Genetics

• Living organisms that belong to the same species, or type, differ from one another in a large number of ways
• Variation


  Differences between individuals for a particular characteristic
• Causes of variation


  • Differences in the genetic makeup (inherited from parents)
  • Environmental factors
• Some types of variation may be due to only one of the two causes (genetic or environmental)
• Eye colour


  Determined by genetics (inherited)
• Height in humans


  Affected by both genetic (potential based on parents) and environmental (factors like malnutrition and disease) influences
• Differences in the appearance of identical twins are due to environmental factors such as: diet, healthcare, life experiences
• All living organisms produced by sexual reproduction are different from each other because their genes are different
• Mutations that change the genetic makeup of organisms also cause variation
• Environmental variation


  Variation caused by the surrounding environment, particularly obvious in plants
• Environmental factors affecting plants

  • Temperature
  • Sunlight
  • Moisture levels
  • Soil composition
• Continuous variation


  Feature varies over a continuous range of values, can have any value within a range
• Examples of continuous variation in humans


  • Height
  • Body mass
  • Chest circumference
  • Hand span
• Discontinuous variation


  Feature can only take one of several options, population can be clearly divided into discrete groups or categories
• Examples of discontinuous variation in humans

  • Ability/inability to roll the tongue
  • Presence/absence of ear lobes
  • Blood groups
• Young plants and animals resemble their parents due to inheritance of family characteristics
• Genetics


  The scientific study of inheritance
• Chromosomes


  Structures in the nucleus containing long strands of DNA and large numbers of genes
• Homologous pair


  A pair of chromosomes that carry genes for the same characteristics
• We have two of each chromosome because we inherit one from our mother and one from our father
• Alleles


  Different forms of the same gene
• Dominant allele


  Always expresses itself, works whether paired with another dominant or recessive allele
• Recessive allele


  Only works when paired with another recessive allele, masked by a dominant allele
• TT - tall, tt - dwarf, Tt - tall (dominant T allele is expressed)
• Letters like B/b, R/r, G/g are better to use for alleles than S/s, W/w, P/p as they are more distinct
• Homozygous


  Only one type of allele present (e.g. TT or tt)
• Heterozygous


  Different alleles present (e.g. Tt)
• Genotype is the type of genes present, phenotype is the physical characteristic that results
• Chromosomes consist of DNA and have a large number of genes along their length
• Organisms with only one form of a gene are homozygous, those with both alternative alleles are heterozygous
• Recessive allele


  Only expressed as a physical characteristic when two are present
• Dominant allele


  Can hide the effect of recessive alleles
• Genotype


  The genes a plant or animal has
• Phenotype


  The physical characteristics of a plant or animal
• Monohybrid cross


  Breeding experiment considering only one characteristic
• Gametes (sex cells) only have one allele, not the full pair
• Heterozygous parents produce gametes with 50% chance of each allele
• Offspring genotype ratios from a monohybrid cross are probabilities, not guaranteed outcomes
• Punnett square


  Diagram used to show the possible genotypes of offspring from a cross
• Sex chromosomes


  Determine whether an individual is male (XY) or female (XX)