Genes (DP IB Biology: SL)

• A gene is a section or length of DNA that codes for a polypeptide
• Genes are heritable factors that influence specific characteristics (via the polypeptides produced)
  • Characteristic means a feature of an organism like height in pea plants or blood group in humans
  • Heritable means genes are factors that pass from parent to offspring during reproduction

• A gene occupies a specific position on a chromosome
• The gene for a particular characteristic is always found at the same position or locus (plural is loci) on a particular chromosome

A gene consists of a length of DNA found in the nucleus. This length of DNA causes a specific characteristic by coding for specific polypeptides.

• A single chromosome contains several hundred or thousands of genes
  • Dependent on the length of the chromosome

• Through experiments and genetic mapping techniques, scientists were able to work out the specific physical location of a gene on a chromosome
• The location of a gene on a chromosome is known as its locus (the plural of locus is loci)
• Each gene occupies a specific locus so that the gene for a particular characteristic is always found at the same position on a particular chromosome

• Species vary in the number of genes they have
• The number of genes a species has is not related to the size/complexity or even the sophistication of the organism
  • Because genes can vary in length

• Counting the exact number of genes in a species is difficult, so you may see conflicting numbers in different sources
• Humans have around 20,000 genes
• Dogs have 19,000 genes, which is less than humans
• A water flea has more than a human with 31,000 genes
• E. coli, a bacterium, has only 4,300 genes
• A rice plant has 41,500 genes

Comparing the Number of Genes between Different Organisms Table