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					 Intro to
 1. What is a trait?
 2. What does it mean to have a
  homozygous dominant trait/
 3. What is the P generation?
      Genetics: The
       scientific study of
      Gregor Mendel: The
       father of genetics.
          Monk who studied
          Originated concept of
           fertilization - when
           male and female
           reproductive cells join.
Genes and Dominance
      Mendel studied                  The seven traits:
       seven different pea                 Seed shape
       plant traits:                       Seed color
          Trait: A specific               Seed Coat Color
           characteristic that             Pod Shape
           varies from one
                                           Pod Color
           individual to another.
                                           Flower Position
                                           Plant Height

               Types of Traits

   Dominant - a trait (gene) which can mask the
    recessive form of the trait. (ex. Tall Pea Plant)
    Dominant traits are represented with a capital letter.
   Recessive - a trait (gene) that can be masked by the
    dominant form of the trait. (ex. Short Pea Plant)
    Recessive traits are represented with the lower case
    letter of the dominant trait. (t)
   Alleles - The different forms of a gene
    The Principle of Dominance
   The principle of Dominance states that some
    alleles are dominant and others are recessive.

       The Principle of Probability
    Living things have a minimum of two genes for every trait.
     Genes are located on chromosomes.
    When two genes are present that call for the same trait it is
     called homozygous.
        Homozygous Dominant is two dominant genes.
         (Represented by TT)
        Homozygous Recessive is two recessive genes.
         (Represented by tt)
    When two genes call for different traits it is called
     heterozygous. (Represented by Tt)

   Mendel had a
    question: Do
    recessive alleles
    disappear, or are
    they still present in
    his plants.
   To answer this
    questions, he
    developed the
    Monohybrid Cross.
           Monohybrid Cross
   The P1 generation is the
    parent generation
   Genotype - the type of
    genes an organism has
    for a trait
   Phenotype - the
    characteristics which
    the organism has.
                               tt   TT

           Monohybrid Cross
   A Punnett square
    can be used to
    determine the
    results of a
    monohybrid cross.
   Parents give copies
    of their genes to the
    next generation.
   Cross #1 tall pea plant
    (TT) X short pea plant (tt)
           Results of the P1 Cross
   F1 generation is the
    offspring of the parent           T    T
   Genotype Tt               t
                                      Tt       Tt
   Genotypic Ratio 4:0
   Phenotype Tall
   Phenotypic Ratio 4:0
                                  t   Tt       Tt
   A cross of the F1
    offspring produces the
    F2 generation
   11.2
    Results of the F1 Cross
   F2 generation is the
    offspring of the F1         T    t
   Genotype TT, Tt, tt
   Genotypic Ratio 1:2:1   T   TT       Tt
   Phenotype Tall, Short
   Phenotypic Ratio 3:1
                            t   Tt       tt

     Some Crosses to Try
   Cross TT X Tt
       Find the genotype, genotypic ratio, phenotype,
        and phenotypic ratio.
   Cross Tt X tt
       Find the genotype, genotypic ratio, phenotype,
        and phenotypic ratio.

Independent Assortment
   The Two-factor
    Cross: Mendel
    crossed true-          RRYY x rryy
    breeding plants that
    produced only round       And
    yellow peas with       RrYy x RrYy
    plants that produced
    round yellow seeds.
   Try to complete the
    two-factor crosses:
Independent Assortment
                  The principle of
                   independent assortment
                   states that genes for
                   different traits can
                   segregate independently
                   during the formation of
                  Independent assortment
                   helps account for the
                   many genetic variations
                   observed in plants,
                   animals, and other
    Beyond Dominant and
    Recessive Genes
   Some alleles are neither
    dominant nor recessive,
    and many traits are
    controlled by multiple
    alleles or multiple genes
       Incomplete dominance
       Codominance
       Multiple Alleles
       Polygenetic Traits

    Beyond Dominant and
    Recessive Genes
   Incomplete Dominance: The heterozygous
    phenotype is somewhere in between the two
    homozygous phenotypes
       Ex: A Pink Flower is formed from dominant Red and
        White Flowers

Beyond Dominant and
Recessive Genes
                 Codominance: both
                  alleles contribute to
                  the phenotype.
                     Ex: Chicken Feathers.
                      The allele for black
                      feathers is
                      codominant with the
                      allele for white
                      feathers. The
                      offspring have
                      speckled white and
                      black feathers.
Beyond Dominant and
Recessive Genes
   Multiple Alleles: Many
    genes have more
    than two alleles.
       Ex: More than two
        possible alleles exist
        in a population.
           • Rabbits have 4 colors
             for coats.
           • Human genes for blood

Beyond Dominant and
Recessive Genes
   Polygenic Traits: Traits controlled by two or more
   “Many Genes”
          Ex: Wide Range of Skin Color or Height

   Type of cell
    division where one
    body cell produces
    four gametes,
    each containing
    half the number of
    chromosomes as
    the original cell.
Chromosome Number
             Diploid - a cell with two of
              each kind of chromosome.
              Represented as 2n.
             Haploid - a cell with one of
              each kind of chromosome.
              Represented as n.
             Homologous chromosomes
              - pairs of chromosomes that
              have genes for the same
    Important players in Meiosis
   Gametes - sex cells
       Sperm - Male gamete
       Egg - Female gamete
•   Zygote - a fertilized egg cell
•   In humans gametes contain 23 chromosomes

Important Steps in Meiosis
   Crossing Over:
    During Prophase I of
    meiosis, homologous
    chromosomes may
    exchange genetic
    material. This results
    in new combinations
    of alleles on

Difference Between Mitosis and

   Mitosis results in the production of two
    genetically identical diploid cells, whereas
    meiosis produces four genetically different
    haploid cells.

Mistakes in Meiosis
•   Nondisjunction - the failure of homologous
    chromosomes to separate during meiosis.
    •   Trisomy - meiosis which results in one extra
    •   Monosomy - meiosis which results in one
        missing chromosome
    •   Triplody - meiosis which results in an extra
        set of chromosomes