GCSE | BIOLOGY /6/ 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
• Cells in the reproductive organs divide by meiosis to form gametes
• DNA is composed of a chemical called DNA, a polymer made up of two strands in a double helix structure
• Gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes
• 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
• Chromosomes are structures made up of long molecules of DNA found in the nucleus of a cell
• DNA is made up of nucleotides, each consisting of a sugar molecule, a phosphate molecule, and one of the four types of organic bases (A, C, G, T)
• The order of bases in DNA codes for amino acids, which join together to make proteins
• Proteins can be enzymes, hormones, or structural 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
• Phenotype:

  • The physical characteristics observed in the individual, e.g. eye color
• Family trees show the inheritance of different phenotypes over generations in the same family
• Inherited Disorders:

  • Genetic disorders are caused by inheriting certain alleles
  • 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, 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:

  • Variation depends on genotype 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 the formation of a new species
• Selective Breeding:

  • Humans choose which organisms to breed for certain desirable 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 modification in medicine involves gene therapy to cure inherited disorders
  • Perceived benefits and risks of genetic engineering
• Cloning:

  • Creating genetically identical copies of an organism
  • Methods of plant cloning: tissue culture and cuttings
  • Cloning in animals: 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 even bring back species that have become extinct
  • The study of clones could help research into embryo development
• Risks of cloning:

  • The gene pool is reduced through producing clones, meaning it is less likely that the population will survive if a disease arises with low diversity in the population
  • Clones have a low survival rate, and tend to have some 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 to reproductive age and breed successfully – called survival of the fittest
  • The beneficial characteristics are then passed on to the next generation
  • Over many generations the frequency of alleles for this advantageous characteristic increase within the population
• Charles Darwin:

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

  • Changes that occurred during the lifetime of an organism were passed onto offspring
  • If an individual continually repeated an action, the characteristic that allowed it to do this would develop further
  • Lamarck’s theory was proven wrong when it was understood that changes caused by the environment were not passed on in the sex cells
• Speciation:

  • The process of a new species developing through the selection of different alleles
  • Increases genetic variation until the new population cannot breed with those in the old population to produce fertile offspring
  • Alfred Russel Wallace developed the theory of speciation and evolution by natural selection
  • Different alleles may be advantageous in the new environment, leading to them being selected for
• Gregor Mendel:

  • Trained in mathematics and natural history in Vienna
  • Observed the characteristics passed on to the next generations in plants
  • Carried out breeding experiments on pea plants
  • Offspring have some characteristics that their parents have because they inherit ‘hereditary units’ from each
• Fossils:

  • The remains of organisms from many years ago, found in rocks
  • Used to show how the anatomy of organisms has changed over time
  • Cannot be used to tell us how life started on Earth due to most early life forms being soft-bodied and few fossils of them
• Extinction:

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

  • Bacteria labelled resistant when they are not killed by antibiotics
  • Mutations during reproduction can result in new genes, such as the gene for antibiotic resistance
  • Exposure to antibiotics creates a selection pressure, leading to the increase of antibiotic resistant bacteria