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

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

Ø      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)

Genotypes

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.

     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  

   Punnett Square

Ø      Used to help solve genetics problems

   Mendel’s Laws

Ø      Results of Monohybrid Crosses

Ø      Inheritable factors or genes are responsible for all heritable characteristics

Ø      Phenotype is based on Genotype

Ø      Each trait is based on two genes, one from the mother and the other from the father

Ø      True-breeding individuals are homozygous ( both alleles) are the same

        1.  Law of Dominance

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

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

Ø      RR x rr yields all Rr (round seeds)

        2.  Law of Segregation

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

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

        3.  Law of Independent Assortment

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

Ø      This law can be illustrated using dihybrid crosses.

   Dihybrid Cross

Ø      A breeding experiment that tracks the inheritance of two traits.

Ø      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       X        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

              c^ch = chinchilla, dominant to c^h and c alleles

              c^h = 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