B6 - Inheritance, Variation, and Evolution

• DNA is a polymer found in chromosomes, in nuclei, which contains genetic information that determines the inherited characteristics you have
• A gene is a small section of DNA that codes for a specific protein
• Only 20 different amino acids are used, but in different combinations and orders they form thousands of different proteins
• Genome - the entire set of genetic material in an organism
• Understanding the human genome:

  • Allows us to identify genes that are linked to different types of disease, making it easier to find treatments
  • Allows us to trace the origins and migration of different populations, based on small adaptations
• Sexual reproduction - the fusion of male and female gametes to create a unique offspring with a mixture of their parents' genes
• Asexual reproduction - one parent replicates itself to produce genetically identical offspring
• Gametes are produced by meiosis
• Meiosis:

  • Cell starts with 23 single-armed chromosomes containing half the genetic info needed
  • It duplicates the genetic info creating two-armed chromosomes where the two arms are identical. The chromosomes sort themselves into pairs
  • Pairs line up and are pulled apart as cell divides so the two new cells each have one of the chromosomes per pair
  • These new cells aren't identical
  • They divide again, the same way as mitosis, to produce 4 gametes that are different
• After the two gametes have fused, the new zygote divides by mitosis to reproduce itself, then these cells start to differentiate and specialise
• 23 pairs of chromosomes in each human body cell
• Males - XY
• Females - XX
• Genetic diagrams are used to show the probability of inheriting a certain characteristic
• Alleles:

  • Versions of genes represented by letters in punnet squares
  • We have two alleles of each gene, found on each chromosome in a pair
• Two alleles the same - homozygous
• Two alleles different - heterozygous
• If someone is heterozygous, the dominant allele (capitalised) will be the one that is shown, rather than the recessive (lowercase)
• For an organism to display a recessive characteristic, both alleles must be recessive (e.g bb)
• For an organism to display a dominant characteristic, there must be at least one dominant allele (e.g Bb, BB)
• Genotype - the combination of alleles
• Phenotype - the characteristics that are shown
• Cystic fibrosis:

  • Genetic disorder
  • Affects cell membranes - producing mucus
  • Caused by recessive alleles
  • People with only one copy are carriers and unaffected
  • Both parents must be carriers or affected to give it to their child
• Polydactyly:

  • Genetic disorder giving people extra fingers or toes
  • Caused by a dominant allele
  • Only one parent must have it for the child to be able to have it
• Embryonic screening:

  • Before embryos are implanted in IVF, it is possible to screen their genes for genetic disorders
  • DNA can also be gathered from embryos in the womb
  • The faulty embryos for IVF are destroyed, and parents are given the choice of termination
• Embryonic screening pros:

  • Parents get the choice to terminate affected embryos
  • Treating disorders costs the government and taxpayers a lot of money
• Embryo screening cons:

  • Against religious views about interfering with God's will
  • Risk to mothers and babies
• Organisms of the same species have slight variations
• Genetic variation - variation caused by genotype e.g eye colour, height
• Environmental variation - variation caused by interaction with the environment e.g hair dye, piercings
• Most variation is caused by a mixture of genetic and environmental factors.
• Mutations:

  • A rare, random change in an organism's DNA
  • Often these mutations have no effect on survival and only affect that individual
  • Sometimes these mutations are useful for survival and are passed down due to natural selection
• Theory of Evolution - all of today's species have evolved from simple life forms that started to develop over 3 billion years ago
• Natural selection:

  • Organisms must compete for limited resources
  • Organisms with helpful mutations will be more likely to access the resources
  • They will survive long enough to breed where others won't
  • They can pass down their genes and the helpful mutation becomes the new normal
• Discoveries such as genetics, fossils, and antibiotic-resistant bacteria have developed and supported Darwin's theory
• Speciation - when an organism evolves and mutates far enough from its ancestor to become a distinct new species
• Extinction reasons:

  • Environment changes too quickly (e.g habitat destruction)
  • A new predator kills them all (e.g humans hunting)
  • New disease kills them all
  • Can't compete with another species for resources
  • Catastrophic event kills them all (e.g volcano)
• Selective breeding - humans breeding together animals/plants with desired characteristics to get offspring with the features
• Selective breeding:

  1. Select organisms with desired characteristics
  2. Breed them together
  3. Select best offspring and breed them together
  4. Repeat over several generations to ensure all offspring have the characteristic
• Selective breeding pros:

  • Greater meat/milk yield
  • Disease resistance
  • Temperament
  • Appearance