bio (mendel)

• What genetic principles account for the passing of traits from parents to offspring?
• Blending hypothesis


  Genetic material from the two parents blends together (like blue and yellow paint blend to make green)
• Particulate hypothesis


  Parents pass on discrete heritable units (genes)
• Mendel documented a particulate mechanism through his experiments with garden peas
• Mendel's approach


  • He used the scientific approach to identify two laws of inheritance
  • He discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
• Advantages of pea plants for genetic study

  • There are many varieties with distinct heritable features, or characters (such as flower color)
  • Character variants (such as purple or white flowers) are called traits
  • Mating of plants can be controlled
  • Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)
  • Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another
• Mendel chose to track only those characters that varied in an either-or manner
• Mendel used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)
• Typical experiment


  1. Mendel mated two contrasting, true-breeding varieties, a process called hybridization

  2. The true-breeding parents are the P generation

  3. The hybrid offspring of the P generation are called the F1 generation

  4. When F1 individuals self-pollinate, the F2 generation is produced
• Law of Segregation


  The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
• Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

  
• Dominant trait


  The purple flower color in Mendel's experiments

  
• Recessive trait


  The white flower color in Mendel's experiments
• Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits

  
• Heritable factor


  What Mendel called a "heritable factor" is what we now call a gene
• Concepts in Mendel's model

  • Alternative versions of genes account for variations in inherited characters
  • Organisms inherit two alleles, one from each parent
  • If the two alleles at a locus differ, the dominant allele determines the organism's appearance
  • The two alleles for a heritable character separate (segregate) during gamete formation
• Alleles


  Alternative versions of a gene
• Homozygous


  An organism with two identical alleles for a character
• Heterozygous


  An organism that has two different alleles for a gene
• Phenotype


  An organism's physical appearance
• Genotype


  An organism's genetic makeup
• Testcross


  1. Breeding the mystery individual with a homozygous recessive individual

  2. If any offspring display the recessive phenotype, the mystery parent must be heterozygous
• Mendel derived the law of segregation by following a single character
• Monohybrid


  Individuals that are heterozygous for one character
• Monohybrid cross


  A cross between monohybrid individuals
• Dihybrid


  Individuals that are heterozygous for two characters
• Dihybrid cross


  A cross between dihybrid individuals
• Mendel identified his second law of inheritance by following two characters at the same time
• A dihybrid cross can determine whether two characters are transmitted to offspring as a package or independently
• Mendel's dihybrid cross results in a phenotypic ratio of approximately 9:3:3:1
• Dihybrid cross


  1. Produces dihybrids in the F1 generation, heterozygous for both characters

  2. Can determine whether two characters are transmitted to offspring as a package or independently
• The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation
• Strictly speaking, the law of independent assortment applies only to genes on different, nonhomologous chromosomes
• Genes located near each other on the same chromosome tend to be inherited together
• Probability


  The rules that govern Mendelian inheritance
• Multiplication rule


  The probability that two or more independent events will occur together is the product of their individual probabilities
• Addition rule


  The probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities
• A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously
• In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together
• Complete dominance


  Phenotypes of the heterozygote and dominant homozygote are identical