genetics

• Sexual reproduction


  Involves the joining of male and female gametes, each containing genetic information from the mother or father
• Gametes


  • Sperm and egg cells in animals
  • Pollen and egg cells in flowering plants
  • Formed by meiosis, as they are non identical
  • Normal cell has 46 chromosomes, two sets of 23 pairs, one from each parent
  • Each gamete has 23 chromosomes, they fuse in fertilisation
  • Genetic information from each parent is mixed, producing variation in the offspring
• Asexual reproduction


  • Involves one parent with no gametes joining
  • Happens using the process of mitosis, where two identical cells are formed from one cell
  • There is no mixing of genetic information
  • Leads to clones, which are genetically identical to each other and the parent
• Advantages of sexual reproduction

  • Produces variation in offspring
  • Decreases the chance of the whole species becoming extinct
  • Allows selective breeding
• Advantages of asexual reproduction

  • Only one parent is needed
  • Uses less energy and is faster as organisms do not need to find a mate
  • Allows for rapid reproduction in favourable conditions
• Meiosis


  • The formation of four non-identical cells from one cell
  • The cell makes copies of its chromosomes, then divides twice to produce four genetically different gametes with 23 chromosomes each
  • These gametes join at fertilisation to produce a cell with 46 chromosomes, the normal number
• DNA


  • A chemical that contains genetic material
  • A polymer made up of nucleotides, each with a sugar, phosphate and one of four organic bases (A, C, G, T)
  • Two DNA strands twisted together in a double helix, with complementary base pairing (A-T, C-G)
  • The order of bases forms a genetic code
• Gene


  A short section of DNA that codes for a specific protein
• Genome


  All the genetic information (DNA) of a single organism
• Extracting DNA from fruit

  1. Gently mix water, salt and washing up liquid, heat to 50C

  2. Pulverise kiwi fruit, add solution

  3. Filter and add pineapple juice

  4. Add ethanol to precipitate the DNA
• Protein synthesis


  1. DNA contains genetic code for protein, mRNA is transcribed from DNA in nucleus

  2. mRNA moves to ribosomes in cytoplasm

  3. tRNAs bring amino acids to ribosomes, which join them into a polypeptide chain

  4. Polypeptide folds into final 3D protein structure
• Genetic variants


  • Small changes in the order of DNA bases
  • Can affect protein structure if in coding DNA, or gene expression if in non-coding DNA
• Mutations


  • Changes to DNA sequence, such as base insertion, deletion or substitution
  • Can change amino acid sequence and protein structure, or affect gene expression
• Mendelian genetics


  • Gregor Mendel's work on inheritance of characteristics in pea plants
  • Offspring inherit 'hereditary units' (genes) from each parent, which can be dominant or recessive
• Alleles


  Different versions of the same gene
• Mendel's work was not recognised until after his death, as genes and chromosomes were not yet discovered
• Gregor Mendel


  Trained in mathematics and natural history in Vienna, worked in the monastery gardens and observed the characteristics passed on to the next generation in plants, carried out breeding experiments on pea plants
• Mendel's conclusions


  Offspring have some characteristics that their parents have because they inherit 'hereditary units' from each, one unit is received from each parent, units can be dominant or recessive, and cannot be mixed together
• Mendel was not recognised till after his death as genes and chromosomes were not yet discovered, so people could not understand
• 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 contribute to a characteristic
• 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 the corresponding phenotype to be observed
• Recessive allele


  Two copies are needed for it to be expressed and for the corresponding the phenotype to be observed
• Homozygous


  When both inherited alleles are the same (i.e. two dominant alleles or two recessive alleles)
• Heterozygous


  When one of the inherited alleles is dominant and the other is recessive
• Genotype


  The combination of alleles an individual has, e.g. Aa
• Phenotype


  The physical characteristics that are observed in the individual, e.g. eye colour
• Zygote


  The stage of development immediately after fertilisation - a diploid (2n) cell formed from the fusion of two haploid gametes
• Monohybrid (single gene) cross


  Looks at the probability of the offspring of two parents having certain genotypes and phenotypes, using the alleles the two parents have for a gene and a Punnett square diagram
• Uppercase letters are used to represent dominant characteristics, lowercase letters represent recessive characteristics
• Combining the alleles results in a 1/4 chance of having an offspring who is homozygous dominant, and no chance of having a homozygous recessive offspring
• Family pedigrees


  Used to show how a condition (or more specifically, the allele which causes it) are passed down through different generations
• Squares represent males, circles represent females, black shapes represent affected individuals, white shapes represent unaffected individuals, a line through the shape means the individual is deceased
• Sex determining genes


  Found on the 23rd chromosome pair, which can look different (Y chromosomes are much smaller than X chromosomes)
• Sex inheritance


  Females have two X chromosomes, so only pass on X chromosomes in their eggs, males have one X chromosome and one Y chromosome, so can pass on X or Y chromosomes in their sperm
• Codominance


  When two dominant alleles can be expressed together in the same individual
• Multiple alleles


  When three or more alleles can be present at the same loci (but not necessarily expressed at once)
• ABO blood group system

  • A and O alleles: becomes type A, A and B alleles: becomes type AB (codominantly expressed), A and A alleles: becomes type A, B and B alleles: becomes type B, O and O alleles: becomes type O