Chapter 1

• Monohybrid crosses


  Cross in which an experimenter is observing only one character
• Dihybrid crosses


  An individual that is heterozygous for two genes at the same time
• Law of Segregation


  Two alleles for each trait separate (segregate) during gamete formation, and then unite at random, one from each parent, at fertilization
• Law of Independent Assortment


  During gamete formation different pairs of alleles segregate independently of each other
• Dominant


  Trait that appears in F1
• Recessive


  Trait that is hidden in F1
• Alleles


  Alternative forms of traits
• Genotype


  Genetic composition of an individual
• Phenotype


  Observable characteristics of an organism
• Homozygous


  Individual possesses two identical copies of a gene
• Heterozygous


  Individual carries different alleles of a gene
• Testcross


  Mating in which an individual showing the dominant phenotype (but unknown genotype) is crossed with an individual with the recessive phenotype
• Garden pea


  • Vigorous growth
  • Self fertilization
  • Easy to cross fertilize
  • Produced large number of offspring each generation
• Traits Mendel studied


  • Purple vs. white flowers
  • Yellow vs. green peas
  • Round vs. wrinkled seeds
  • Long vs. short stem length
• Pure-bred (true breeding) lines


  Variety that continue to produce the same characteristic after several generations of self-fertilization
• Reciprocal cross


  Same outcome
• Probability and Mendel's Results


  1. Chance of Y sperm uniting with a Y egg

  2. Chance of Yy offspring

  3. Formation of egg and sperm are independent events

  4. Probability that a particular combination of maternal and paternal alleles will occur simultaneously in the same zygote is the product of independent probability

  5. Sum of individual probabilities for mutually exclusive events
• Contrary, two different fertilization events are mutually exclusive
• If Y combine with Y, it cannot also combine with y in the same zygote
• Further crosses verify the Law of Segregation by confirming predicted ratios (3:1)
• Dihybrid crosses produce a predictable ratio of phenotypes
• Dihybrid cross shows parental and recombinant types
• Using the product rule for assessing the probability of independent events

  1. Yellow-to-green ratio will be 3/4: 1/4

  2. Round-to-wrinkled ratio will be 3/4:1/4

  3. Probability of yellow round: 3/4 x 3/4 = 9/16

  4. Probability of green round: 1/4 x 3/4= 3/16

  5. Probability of yellow wrinkled: 3/4 x 1/4= 3/16

  6. Probability of green wrinkled: 1/4 x 1/4= 1/16
• Branched-line diagrams


  • Showing 9:3:3:1 ratio of phenotypes
• Key Concepts


  • Traits, genes and alleles
  • Genotype / phenotype
  • Dominant and recessive forms
  • True breeding (pure breeding) lines
  • Reciprocal cross
  • Test cross
  • Rules of probability: Independent vs mutually exclusive events, Product rule and sum rule
  • Punnett square
  • Monohybrid cross, 3:1 ratio
  • Dihybrid cross: independent assortment, 9:3:3:1 ratio
  • Parental vs recombinant type
• The third step is to determine whether the trait is dominant, recessive, or co-dominiant
• A gene pool consists of all the alleles present at a particular locus in a population.
• The second step is to determine how many alleles are present at the gene locus being investigated.
• Genetic variation refers to differences among individuals or populations with respect to their genes, chromosomes, or other heritable characteristics.
• The first step is to identify the trait or characteristic that will be studied.
• The genome is the complete set of genetic material (DNA) found within an organism.
• If both parents have the same genotype, they are homozygous.
• If one parent has two different alleles, it is heterozygous.
• If one parent has two different alleles, it is heterozygous.
• If one parent has two different alleles, it is heterozygous.
• Hardy–Weinberg equilibrium refers to the proportions of genotypes that would occur if there were no evolutionary forces acting on them.
• Hardy–Weinberg equilibrium refers to the proportions of genotypes that would occur if there were no evolutionary forces acting on them.
• Incomplete dominance occurs when neither parental genotype produces a fully expressed phenotype.
• Incomplete dominance occurs when neither parental genotype produces a fully expressed phenotype.