DNA and inheritance

• DNA


  Double-stranded polymer of nucleotides, wound to form a double helix
• DNA structure

  • Each strand consists of alternating sugar and phosphate molecules
  • The two strands are joined by a pair of bases
  • There are four bases: A (adenine), T (thymine), C (cytosine) and G (guanine)
  • A pairs with T and C pairs with G
• Gene


  A section of DNA that codes for a protein
• Triplet


  • A sequence of three bases in a gene
• Amino acid


  Each triplet codes for an amino acid
• Protein

  Order of amino acids determines the structure and function of the protein formed
• Genetic profiling

  1. Sample of cells collected

  2. DNA extracted from cells

  3. DNA sample cut into fragments using enzymes

  4. Fragments separated into bands, creating a genetic profile
• Genetic profiling

  Comparing DNA to view similarities in the DNA sequences

  
• Gene

  A length of DNA that codes for the production of a particular protein
• Allele

  A version of a gene
• Gamete

  Reproductive cells (e.g. egg and sperm cells) that contain a single copy of each chromosome
• Dominant

  Describes an allele that is always expressed, represented with a capital letter
• Recessive

  An allele that is only expressed in the absence of a dominant allele, represented with a small letter
• Homozygous

  Having two identical alleles of a gene
• Heterozygous

  Having two different alleles of a gene
• Genotype

  An organism's genetic composition, describes all alleles
• Phenotype

  An organism's observable characteristics
• Selfing


  An artificial method of self-pollinating plants
• Most characteristics of an organism are determined by multiple genes interacting, however some are determined by a single gene
• Monohybrid inheritance


  The inheritance of a single gene
• Punnett square


  Can be used to illustrate single gene inheritance

  
• Monohybrid inheritance examples


  • PKU is a recessive condition
  • Cystic fibrosis is a recessive condition
• Sex chromosomes


  One of the 23 pairs of chromosomes that determines sex
• Sex determination in humans

  • Males have an X and a Y chromosome (genotype XY)
  • Females have two X chromosomes (genotype XX)
  • Meiosis produces gametes with half the number of chromosomes and thus a single sex chromosome
  • All egg cells contain an X chromosome only whilst male sperm cells may contain an X or a Y chromosome
  • The baby's gender depends on which sperm cell fertilises the egg

  
• Sex determination in humans

  Punnett square can be used to illustrate
• Genetic engineering


  Modification of the genome of an organism by the insertion of a desired gene from another organism
• Genetic engineering

  • Enables the formation of an organism with beneficial characteristics
  • Genetically engineered organisms are known as GM organisms
• Benefits of genetic engineering

  • Increased crop yields for growing population
  • Useful in medicine
  • GM crops produce scarce resources
  • GM crops can produce oils which can be used as biofuels
• Risks of genetic engineering

  • Long-term effects of consumption of GM crops are unknown
  • Negative environmental impacts
  • GM seeds are expensive, LEDCs may be unable to afford them or may become dependent on businesses that sell them
  • GM plants could become a pest themselves