Inheritance, variation and evolution

• Variation

  Differences in the characteristics of individuals in a population
• Causes of variation

  • Differences in the genes they have inherited
  • The environment in which they have developed
  • A combination of genes and the environment
• Mutation

  A change in a DNA sequence
• Mutations occur continuously
• Very rarely a mutation will lead to a new phenotype
• Some mutations may change an existing phenotype and most have no effect
• If a new phenotype is suited to an environmental change


  It can lead to a relatively rapid change in the species - this is the theory of evolution by natural selection
• Selective breeding (artificial selection)

  The process by which humans breed plants and animals for particular genetic characteristics
• Process of selective breeding

  1. Choose parents with the desired characteristic from a mixed population

  2. Breed them together

  3. Choose offspring with the desired characteristic and breed them together

  4. Continue over many generations until all offspring show the desired characteristic
• Characteristics targeted in selective breeding


  • Usefulness
  • Appearance
• Characteristics targeted in selective breeding

  • Disease resistance in food crops
  • Animals that produce more meat or milk
  • Domestic dogs with a gentle nature
  • Larger or unusual flowers
• Genetic engineering

  A process that involves changing the genome of an organism by introducing a gene from another organism to produce a desired characteristic
• Genetic engineering applications

  • Bacterial cells producing useful substances like human insulin
  • Plant crops engineered to be resistant to diseases, insects, or herbicides, or to produce bigger and better fruits and higher crop yields
• Benefits of genetic engineering

  • Potential to overcome some inherited human diseases
  • Crops can be engineered to be resistant to herbicides, make their own pesticides, or be better adapted to environmental conditions
  • Can lead to higher value of crops as GM crops have bigger yields than normal
• Risks of genetic engineering

  • Genes from GM plants and animals may spread to other wildlife, which could have devastating effects on ecosystems
  • Potential negative impacts on populations of wild flowers and insects
  • Ethical concerns, for example, in the future people could manipulate the genes of fetuses to ensure certain characteristics
  • Some people believe the long-term effects on health of eating GM crops have not been fully explored
• Sexual reproduction

  Cell division through meiosis, joining (fusion) of male and female sex cells (gametes), produces non-identical offspring that are genetically different to parents, results in wide variation within offspring and species
• Asexual reproduction

  Cell division through mitosis, no fusion of gametes, produces offspring that are genetically identical to parent (clones), no mixing of genetic information
• Meiosis

  A type of cell division that makes gametes in the reproductive organs, halves the number of chromosomes in gametes, and fertilisation (joining of two gametes) restores the full number of chromosomes
• DNA

  Genetic material in the nucleus of a cell, made up of two strands forming a double helix
• Gene

  A small section of DNA on a chromosome that codes for a specific sequence of amino acids, to produce a specific protein
• Genome

  The entire genetic material of an organism
• The whole human genome has been studied, and this has allowed scientists to search for genes linked to different diseases, understand and treat inherited disorders, and trace human migration patterns from the past
• Inherited disorders

  Disorders due to the inheritance of certain alleles, such as polydactyly (extra fingers or toes) caused by a dominant allele, and cystic fibrosis caused by a recessive allele
• Embryo screening and gene therapy may alleviate suffering from inherited disorders, but there are ethical issues surrounding their use
• Allele

  Different forms of the same gene
• Dominant allele

  An allele that only needs one copy present to be expressed (it is always expressed)
• Recessive allele

  An allele that needs two copies present to be expressed
• Homozygous

  When an individual carries two copies of the same allele for a trait
• Heterozygous

  When an individual carries two different alleles for a trait
• Genotype

  The combination of alleles an individual has
• Phenotype

  The physical expression of the genotype - the characteristic shown
• Genetic cross

  Considering the offspring that might result from two known parents, using Punnett squares to predict the outcome for both genotypes and phenotypes
• Normal human body cells contain 23 pairs of chromosomes, one of these pairs determines the sex of the offspring
• In human females the sex chromosomes are the same (XX, homozygous), and in males they are different (XY, heterozygous)
• The probability of offspring being male or female is always 50% in humans as there are two XX outcomes and two XY outcomes
• Evolution

  The gradual change in the inherited characteristics of a population over time, occurring through the process of natural selection and may result in the formation of new species
• Fossils

  The remains of organisms from millions of years ago, found in rocks, formed from parts of an organism that do not decay or are replaced by minerals, or the preservation of traces of organisms
• Benefits of the fossil record

  • Can tell scientists how individual species have changed over time
  • Fossils allow us to understand how life developed over the Earth's history
  • Fossils can be used to track the movement of a species or its ancestors across the world
• Problems with the fossil record

  • Many early organisms were soft-bodied, so most decayed before producing fossils
  • There are gaps in the fossil record as not all fossils have been found and others have been destroyed by geological or human activity - this means scientists cannot be certain about how life began on Earth
• Extinction

  When there are no remaining individuals of a species still alive