Inheritance, Variation & Evolution

• DNA


  Deoxyribonucleic acid, the chemical that all of our genetic material is made of
• DNA

  • It's a polymer made up of lots of similar units stuck together
  • It has two strands which are stuck together to form a double helix
• Chromosome

  A really tight coil of DNA, each cell has 46 chromosomes
• Sex chromosomes

  The 23rd pair, consisting of an X chromosome and a Y chromosome
• Chromosomes only look like an X shape just before cell division
• Gene

  A small section of DNA that codes for a particular type of protein
• There are 20 different types of amino acids, which can be combined in many different sequences to form thousands of different proteins
• Genome

  The entire set of genetic material in an organism
• Scientists have worked out the complete human genome, which allows them to identify genes linked to diseases and develop better treatments
• Genomes can be used to trace the migrations of human ancestors
• Allele

  Different versions of the same gene
• Homozygous


  Having two of the same alleles for a particular gene
• Heterozygous

  Having two different alleles for a particular gene
• Dominant allele

  The allele that gets expressed when present, regardless of whether the organism is homozygous or heterozygous for that allele
• Recessive allele


  The allele that only gets expressed when the organism is homozygous for that allele
• Genotype


  The entire collection of alleles that an organism has
• Phenotype

  The characteristics that an organism displays, which are determined by its genotype
• Characteristics are often determined by the interaction of multiple genes, not just a single gene
• Genes code for different forms of the same protein, which are called alleles
• If an organism is heterozygous for a trait, the dominant allele will be expressed and the recessive allele will not be expressed
• The only way to have a recessive trait expressed is if the organism is homozygous for the recessive allele
• Organisms with the same genotype may have different phenotypes, and organisms with different genotypes may have the same phenotype
• Main ways organisms can reproduce

  • Sexual reproduction
  • Asexual reproduction
• Some organisms like plants can reproduce through either sexual or asexual reproduction
• Most animals have to rely on sexual reproduction
• Bacteria all reproduce asexually
• Sexual reproduction


  Involves the fusion of male and female gametes (fertilization)
• Offspring from sexual reproduction contain a mixture of their parents' genes and are genetically different
• Gametes


  Cells like sperm and eggs in animals, or pollen and eggs in flowering plants
• Gametes only have half the genetic material of a normal cell
• Meiosis


  The process by which gametes are produced with half the genetic material
• Sexual reproduction doesn't have to involve sex or intercourse, it just refers to the fusion of male and female gametes
• Asexual reproduction


  Takes place with only one parent and doesn't involve any gametes
• Asexual reproduction results in offspring that are identical clones with no genetic variation
• Asexual reproduction in eukaryotic organisms

  Achieved through mitosis
• Asexual reproduction in prokaryotic organisms

  Achieved through binary fission