Inheritance, Variation and Evolution

• Meiosis is the formation of four non-identical cells from one cell
• Mitosis is the formation of two identical cells from one cell
• Sexual reproduction involves the joining of male and female gametes, each containing genetic information from the mother or father
• Gametes are formed by meiosis, as they are non-identical
• A normal cell has 46 chromosomes, with two sets of chromosomes (23 pairs)
• Each gamete has 23 chromosomes and they fuse in fertilisation, mixing genetic information from each parent
• Asexual reproduction involves one parent with no gametes joining, forming two identical cells from one cell
• Meiosis is the formation of four non-identical cells from one cell, with gametes only having one copy of each chromosome
• Gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes
• DNA is composed of a chemical called DNA, a polymer made up of two strands in a double helix structure
• A gene is a small section of DNA on a chromosome that codes for a specific protein
• The genome is all the genes coding for all of the proteins within an organism
• DNA is made up of nucleotides, each consisting of a sugar molecule, a phosphate molecule, and one of the four organic bases (A, C, G, T)
• The order of bases in DNA codes for amino acids, which join to make proteins
• Protein synthesis involves DNA being transcribed into mRNA, which then codes for amino acids to form proteins
• Mutations can change the sequences of bases in DNA, affecting the structure and function of proteins
• Genetic inheritance involves terms like gamete, chromosome, gene, alleles, dominant allele, recessive allele, homozygous, and heterozygous
• Phenotype to be observed:

  • Homozygous: when both inherited alleles are the same (i.e. two dominant alleles or two recessive alleles)
  • Heterozygous: when one inherited allele is dominant and the other is recessive
• Genotype:

  • The combination of alleles an individual has, e.g. Aa
• Family trees show the inheritance of different phenotypes over generations in the same family
• Inherited Disorders:

  • Polydactyly: having extra fingers or toes, caused by a dominant allele
  • Cystic fibrosis: a disorder of the cell membranes, resulting in thick mucus in the airways and pancreas, caused by a recessive allele
  • Embryonic screening allows scientists to observe whether the child will have a genetic condition or not
• Sex Determination:

  • Human body cells have 23 pairs of chromosomes
  • The 23rd pair carries sex determining genes, with X chromosomes and Y chromosomes
  • Females have two X chromosomes, males have one X and one Y chromosome
  • Punnett squares can be used to show sex inheritance, with a 50% chance of the child being male and a 50% chance of being female
• Variation and Evolution:

  • The phenotype of an organism depends on genotype (inherited genes) and environment
  • Genetic variation is introduced by mutations in the sequence of DNA
  • Evolution is a change in inherited characteristics of a population over time through natural selection, leading to speciation
• Selective Breeding:

  • Humans choose which organisms to breed to produce offspring with desired characteristics
  • Selective breeding can lead to inbreeding and reduction of the gene pool
• Genetic Engineering:

  • Modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic
  • Genetic engineering can be used in agriculture, medicine, and to cure inherited disorders
• Cloning:

  • Cloning is creating genetically identical copies of an organism
  • Methods of plant cloning include tissue culture and cuttings
  • Cloning in animals can be done through embryo transplants and adult cell cloning
• Cloning:

  • Through the stimulation of an electric shock, the egg cell begins to divide to form an embryo
  • The embryo is implanted into the womb of a female
  • The offspring born is a clone of the adult body cell
• Benefits of cloning:

  • Produces lots of offspring with a specific desirable feature
  • Can help extremely endangered species or bring back extinct species
  • The study of clones could help research into embryo development
• Risks of cloning:

  • The gene pool is reduced through producing clones, leading to low diversity in the population
  • Clones have a low survival rate and tend to have genetic problems
  • It may lead to human cloning
• Theory of Evolution:

  • Variation exists within species as a result of mutations in DNA
  • Organisms with characteristics most suited to the environment are more likely to survive and breed successfully (survival of the fittest)
  • Beneficial characteristics are passed on to the next generation
  • Over many generations, the frequency of advantageous alleles increases within the population
• Charles Darwin:

  • Scientist and naturalist
  • Put forward the theory of evolution
  • Supported by experimentation, geology, and fossils discovered on a world expedition
  • Published 'On the Origin of Species' in 1859
• Jean-Baptiste Lamarck's theory:

  • Changes occurring during an organism's lifetime are not passed onto offspring
  • Characteristics developed through repeated actions are not inherited
  • Lamarck's theory was proven wrong as changes caused by the environment are not passed on in sex cells
• Speciation:

  • The process of a new species developing through the selection of different alleles
  • Increases genetic variation until new population cannot breed with the old population to produce fertile offspring
  • Alfred Russel Wallace developed the theory of speciation and evolution by natural selection
• Gregor Mendel:

  • Trained in mathematics and natural history
  • Observed characteristics passed on to next generations in plants
  • Conducted breeding experiments on pea plants
  • Concluded that offspring inherit hereditary units from each parent
• Evidence for Evolution:

  • Fossils: remains of organisms found in rocks
  • Antibiotic resistance in bacteria: advantageous mutations selected for in bacterial populations
• Fossils:

  • Remains of organisms from millions of years ago found in rocks
  • Formed from parts that have not decayed, replaced by minerals, or preserved traces
  • Used to show how anatomy of organisms has changed over time
• Extinction:

  • When an entire species has died out
  • Factors contributing to extinction include changes in the environment, new predators, diseases, competition, catastrophic events, and habitat destruction
• Resistant Bacteria:

  • Bacteria labelled resistant when not killed by antibiotics
  • Mutations during reproduction result in new genes like antibiotic resistance
  • Exposure to antibiotics creates selection pressure for resistant genes to survive and reproduce
• How to slow the development of resistance in bacteria:

  • Antibiotics should not be given for viral or non-serious infections
  • Specific antibiotics should be given for specific bacteria
  • Patients should complete their course of antibiotics
  • Antibiotics should be used less in agriculture
• Classification of Living Organisms:

  • Involves putting organisms into groups based on structure and characteristics
  • The Linnaean system: divided living things into kingdoms, phylum, class, order, family, genus, species
  • The binomial system gives each organism a name using genus and species