inheritance, variation and evolution

• 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 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

  Students should be able to explain how meiosis halves the number of chromosomes in gametes and fertilisation restores the full number of chromosomes.

  Cells in reproductive organs divide by meiosis to form gametes.
• Meiosis

  When a cell divides to form gametes:

  • copies of the genetic information are made
  • the cell divides twice to form four gametes, each with a single set of chromosomes
  • all gametes are genetically different from each other.
•  Meiosis

  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.
• Advantages of sexual reproduction:

  • produces variation in the offspring
  • if the environment changes variation gives a survival advantage by natural selection
  • natural selection can be speeded up by humans in selective breeding to increase food production.
• Advantages of asexual reproduction:

  • only one parent needed
  • more time and energy efficient as do not need to find a mate
  • faster than sexual reproduction
  • many identical offspring can be produced when conditions are favourable.
• Some organisms reproduce by both methods depending on the circumstances.

  • Malarial parasites reproduce asexually in the human host, but sexually in the mosquito.
  • Many fungi reproduce asexually by spores but also reproduce sexually to give variation.
  • Many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils.
• 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 is contained in structures called chromosomes.

  A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.
• 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.
• Students should be able to discuss the importance of understanding the human genome.

  This is limited to the:

  • search for genes linked to different types of disease
  • understanding and treatment of inherited disorders
  • use in tracing human migration patterns from the past.
• DNA structure (biology only)

  Students should be able to describe DNA as a polymer made from four different nucleotides. Each nucleotide consists of a common sugar and phosphate group with one of four different bases attached to the sugar.

  DNA contains four bases, A, C, G and T.

  A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein.
•  DNA structure (biology only)

  The long strands of DNA consist of alternating sugar and phosphate sections. Attached to each sugar is one of the four bases.

  The DNA polymer is made up of repeating nucleotide units.
• mutations

   Mutations occur continuously. Most do not alter the protein, or only alter it slightly so that its appearance or function is not changed.

  (HT only) A few mutations code for an altered protein with a different shape. An enzyme may no longer fit the substrate binding site or a structural protein may lose its strength.
• mutations

   Not all parts of DNA code for proteins. Non-coding parts of DNA can switch genes on and off, so variations in these areas of DNA may affect how genes are expressed.
• Some characteristics are controlled by a single gene, such as: fur colour in mice; and red-green colour blindness in humans. Each gene may have different forms called alleles.

  The alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as a phenotype.
• A dominant allele is always expressed, even if only one copy is present. A recessive allele is only expressed if two copies are present (therefore no dominant allele present).
• If the two alleles present are the same the organism is homozygous for that trait, but if the alleles are different they are heterozygous.

  Most characteristics are a result of multiple genes interacting, rather than a single gene.
•  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 determine sex.

  • In females the sex chromosomes are the same (XX).
  • In males the chromosomes are different (XY).
• 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)
  • a combination of genes and the environment.
• mutations

  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 (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics. Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals
• 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.
• 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.

  Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects.
• Genetic engineering

  Plant crops have been genetically engineered to be resistant to diseases or to produce bigger better fruits.

   

  Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes.
• In genetic engineering, genes from the chromosomes of humans and other organisms can be ‘cut out’ and transferred to cells of other organisms.

  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.

  Modern medical research is exploring the possibility of genetic modification to overcome some inherited disorders.
• the process of genetic engineering

  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 characteristics.
• Cloning (biology only)

  Tissue culture: using small groups of cells from part of a plant to grow identical new plants. This is important for preserving rare plant species or commercially in nurseries.
•  Cloning (biology only)

  Cuttings: an older, but simple, method used by gardeners to produce many identical new plants from a parent plant.
• Cloning (biology only)

  Embryo transplants: splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers.
• Cloning (biology only)

  Adult cell cloning:

  • The nucleus is removed from an unfertilised egg cell.
  • The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell.
  • An electric shock stimulates the egg cell to divide to form an embryo.
  • These embryo cells contain the same genetic information as the adult skin cell.
  • When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.
• Theory of evolution (biology only)

  Charles Darwin, as a result of observations on a round the world expedition, backed by years of experimentation and discussion and linked to developing knowledge of geology and fossils, proposed the theory of evolution by natural selection.
• Charles Darwin proposed the theory of natural selection

  • Individual organisms within a particular species show a wide range of variation for a characteristic.
  • Individuals with characteristics most suited to the environment are more likely to survive to breed successfully.
  • The characteristics that have enabled these individuals to survive are then passed on to the next generation.
• Darwin published his ideas in On the Origin of Species (1859). There was much controversy surrounding these revolutionary new ideas.

  The theory of evolution by natural selection was only gradually accepted because:

  • the theory challenged the idea that God made all the animals and plants that live on Earth
  • there was insufficient evidence at the time the theory was published to convince many scientists
  • the mechanism of inheritance and variation was not known until 50 years after the theory was published
• Other theories, including that of Jean-Baptiste Lamarck, are based mainly on the idea that changes that occur in an organism during its lifetime can be inherited. We now know that in the vast majority of cases this type of inheritance cannot occur.

  example of giraffes' neck length becoming longer as they stretch -> then the offspring inherits it