Genes and Heredity Notes

  1. Background (pp. 570N; 238C)
    1. was known for 1000s of years that traits were inherited
    2. Gregor Mendel (1822-1884) - Austrian monk who experimented with garden peas and developed the foundation of modern genetics. Noticed that peas had several traits and always showed only one of a pair rather than a blend which was previously believed. He crossed plants with different traits to see what the offspring would look like. (Fig. 24.3 p. 572N)
    3. Mendel found that no matter what combinations he tried, one trait always dominated and masked the other. It didn't matter if the trait came from the male or female parent. The traits were controlled by factors which were later known as genes
    4. Mendel's Laws of Heredity (p 573N; 240C)
      1. inherited traits are controlled by genes that occur in pairs. These two versions are called alleles
      2. an organism inherits an allele for each trait from each parent (2 alleles for each trait total)
      3. one allele masks the presence of the other. Called the principle of dominance. Dominant (R) vs recessive (r)
      4. alleles separate during meiosis I. Called the law of segregation
    5. Vocabulary
      1. homozygous - both alleles for a trait are the same
      2. heterozygous - the alleles for trait are different
      3. genotype - the actual genetic makeup for a trait
      4. phenotype - the way in which the genotype is expressed
  2. Monohybrid Cross (p. 575N)
    1. 3:1 ratio


e.g., round seed wrinkled

P RR x rr

gametes R R r r

F1 Rr (all round)



use F1 as Parents

P Rr x Rr

gametes R r R r

F2 RR, Rr, Rr, rr (round, round, round, wrinkled; 3:1)



    1. Punnett square can be used to show genotype, phenotype, and probability


e.g., heterozygous purple x white



purple



    1. Test cross (p. 577N)
  1. Dihybrid Cross (p. 583N; 244C)
    1. Mendel wondered if traits always traveled together or if were inherited separately. e.g., if yellow round x green wrinkled would all offspring be yellow round or green wrinkled or would some be yellow wrinkled and some green round
    2. 9:3:3:1 ratio
    3. showed that each trait is inherited independently
  2. Probability = the chance that an event will occur = #correct outcomes / # total outcomes. Expressed on a scale from 0 to 1 where 0 = zero chance and 1 = 100% chance i.e. every outcome is the right one
    1. Independent events - previous events don't affect future events. e.g., the chance of getting heads in a coin toss is ½; the chance of getting heads a second time is ½
    2. Rule of Multiplication - chance of two events occurring together is P1 x P2