B1 - OCR 21st Century Science

• Most cells in your body have a nucleus which contains your genetic material.
• The human cell contains 23 pairs of chromosomes. They are all known and numbered.
• Chromosomes carry genes. Different genes control the development of characteristics. Example: hair colour.
• A gene is a short length of chromosome, Which is one molecule of DNA
• DNA is coiled up to form the arms of the chromosomes.
• Genes exist in different versions. Each version gives a different characteristic, like blue or brown eyes. The different versions of the same gene are call alleles.

1. Each gene is a code for making a certain protein which are the building blocks for cells. Having different versions of proteins mean that we end up with different characteristics.
2. Some proteins are for structural purposes which are part of making skin, hair, blood and the cytoplasm in our cells.
3. Other proteins are enzymes which control the chemical reactions in our body. Example: respiration. They also help by breaking down food molecules.

Chromosomes come in pairs because we get one from each parent.

• The sex cells (the sperm and the egg) are different from the other cells in your body as they only contain 23 single chromosomes instead of 23 pairs.
• When the sperm fertilises the egg, the 23 chromosomes in the egg cell and the 23 chromosomes in the sperm cell combine.
• The fertilised egg then has 43 chromosomes (or 23 pairs) like a normal body cell.

Children look like both parents but never Identical.

• Children get some alleles from each of their parents.
• Every person is unique , no one looks the same.
• There are millions of different allele combinations (about 8 million).

So no two people will look the same

Genetic Diagrams show the chances of a child having a certain Characteristics.

• Alleles are different versions of genes.
• To represent a dominant allele we use a CAPITAL LETTER.
• To represent a recessive allele we use a little letter.
• You can either be Homozygous which means you have two alleles the same. (eg bb, BB)
• Or you can be Heterozygous which means you have two alleles that are different. (eg Bb)
• A dominant allele will always overrule a recessive allele.

• Gender is controlled by a certain set of chromosomes.
• All men have an X and Y chromosome (the Y causes male characteristics to develop)
• All woman have two X chromosomes (the X causes female characteristics)
  This means that the Y chromosome is dominant.
• There is an equal chance of giving birth to a boy or a girl.