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Biology 2014-2015

Mendelian Genetics

SWS Biology
SWS Life Science

Sec. 11-1, 11-2, 11-3


Gregor Johann Mendel (1822-1884)

v     Austrian monk

v     Studied the inheritance of traits in pea plants

v     Developed the laws of inheritance

  • Mendel's work was not recognized until the turn of the 20th century
  • Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants
  • He found that the plants' offspring retained traits of the parents
  • Called the “Father of Genetics"

Genetic Terminology

v     Trait - any characteristic that can be passed from parent to offspring

      Factors that influence traits are: genes you inherit and your environment

      Genes for a trait occur in pairs: one from each parent

v     Heredity - passing of traits from parent to offspring

v     Genetics - study of heredity

Designer “Genes”

v     Alleles - two forms of a gene (dominant & recessive)

v     Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R)

v     Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)


v     Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (ex. RR or rr); also called pure 

v     Heterozygous genotype - gene combination of one dominant & one recessive allele    (ex. Rr); also called hybrid

v     Genotype - gene combination for a trait (ex. RR, Rr, rr)

v     Phenotype - the physical feature resulting from a genotype (ex. red, white)

   Generation “Gap”

v     Parental P1 Generation = the parental generation in a breeding experiment.

v     F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation)

     From breeding individuals from the P1 generation

v     F2 generation = the second-generation offspring in a breeding experiment.
(2nd filial generation)

              From breeding individuals from the F1 generation

Types of Genetic Crosses

v     Monohybrid cross - cross involving a single trait
          ex. flower color  

v     Dihybrid cross - cross involving two traits
          ex. flower color & plant height

   Punnett Square

      Used to help solve genetics problems

   Mendel’s Laws

      Results of Monohybrid Crosses

      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

        1.  Law of Dominance

      In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.

      All the offspring will be heterozygous and express only the dominant trait.

      RR x rr yields all Rr (round seeds)

        2.  Law of Segregation

      During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.

      Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

        3.  Law of Independent Assortment

      Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

      This law can be illustrated using dihybrid crosses.

   Dihybrid Cross

      A breeding experiment that tracks the inheritance of two traits.

      Mendel’s “Law of Independent Assortment”

              a. Each pair of alleles segregates

independently during gamete formation

              b. Formula:  2n (n = # of heterozygotes)

   Beyond Dominant and Recessive Alleles

1.  Incomplete Dominance

     - one allele is not completely dominant over another


              RR              WW                  RW

         (red-flowered)         (white-flowered)           (pink-flowered)


     -heterozygous phenotype is somewhere in between the homozygous phenotypes

     -the colors blend together to give a different color

2.  Codominance

     - both alleles contribute to the phenotype

     - In certain chickens, black feathers are codominant with the allele for white feathers.

     - Heterozygous chickens have black and white speckled feathers

     - the black and white colors appear separately

3.  Multiple Alleles              

     - more than two possible alleles exist in a population

     - coat color in rabbits is determined by a single gene with four different alleles

     - four known alleles are:

              C = full color, dominant to all other alleles

              cch = chinchilla, dominant to ch and c alleles

              ch = Himalayan, dominant to c allele

              c = albino, recessive to all other alleles

4.  Polygenic Traits

     - traits controlled by two or more genes

     - polygenic traits often show a wide range of phenotypes

     - skin color in humans comes about partly because more than four different genes probably control this trait