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.

  • Sperm and egg cells in animals
  • Pollen and egg cells in flowering plants
• Gametes are formed by meiosis, as they are non identical.
• A normal cell has 46 chromosomes. There are two sets of chromosomes(i.e. 23 pairs). In each pair, one chromosome is from the father and the second set is from the mother
• Each gamete has 23 chromosomes and they fuse in fertilisation.
• The genetic information from each parent is mixed, producing variation in the offspring.
• Asexual reproduction involves one parent with no gametes joining. It happens using the process of mitosis, where two identical cells are formed from one cell.
• There is no mixing of genetic information in asexual reproduction.
• Asexual reproduction leads to clones, which are genetically identical to each other and the parent.
• Examples of organisms that reproduce by asexual reproduction are bacteria, some plants and some animals.
• Meiosis is the formation of four non-identical cells from one cell. Cells in the reproductive organs divide by meiosis to form gametes. Gametes only have one copy of each chromosome.
• Meiosis:

  • The cell makes copies of its chromosomes, so it has to double the amount of genetic information
  • The cell divides into two cells, each with half the amount of chromosomes(46)
  • The cell divides again producing four cells, each with a quarter of the amount of chromosomes(23)
  • These cells are called gametes and they are all genetically different from each other because the chromosomes are shuffled during the process, resulting in random chromosomes ending up in each of the four cells
• Gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes, the normal number.

  • This cell divides by mitosis to produce many copies.
  • More and more cells are produced, and an embryo forms.
  • The cells begin to take on different roles after this stage(differentiation)
• One advantage of sexual reproduction is it produces variation in offspring.

  • This means that if the environment changes it is likely that an organism in the species will have a characteristic that allows them to survive(called a survival advantage)
  • Although some individuals may die, variation decreases the chance of the whole species becoming extinct
• One advantage of sexual reproduction is it allows us to use selective breeding.

  • This type of reproduction mixes the genetic information from two organisms.
  • Organisms with different desirable characteristics can be bred to produce offspring with even more desirable characteristics.
  • This speeds up natural selection.
  • An example is to increase food production by breeding two animals with lots of meat.
• Advantages of asexual reproduction.

  • Only one parent is needed.
  • Uses less energy and is faster as organisms do not need to find a mate.
  • In favourable conditions lots of identical offspring can be produced.
• Examples of organisms that use both methods to reproduce are below:

  • Malarial parasites
  • Some fungi
  • Some plants
• Malarial parasites:

  • Causes malaria, spread by mosquitos and transferred to humans through a bite.
  • They reproduce sexually in the mosquito.
  • They reproduce asexually in the human host(in the liver and blood cells).
• Some fungi:

  • Many species can undergo both types of reproduction, releasing spores which land and become new fungi.
  • Spores produced asexually are genetically identical.
  • Spores are produced sexually when the conditions change, in order to increase variation and avoid extinction
• Some plants:

  • Many reproduce sexually using pollen, which must reach the egg cells in the female parts of another flower. This is called pollination, and it forms seeds.
  • Strawberry plants reproduce asexually, as they produce runners. New identical plants grow off the runner.
  • Daffodils reproduce asexually. They grow from bulbs. New bulbs can grow from the main one, producing a new identical plant.
  • It is advantageous in plants as it means they can reproduce even if the flowers have been destroyed by frost or other animals
• 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 which wrap around each other like a rope - in a structure called a double helix.
• The DNA in the nucleus is contained in structures called chromosomes.
• Between the two strands in DNA are the four nitrogenous bases lined up in single rows - these come together to form a series of complementary pairs.
• A gene is a small section of DNA on a chromosome - a triplet of bases that codes for a specific protein.
• Each gene codes for a particular sequence of amino acids, together a chain of amino acids can join to make a protein.
• The genome is all the genes coding for all of the proteins within an organism.
• The whole human genome has now been studied and this has improved our understanding of the genes linked to different types of disease, the treatment of inherited disorders and has helped in tracing human migration patterns from the past.
• Understanding the human genome will have great importance for medicine in the future.
• Gamete = an organism's reproductive cell (egg in female and sperm in males) which has half the number of chromosomes(23)
• Chromosome = A structure found in the nucleus which is made up of a long strand of DNA.
• Gene = a short section of DNA that codes for a protein, and therefore contributes to a characteristic.
• Some characteristics are controlled by a single gene, such as fur colour in mice and red-green colour blindness in humans. However, most characteristics are the result of many different genes interacting.
• Alleles = the different forms of the gene - humans have two alleles for each gene as they inherit one from each parent.
• Dominant allele = only one (out of the two alleles) is needed for it to be expressed and for the corresponding phenotype to be observed
• Recessive allele = two copies are needed for it to be expressed and for the corresponding phenotype to be observed.
• Homozygous = when both inherited alleles are the same (i.e. two dominant alleles or two recessive alleles)
• Heterozygous = when on of the inherited alleles is dominant and the other is recessive.
• Genotype = the combination of alleles an individual has, e.g. Aa