B1 Revision - You and Your Genes (OCR 21st Century)

B1 Revision - You and Your Genes (OCR 21st Century)

1. Most cells in your body have a nucleus

   • This contains your genetic material
     • Your genetic material is arranged into chromosomes.
       • There are 23 pairs of chromosomes in the human cell nucleus.
         • Each chromosome is one very long molecule of DNA.
         • A gene is a short length of a chromosome.
           • Genes control the development of different characteristics - for example, hair colour
           • Genes can exist in different versions.
             • Each version gives a different form of a characteristic, like blue or brown eyes.
               • The different versions of the same gene are called alleles.
           • Genes are instructions for making different proteins.
             • Proteins are the building blocks of cells. Having different versions of proteins means we end up with different characteristics.
               • Structural proteins: part of things like skin, blood, hair, and the cytoplasm in cells.
                 • Collagen is a structural protein that is found in tendons, bones and cartilage.
               • Functional proteins: Enzymes are proteins that help with digestion by breaking down food molecules - amylase is a digestive enzyme that breaks down starch to maltose.

2. An organism's genotype describes the genes it's got.

   • An organism's genotype is all the genes it has.
   • The characteristics an organism displays are it's phenotype.
   • Some characteristics are controlled only by genes.
     • This can only be one gene, or quite often, the characteristic is controlled by several genes working together: eg, eye colour.
   • However some characteristics are controlled only by environmental factors such as scars. They have nothing to do with genes.
   • Some characteristics are controlled by genes and the environment, for example, weight

3. Genes and variation

   • In body cells (all your cells apart from sex cells), chromosomes come in pairs.
     • One chromosome in every pair has come from each parent.
       • The sex cells (sperm and egg) are different from ordinary body cells because they contain just 23 single chromosomes - one from each pair.
         • When the sperm fertilises the egg, the 23 chromosomes from the egg combine with the 23 from the sperm.
           • The fertilised egg now has 23 pairs of chromosomes, just like an ordinary body cell.
             • The two chromosomes in a pair always carry the same genes.
               • Each gene is always found in the same place on the two chromosomes.
                 • Because the two chromosomes in a pair came from different parents, they might have different alleles of these genes. Alleles are different versions of the same gene.
   • Children resemble both parents, but are identical to neither.
     • Half a child's chromosomes come from each parent. That means children get some alleles from each of their parents, which is why some children look like a bit of both of their parents.
       • Every child has a unique combination of alleles, which is why no two people in the world are the same - apart from identical twins.