Inheritance, variation and evolution

• sexual reproduction

  Sexual reproduction involves the joining (fusion) of male and female gametes:

  • sperm and egg cells in animals

  • pollen and egg cells in flowering plants.

  In sexual reproduction there is mixing of genetic information which leads to variety in the offspring. The formation of gametes involves meiosis.
• asexual reproduction

  Asexual reproduction involves only one parent and no fusion of gametes. There is no mixing of genetic information. This leads to genetically identical offspring (clones). Only mitosis is involved.
• meiosis

  Cells in reproductive organs divide by meiosis to form gametes.

  When a cell divides to form gametes:

  • copies of genetic information are made
  • the cell divides twice to form four gamates,each with a single set of chromosomes
  • all gamates are genetically different from each other
• Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases. As the embryo develops cells differentiate.
• DNA

  The genetic material in the nucleus of a cell is composed of a chemical called DNA.

  DNA is a polymer made up of two strands forming a double helix.

  The DNA contained in structures called chromosome.
• gene

  A gene is a small section of DNA on a chromosome.Each gene codes for a particular sequence of amino acids,to make protein.
• genome

  The genome of an organism is the entire genetic material of that organism.The whole human genome has now been studied and this will have great importance for medicine in the future.
• importance of understanding the human genome

  This is limited to the:

  • search for genes linked to different types of diseases
  • understanding and treatment of inherited diseases
  • use in tracing human mig patterns from the past
• gamate


  Sex cells (sperm and egg cells) with half the usual number of chromosomes
• chromesome


  long, coiled molecule of DNA that carries genetic information in the form of genes.
• gene

  A section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein.
• allele


  a version of a gene
• dominant

  Describes an allele that is always expressed. Represented by a capital letter.
• recessive


  describes an allele that is only expressed in absence of a dominant allele.represented by a small letter.
• homozygous

  When someone has two identical alleles of a gene e.g. ff.
• heterozygous

  When someone has two different alleles of a gene e.g. Ff.
• genotype

  An organism's genetic composition. Describes all alleles.
• phenotype

  An organism's observable characteristics. Due to interactions of the genotype and the environment.
• inherited disorders

  Some disorders are inherited. These disorders are caused by the inheritance of certain alleles.

  • Polydactyly (having extra fingers or toes) is caused by a dominant allele.

  • Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
• sex determination

  Ordinary human body cells contain 23 pairs of chromosomes.

  22 pairs control characteristics only, but one of the pairs carries the genes that determin sex.

  • in females the sex chromosomes are the same (XX)
  • in males the sex chromosomes are different (XY)
• variation

  Differences in the characteristics of individuals in a population is called variation and may be due to differences in:

  • the genes they have inherited (genetic causes)

  • the conditions in which they have developed(environmental causes)
• Mutations occur continuously. Very rarely a mutation will lead to a new phenotype. If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species.
• evolution

  The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.
• If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring they have formed two new species.
• selective breeding

  Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics.
• process of selective breeding

  Selective breeding involves choosing parents with the desired characteristic from a mixed population. They are bred together.

  From the offspring those with the desired characteristic are bred together. This continues over many generations until all the offspring show the desired characteristic.
• Results of selective breeding

  The characteristic can be chosen for usefulness or appearance:

  • Disease resistance in food crops.
  • Animals which produce more meat or milk.
  • Domestic dogs with a gentle nature.
  • Large or unusual flowers.
• disadvantage of selective breeding

  Selective breeding can lead to 'inbreeding' where some breeds are particularly prone to disease or inherited defects.
• plant crops have been genetically engineered to be res to diseases or to produce bigger better fruits
• In genetic engineering, genes from the chromosomes of humans and other organisms can be 'cut out' and transferred to cells of other organisms.
• GM crops

  Crops that have had their genes modified in this way are called genetically modified (GM) crops. GM crops include ones that are resistant to insect attack or to herbicides. GM crops generally show increased yields.
• Concerns about GM crops include the effect on populations of wild flowers and insects. Some people feel the effects of eating GM crops on human health have not been fully explored.
• gentic engineering steps

  In genetic engineering:

  • enzymes are used to isolate the required gene; this gene is inserted into a vector, usually a bacterial plasmid or a virus

  • the vector is used to insert the gene into the required cells genes are transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristic.
• evolution

  The gradual change in the inherited traits within a population over time. Occurs due to natural selection.
• natural selection

  The process by which the frequency of advantageous traits passed on in genes gradually increases in a population over time.
• charles darwin’s theory

  Evidence for Darwin's theory is now available as it has been shown that characteristics are passed on to offspring in genes. There is further evidence in the fossil record and the knowledge of how resistance to antibiotics evolves in bacteria.
• fossils


  fossils are the ‘remains’ of organisms from millions of years ago,which are found in rocks.

  we can learn from fossils how much or how little different organisms have chnaged as life developed on earth.
• extinction 


  extinctions occur when there are no remaining individuals of a species still alive.
• resistant bacteria

  Bacteria can evolve rapidly because they reproduce at a fast rate.

  Mutations of bacterial pathogens produce new strains. Some strains might be resistant to antibiotics, and so are not killed. They survive and reproduce, so the population of the resistant strain rises. The resistant strain will then spread because people are not immune to it and there is no effective treatment.
• MRSA is resistant to antibiotics.