies of this allele to have blue eyes.
• A dominant allele always shows, even if the individual only has one copy of it. For example, the allele for brown eyes is dominant. You only need one copy of it to have brown eyes. Two copies will still give you brown eyes.

Individuals A and B have brown eyes - only individual C has blue eyes

Only individual C will have blue eyes, because the allele for blue eyes is recessive.

Individual A is called a carrier because, even though they have brown eyes, they still carry the allele for blues eyes and can pass this allele on to future generations.

Cystic fibrosis

Cystic fibrosis (CF) is caused by a recessive allele. In the genetic diagram below, it is written as f.

People with CF produce abnormally thick and sticky mucus in their lungs and airways. As a result, they are more likely to get respiratory infections. Daily physiotherapy helps to relieve congestion, while antibiotics can fight infection. CF also affects the gut and pancreas, so food is not digested efficiently.

Inheriting copies of the allele

You need to inherit two copies of the faulty allele to be born with CF. If you have just one copy, you are a carrier, but will not experience any symptoms. If two carriers have a child together, there is a one in four chance of it inheriting the disorder.

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