Inheritance, Variation and Evolution.



  • DNA
    • Deoxyribonucleic Acid - the chemical that makes up all of the genetic material in a cell
    • Contains coded information - instructions to put an organism together and make it work
    • Determines inherited characteristics
    • Found in the nucleus of animal and plant cells in long structures called chromosomes
    • Chromosomes usually come in pairs
    • DNA is a polymer - made up of two strands coiled together in a double helix
  • Genes
    • Small section of DNA found on a chromosome
    • Code for a particular sequence of amino acids which form a specific protein
    • Only 20 amino acids are used, but they can form 1000s of different proteins
    • Tell cells in which order they need to put the amino acids together
    • DNA determines what proteins a cell produces (e.g. haemoglobin, keratin etc.)
    • The proteins the cell produces determine what type of cell it is e.g. red blood cell etc.)
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The Genome

The entire set of genetic material that makes up an organism

Research into the genome benefits science and medicine:

  • Allows scientists to identify genes linked to types of diseases
  • Leads to greater understanding and treatments for gentic disorders
  • Can trace human migrations patterns from the past
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The Structure of DNA

  • Polymer made up of repeating units called nucleotides
  • DNA is made from four different nucleotides
  • Each nucleotide consists of one phosphate molecule, one sugar molecule and one 'base'
  • The sugar and phosphate molecules alternate
  • One of four different bases - A,C,G,T - is attached to each sugar molecule
  • Each base links to a base on the opposite strand of the double helix
  • Complementary base pairing - A always pairs up with T, C always pairs up with G
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Protein Synthesis

  • The order of bases in a gene decides the order of amino acids in a protein
  • Each amino acid is coded for by a sequence of three bases in the gene
  • The protein that is synthesised depends on the order of the gene's bases
  • Some parts of DNA do not code for proteins - these are called non-coding DNA
  • Non-coding DNA switches genes on and - so they can control whether a gene is expressed (used to make a protein)
  • Proteins are synthesised in the cell cytoplasm on ribosomes
  • To synthesise specific proteins, ribosomes use the genetic code in the DNA
  • However, DNA is in the cell nucleus and cannot move out of it, so mRNA is needed
  • Transcription - mRNA (or messenger RNA) holds a copy of the gene's base sequence and takes it to the cell's cytoplasm
  • Translation - the mRNA then attaches to a ribosome, and amino acids are brought to the ribosome in the form of tRNA (or transfer RNA, carrier molecules)
  • The ribosome orders the amino acids in the way that the bases of the gene specify
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Examples of Protein Functions:

  • Enzymes - act as biological catylsts to speed up chemical reactions in the body
  • Hormones - carry messages around the body, used in the endocrine system
  • Structural Proteins - physically strong
    • e.g. collagen - strengthens connective tissues (tendons, ligaments and cartilage)
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  • A mutation is a random change in an organism’s DNA
  • Mutations can be inherited
  • Mutations occur continuously and spontaneously
  • The chance of mutation is increased by exposure to certain substances or types of radiation
  • Mutations change the sequence of the DNA bases in a gene producing a genetic variant
  • As the sequence of DNA bases codes for the sequence of amino acids, mutation sometimes leads to a change in the protein that is coded for 
  • Most mutations have very little or no effect on the protein that is coded for as some combinations of bases code for the same amino acids
  • Some mutations can seriously affect the protein and affect its ability to perform its functions
  • A mutation in non-coding DNA can alter how genes are expressed
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Sexual Reproduction

  • Involves the fusion of male and female gametes to produce offspring with a unique genome
  • Genetic information from two organisms (a mother and father) is combined to produce offspring which are genetically different to either parent
  • The mother and father produce gametes through meiosis
  • In humans, each gamete has 23 chromosomes (half the number of normal body cells)
  • The egg and sperm cell fuse together in fertilisation to form a cell with a fully number of chromosomes - half from the mother, half from the father
  • The offspring inherits features from both parents, as it has a mixture of chromosomes
  • The mixture of genetic information produces variation in the offspring
  • Flowering plants also reproduce sexually - egg cells are fertilised by pollen, rather than sperm
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Asexual Reproduction

  • Involves only one parent which produces genetically identical offspring
  • Happens by mitosis - an ordinary cell dividing in two, replicating its genetic information
  • The new daughter cells produced have exactly the same genetic information as the parent cell - so they are clones
  • There is no mixing of genetic information
  • As the offspring are identical, there is no genetic variation involved in asexual reproduction
  • Bacteria, some plants and some animals reproduce asexually
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Meiosis and Fertilisation

  • Gametes only have one copy of each chromosome, so that after gamete fusion the new cell has the right number of chromosomes (two copies of each) - fertilisation restores the full number of chromosomes
  • In gametes there are only single chromosomes, but in other cells they are in pairs
  • In humans, meiosis takes place in the cells of the reproductive organs (the ovaries and the testes):
    • Stage 1 - all of the chromosomes in the cell are replicated
    • Stage 2 - the cell then divides into two
    • Stage 3 - both daughter cells then divide into two again
  • By the end of meiosis, the orginal cell has divided into four gametes with single chromosomes
  • Each of the four gametes are genetically different from each other
  • Each gamete has different alleles
  • After gamete fusion (fertilisation), the resulting new cell has 23 pairs of chromosomes and can divide by mitosis - increasing the number of cells
  • As the embryo develops, its cells differentiate
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Sexual Reproduction vs. Asexual Reproduction

  • Advantages of sexual reproduction:
    • produces variation in offspring
    • variation provides a survival advantage by natural selection if the environment changes
    • natural selection can be sped up by humans through selective breeding to increase food production
  • Advantages of asexual reproduction:
    • only one parent needed
    • no need to find mate - more time and energy efficient
    • faster than sexual reproduction
    • many identical offspring can be produced in favourable conditions
  • Some organisms can reproduce through both sexual and asexual reproduction:
    • the parasite that causes malaria reproduces sexually in the mosquito, but asexually when in the human host
    • many funghi reproduce asexually through spores, but also reproduce sexually for variation
    • many plants produce seeds through sexual reproduction, but can also reproduce asexually through 'runners' (e.g. strawberry plant) and bulbs (e.g. daffodils)
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Genetic Inheritance Key Terms

  • Gamete - a sex cell used in sexual reproduction, only contains 23 chromosomes
  • Chromosome - DNA stored in the nuclei of animal and plant cells in very long structures
  • Gene - a small section of DNA found on a chromosome
  • Allele - versions of a gene
  • Dominant - a dominant allele will show in the phenotype, even if there is only one dominant allele present
  • Recessive - needs two homozygous alleles to be shown in the phenotype
  • Homozygous - when a person has two copies of the same allele
  • Heterozygous - when a person has two different alleles of the same gene
  • Genotype - the combination of alleles that a person has
  • Phenotype - the characteristics caused by a person's alleles
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  • Some characteristics (phenotypes) are controlled by a single gene - these include mouse fur colour and red-green colour blindness in humans
  • However most characteristics are controlled by several genes interacting - these include height
  • The alleles present (genotype) operate at a molecular level to develop characteristics that can be expressed as a phenotype
  • Dominant alleles are always expressed, even if only one dominant allele is present
  • Recessive alleles must be homozygous to be expressed, so there must be two of them
  • If the two alleles present are the same, the organism is homozygous for that trait
  • If the two alleles present are different, the organism is heterozygous for that trait
  • Punnett squares are used to show the probability of an individual developing a phenotype
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Cystic Fibrosis and Polydactyly

  • Cystic Fibrosis:
    • Genetic disorder of the cell membranes
    • Controlled by a single gene which has two alleles
    • The allele that causes the defective cell membranes is recessive
      • This means that an individual must inherit a defective allele from both of their parents to develop CF
    • An individual with one normal allele and one defective allele will not have CF, but they are a carrier
  • Polydactyly:
    • Causes the growth of extra fingers and toes
    • Caused be a dominant allele
      • Only one defective allele is required for the devlopment of the polydactyly phenotype
    • As only one dominant, defective allele is required, there are no carriers of polydactyly and it can be passed on even if only one parent has a defective allele
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Embryo Screening

  • During IVF, cells can be removed from the embyros (before they are implanted into the womb) and their genes can be analysed
  • It is also possible to get DNA from an embryo in a womb and test it for disorders
  • Many genetic disorders can be detected in this way (e.g. cystic fibrosis, polydactyly etc.)
  • However, there are a lot of ethical, social and economic concerns regarding embryo screening
  • Embryo screening can also lead to controversial decisions:
    • IVF - screening can lead to detroying embryos with 'bad' alleles
    • From the womb - screening could lead to the decision to terminate the pregnancy
  • Pros of embryo screening:
    • can help people stop suffering (e.g. the suffering that genetic disorders can cause)
    • treating disorders costs the government (and taxpayers) a lot of money
    • there are laws to stop screening going too far (e.g. selecting the sex of their baby)
  • Cons of embryo screening:
    • implication that people with genetic disorders are 'undesirable' - creating prejudice
    • may lead to people wanting to pick the qualities of their baby that they find 'desirable'
    • screening is expensive
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Sex Determination

  • Ordinary body cells contain 23 pairs of chromosomes
  • 22 of the chromosome pairs have genes that control characteristics only
  • 1 of the chromosome pairs has the genes that determine the sex of a person
  • In males, the two sex chromosomes are different - they are XY
  • In females, the two sex chromosomes are the same - they are **
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  • All of the differences in the characteristics of individuals in a population
  • The genome and its interaction with the environment influences the phenotype of an organism
  • There is usually extensive genetic variation within a population of a species
  • One type of variation is genetic:
    • This is caused by the variation in the alleles that indivuals have inherited
  • Another type of variation is environmental:
    • This is caused by adapting to the conditions in which an organism lives and grows
  • The main type of variation is due to a combination of genes and the environment:
    • This is caused by a combination of inherited alleles and the environment
  • Genetic variation is due to mutations
  • Mutations are random changes to the sequence of bases in DNA
  • Mutations take place continuously
  • Most mutations have no effect on the phenotype of an organism, some mutations influence the phenotype but very few mutations actually determine the phenotype
  • Very rarely, mutations can lead to a new phenotype
  • If the environment changes and the new phenotype makes the individual more suited to the new environment, the alleles responsible for the phenotype will be passed on throughout the species, as other less-suited organisms will die - this is natural selection
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  • A change in the inherited characteristics of a population over time, through a process of natural selection, which may result in the formation of a new species
  • The theory of evolution: all modern species have evolved from simple life forms that first started to develop over 3 billion years ago
  • Evolution occurs through the natural selection of phenotypes that increase an organism's ability to survive in its environment
  • If two populations of one species become so different in phenotype that they can no longer reproduce to form fertile offspring, a new species has been formed - this is called speciation
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Selective Breeding

  • Selective breeding is 'artificial selection' - it involves humans selecting plants or animals that they want to breed so that desirable genes for particular characteristics remain in the population
  • Desirable characteristics can be chosen based on usefulness or appearance:
    • disease resistance in crops
    • animals that produce larger quantities of meat and milk
    • domestic dogs with a more gentle nature
    • plants with large or unusual flower
  • The process of selective breeding:
    • parents with the desirable characteristic are chosen from a mixed population
    • the parents are then made to breed
    • from the offspring, those with the desirable characteristic are then bred again
    • this continues over generations, until all offspring share the desired characteristic
  • Drawbacks:
    • reduced gene pool - can lead to inbreeding (where closely related animals/plants are bred)
    • inbreeding can lead to breeds being particularly prone to diseases or inherited defects
    • there is also less variation, so there is a greater chance for a disease to kill the entire population
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Genetic Engineering

  • A process that involves modify the genome of an organism by introducing a gene from another organism to give a desirable characteristic
    • e.g. plant crops are often genetically engineered to be resistant to diseases or to grow bigger/better fruits
    • bacterial cells can also be genetically engineered to produce useful substances (e.g. insulin)
  • Process:
    • an enzyme is used to isolate the desired gene
    • this gene is then attached to a vector (usually a bacterial plasmid)
    • the vector is used to insert the gene into the required cells
    • genes are transferred to the cell of an organism at an early stage in its development, so that it has the ability to develop the desired characteristics
  • Pros:
    • increased yields = increased profit for farmers
    • resistance to insect infestations and herbicides = less waste (money and produce)
    • can provide vital nutrients for those with deficiencies
  • Cons:
    • long-term effects are unknown = may not be safe
    • may affect the number of wild flowers = reduced biodiversity
    • transplanted genes may spread = weeds may also develop herbicide resistance
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Cloning Plants

  • Tissue culture:
    • uses a small group of cells from one part of a plant to grow identical new plants
    • this is important for preserving rare plant species
    • can also be used by plant nurseries to produce lots of stock quickly
    • the cells of the plant are grown in a growth medium with hormones
    • they can be grown quickly, in little space at any time of year
  • Cuttings:
    • gardeners take cuttings from parent plants and plant them to produce genetically identical copies (clones)
    • these clones can be produced quickly and cheaply
    • this is an older, simpler method than tissue culture
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Cloning Animals

  • Embryo transplants:
    • a mother and father with desirable characteristics are made to breed
    • cells from the developing animal embryo are split apart before they have become specialised
    • the identical embryos are then transplanted into host mothers
    • hundreds of identical offspring can be produced every year
  • Adult cell cloning:
    • the nucleus is removed from an unfertilised egg cell
    • the nucleus from an adult body cell (e.g. skin cell) is inserted into the egg cell
    • an electric shock stimulates the egg cell to divide to form an embryo
    • the embryo cells have the same genetic information as the inserted body cell
    • when the embryo is a ball of cells, it is inserted into the womb of an adult female to continue its development
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Pros and Cons of Cloning


  • study of animal clones can lead to greater understanding of the development of the embryo
  • can lead to greater understanding of aging and age-related disorders
  • can help to preserve endangered species


  • reduced gene pool
  • cloned animals may not be as healthy as normal animals
  • may lead to humans being cloned (dangers of human cloning are massive)
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The Theory of Evolution

  • Charles Darwin developed the 'theory of evolution' after observations made on a global expedition - these observations were backed up with knowledge from experiments, discussion, geology and fossils:
    • individual organisms within a population showed a wide range of variation
    • individuals with characteristics most suited to the environment are most likely to survive to breed successfully
    • the characteristics enabling these organisms to survive are passed on to the next generation
  • Darwin published 'The Origin of the Species' in 1859
  • The theory was only gradually accepted as:
    • it challenged the idea that God created all animals and plants that live on Earth
    • there was insufficient evidence to convince scientists at the time
    • the mechanism of inheritance and variation was onlhy discovered 50 years later
  • Lamarck's theory was based around the idea that changes to an organism during its lifetime can then be inherited by offspring
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  • Alfred Russell Wallace independently proposed the idea of evolution by natural selection
  • In 1858, Wallace and Darwin published joint work on the theory of evolution
  • Wallace worked worldwide gathering evidence for evolutionary theory
  • Wallace's most famous work was on colouration and speciation
  • Wallace did a lot of pioneering work on the theory of speciation, but new evidence has led to our current understanding of the concept
  • Speciation is the process where populations develop phenotypes which are so different that they become reproductively isolated - they can no longer interbreed with their original species to produce fertile offspring
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  • Alfred Russell Wallace independently proposed the idea of evolution by natural selection
  • In 1858, Wallace and Darwin published joint work on the theory of evolution
  • Wallace worked worldwide gathering evidence for evolutionary theory
  • Wallace's most famous work was on colouration and speciation
  • Wallace did a lot of pioneering work on the theory of speciation, but new evidence has led to our current understanding of the concept
  • Speciation is the process where populations develop phenotypes which are so different that they become reproductively isolated - they can no longer interbreed with their original species to produce fertile offspring
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  • In the mid-19th century, Gregor Mendel carried out breeding experiments on plants
  • Mendel interbred pea plants - in particular tall and dwarf pea plants
  • He did this continuously - observing the height of the offspring
  • Mendel showed that the height of the pea plants was determined by 'hereditary units' passed on from the parents - he showed that the unit for tall plants was dominant over the unit for dwarf plants
  • He made 3 important conclusions:
    • characteristics in plants are determined by 'hereditary units'
    • hereditary units are passed on to offspring from both parents (one unit from each parent)
    • hereditary units can be dominant or recessive - dominant will always be expressed if present
  • Mendel's work was only recognised after his death:
    • late 1800s - scientists became familiar with chromosomes and cell division
    • early 1900s - link made between chromosomes and 'units', conclusion that 'units' were found on chromosomes - these 'units' are now known as genes
    • 1953 - the structure of DNA was determined - this showed how genes work
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  • The remains of organisms from millions of years ago found in rocks
  • May be formed:
    • from parts of organisms that have not decayed as moisture, oxygen or warmth are absent
    • when parts of the organism are replaced by minerals as they decay
    • as preserved traces of organisms (footprints, burrows or rootlet traces)
  • The earliest lifeforms were mainly soft-bodied, meaning that they did not leave that many fossils - most traces of these were destroyed by geological activity
    • this makes it hard for scientists to be certain about how life on Earth began
  • We can learn how much or how little organisms have developed by studying fossils


  • occurs when there are no remaining individuals of a species left alive, may be due to:
    • environmental changes
    • new predator/new disease
    • inability to compete with other species for food
    • catastrophic event (e.g. volcanic eruption, asteroid)
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  • Traditionally, organisms have been classified into groups based on their structure and characteristics in a system developed by Carl Linnaeus
    • Linnaeus classified organisms into kingdom, phylum, class, order, family, genus and species
    • organisms are named by the binomial system - genus and species
  • Evidence of internal structures has developed due to improvements in microscopes and improved understanding of biochemical processes
    • new models of classification have been proposed
  • Carl Woese proposed a 'three-domain system' based on new evidence from chemical analysis
  • This divides organisms into:
    • archaea (primitive bacteria living in extreme conditions)
    • bacteria (true bacteria)
    • eukaryota (protists, funghi, animals and plants)
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