Mendelian Genetics
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Mendel 1862Mendel 1868Mendel 1880


Genetic Terminology:

  • Trait – any characteristic that can be passed from parent to offspring
  • Heredity – passing of traits from parent to offspring
  • Genetics – study of heredity
  • Alleles – two forms of a gene (dominant & recessive)
  • Dominant – stronger of two genes expressed in the hybrid; represented by a capital letter (R)
  • Recessive – gene that shows up less often in a cross; represented by a lowercase letter (r)
  • Genotype – gene combination for a trait (e.g. RR, Rr, rr)
  • Phenotype – the physical feature resulting from a genotype (e.g. tall, short)
  • Homozygous genotype – gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure 
  • Heterozygous genotype – gene combination of one dominant & one recessive allele    (e.g. Rr); also called hybrid
  • Monohybrid cross – cross involving a single trait
  • Dihybrid cross – cross involving two traits
  • Punnett Square – used to solve genetics problems

Blending Concept of Inheritance:

  • Accepted before Mendel’s experiments
  • Theory stated that offspring would have traits intermediate between those of its parents such as red & white flowers producing pink
  • The appearance of red or white flowers again was consider instability in genetic material
  • Blending theory was of no help to Charles Darwin’s theory of evolution 
  • Blending theory did not account for variation and could not explain species diversity
  • Particulate theory of Inheritance, proposed by Mendel, accounted for variation in a population generation after generation
  • Mendel’s work was unrecognized until 1900

Gregor Mendel:

  • Austrian monk
  • Studied science & math at the University of Vienna
  • Formulated the laws of heredity in the early 1860’s
  • Did a statistical study of  traits in garden peas over an eight year period


drawing of a flower cross-section showing both male and female sexual structures


Why peas, Pisum sativum?

  • Can be grown in a small area
  • Produce lots of offspring
  • Produce pure plants when allowed to self-pollinate several generations
  • Can be artificially cross-pollinate

Picture of Pisum sativum

Mendel’s Experiments:

  • Mendel studied simple traits from 22 varieties of  pea plants (seed color & shape, pod color & shape, etc.)
  • Mendel traced the inheritance of individual traits & kept careful records of numbers of offspring
  • He used his math principles of probability to interpret results
  • Mendel studied pea traits, each of which had a dominant & a recessive form (alleles)
  • The dominant (shows up most often) gene or allele is represented with a capital letter, & the recessive gene with a lower case of that same letter (e.g. B, b)
  • Mendel’s traits included:

         a. Seed shape —  Round (R) or Wrinkled (r)
            b. Seed Color —- Yellow (Y) or  Green (y)
            c. Pod Shape — Smooth (S) or wrinkled (s)
            d. Pod Color —  Green (G) or Yellow (g)
            e. Seed Coat Color —  Gray (G) or White (g)
            f. Flower position — Axial (A) or Terminal (a)
            g. Plant Height — Tall (T) or Short (t)
            h. Flower color — Purple (P) or white (p)

  •  Mendel produced pure strains by allowing the plants to self-pollinate for several generations
  • These strains were called the Parental generation or P1 strain
  • Mendel cross-pollinated two strains and tracked each trait through two
    generations (e.g. TT  x  tt )


                  Trait – plant height

                  Alleles – T tall, t short

    P1 cross    TT  x  tt

    genotype      —    Tt
    ttphenotype    —    Tall
    TTtTtgenotypic ratio –all alike
    TTtTtphenotypic ratio- all alike



  • The offspring of this cross were all hybrids showing only the dominant trait & were called the First Filial or F1 generation
  • Mendel then crossed two of his F1 plants and tracked their traits; known as an F1 cross


              Trait – plant height

              Alleles – T tall, t short

F1 cross    Tt  x  Tt

genotype      —    TT, Tt, tt
Ttphenotype    —    Tall & short
TTTTtgenotypic ratio —1:2:1
tTtttphenotypic ratio- 3:1



  • When 2 hybrids were crossed, 75% (3/4) of the offspring showed the dominant trait & 25% (1/4) showed the recessive trait; always a 3:1 ratio
  • The offspring of this cross were called the F2 generation
  • Mendel then crossed a pure & a hybrid from his F2 generation; known as an F2 or test cross


Trait   –  Plant Height
Alleles – T  tall, t  short

F2 cross       TT  x Tt

F2 cross       tt  x Tt

          genotype – TT, Tt          genotype – tt, Tt
          phenotype  –  Tall          phenotype  –  Tall & short
          genotypic ratio  – 1:1          genotypic ratio  – 1:1
          phenotypic ratio – all alike          phenotypic ratio – 1:1


  • 50% (1/2) of the offspring in a test cross showed the same genotype of one parent & the other 50% showed the genotype of the other parent; always a 1:1 ratio

Problems: Work the P1, F1, and both F2 crosses for all of the other pea plant traits & be sure to include genotypes, phenotypes, genotypic & phenotypic ratios.

  • Mendel also crossed plants that differed in two characteristics (Dihybrid Crosses)
    such as seed shape & seed color
  • In the P1 cross, RRYY  x  rryy, all of the F1 offspring showed only the dominant form for both traits; all hybrids, RrYy


Traits:      Seed Shape & Seed Color

Alleles:     R round                Y yellow
r wrinkled             y green

 P1 Cross:     RRYY          x     r r yy  


ryGenotype:     RrYy
Phenotype:     Round yellow seed
Genotypic ratio:     All alike
Phenotypic ratio:     All Alike


  • When Mendel crossed 2 hybrid plants (F1 cross), he got the following results



Traits:       Seed Shape & Seed Color

Alleles:     R round                Y yellow
r wrinkled             y green

     F1 Cross:     RrYy           x     RrYy                   








r rYY

r rYy


r rYy

r ryy




GenotypesGenotypic RatiosPhenotypesPhenotypic Ratios
RRYY1Round yellow seed
RRyy1Round green seed
r rYY1Wrinkled yellow seed
r rYy2
r ryy1Wrinkled green seed


Problems: Choose two other pea plant traits and work the P1 and F1 dihybrid crosses. Be sure to show the trait, alleles, genotypes, phenotypes, and all ratios. 

Results of Mendel’s Experiments:

  • Inheritable factors or genes are responsible for all heritable characteristics
  • Phenotype is based on Genotype
  • Each trait is based on two genes, one from the mother and the other from the father
  • True-breeding individuals are homozygous ( both alleles) are the same
  • Law of Dominance states that when different alleles for a characteristic are inherited (heterozygous), the trait of only one (the dominant one) will be expressed. The recessive trait’s phenotype only appears in true-breeding (homozygous) individuals


Trait: Pod Color
GGGreen Pod
GgGreen Pod
ggYellow Pod


  • Law of Segregation states that each genetic trait is produced by a pair of alleles which separate (segregate) during reproduction




  • Law of Independent Assortment states that each factor (gene) is distributed (assorted) randomly and independently of one another in the formation of gametes






Other Patterns of Inheritance:

  • Incomplete dominance occurs in the heterozygous or hybrid genotype where the 2 alleles blend to give a different phenotype
  • Flower color in snapdragons shows incomplete dominance whenever a red flower is crossed with a white flower to produce pink flowers

  • In some populations, multiple alleles (3 or more) may determine a trait such as in ABO Blood type
  • Alleles A & B are dominant, while O is recessive




  • Polygenic inheritance occurs whenever many variations in the resulting phenotypes such as in hair, skin, & eye color
  • The expression of a gene is also influenced by environmental factors (example: seasonal change in fur color)