Docstoc

Mendelian Genetics

Document Sample
Mendelian Genetics Powered By Docstoc
					Mendelian Genetics
     Chapter 12, part 1
            Gregor Mendel
• Born in 1822 in Moravia
  (now part of the Czech
  Republic.

• Son of a tenant farmer;
  joined a monastery to
  get an education.

• Deeply interested in
  science, particularly
  heredity.
• At the monastery in
  Brno, Moravia,
  Mendel received the
  support of Abbot
  Napp.

• From 1851-1855,
  studied at the
  University of
  Vienna, but did not
  receive a degree.
• What was understood at the time:

 • Heredity appeared random and
   unpredictable.

 • Many traits seemed to blend in the
   offspring, suggesting a liquid factor
   controlled heredity.

 • Yet some traits, such as red hair, did
   not blend away.
•   With Abbot Napp’s
    encouragement, Mendel
    studied heredity in peas,
    carefully choosing traits that
    did not appear to blend.
    Collected data from 1856 -
    1865.

•   Mendel’s creative
    contribution: he was the first
    to follow single traits from
    generation to generation
    instead of trying to
    document and follow every
    trait in the plants.
•   Mendel presented his
    findings to the Association
    of Natural Research in
    Brno in 1865.

•   Few people recognized
    the significance of
    Mendel’s research. His
    quantitative methods were
    uncommon at the time,
    and the “blending” theory
    was widely accepted.
• In 1868, Mendel
  became abbot of his
  monastery.

• His religious work left
  little time for research,
  which he set aside,
  though he was always
  convinced he had made
  a valuable contribution
  to science.
• Mendel died in 1884. Sixteen
  years later, in 1900, his work
  was rediscovered by Hugo
  de Vries and others looking
  for clues into the puzzle of
  heredity.

• Though criticized in some
  details, the main body of
  Mendel’s work still stands.
      Mendel’s Laws

• A scientific law is an evidence-based
  description of a natural phenomenon in
  a given set of circumstances.
• Mendel’s three Laws of Heredity
  describe what Mendel observed in
  patterns of inherited traits.
Three Laws of Heredity


• Law of Dominance
• Law of Segregation
• Law of Independent Assortment
   Law of Dominance
• Traits are controlled by two factors that
  can be called “dominant” or “recessive.”
• A “dominant” trait shows if the offspring
  inherits at least one dominant factor
  from one parent.
• A “recessive” trait shows only if the
  offspring inherits two recessive factors,
  one from each parent.
                                In this cross between two
                X                  purple-flowered pea
                                plants, one-quarter of the
                                   offspring have white
                                          flowers.

                                    Based just on this
                                  information, which is
                                dominant: white or purple
                                  flowers? How do you
                                         know?

Hint: “Dominance” is not based on numbers of individuals
 with the trait. It is based on the number of copies of the
       allele that must be inherited to show the trait.
 The offspring of a purple-flowered pea plant and a white-
flowered pea plant all have purple flowers. The purple trait
                     is dominant. Why?
           RR                              rr
                        pollen
                                                      Parental
                                                      generation (P)
                        pollen
                   cross-fertilize
       true-breeding,                true-breeding,
      purple-flowered                white-flowered
            plant                         plant

                                                      First-generation
                                                      offspring (F1)
                                 Rr
Offspring of the F1 generation (the hybrids) may be purple-
flowered if they inherit at least one factor for purple flowers,
or may be white flowered if they inherit the white factor from
                         both parents.

                                               First-
             Rr                           Rr
                            X                  generation
                                               offspring (F1)


        RR        Rr        Rr       rr

                                               Second-
                                               generation
                                               offspring (F2)

               3/4 purple         1/4 white
 The purple-flowered trait    The white-flowered trait is
   is dominant because          recessive because an
  each an individual who      individual must inherit two
inherits at least one copy   copies of the white allele (r)
  of the purple allele (R)         to show the white
     shows the purple                  phenotype.
        phenotype.




                                              Same letter,
genotypes:    RR or Rr                rr    different case =
                                              same gene,
phenotype     purple                white    different allele
      Solving problems involving dominance


 Dexter has freckles. So
  does his wife, Darla.
Their son, Derek has no     Dexter                    Darla
   freckles. Is having     freckles                 freckles
freckles a dominant or a
     recessive trait?                   Derek
                                      no freckles
  Law of Segregation

• Each individual has a pair of factors
  controlling each trait, one inherited from
  each biological parent.
• During the formation of gametes (sex
  cells) these two factors separate. Only
  one ends up in each sex cell.
 In modern terms, the homozygous parents in the P
  generation can pass one one kind of allele to their
                     offspring.

   homozygous parent                 gametes
                       gene



    A     A                     A           A




Homologous chromosomes
The heterozygous parents of the F1 generation have two
alleles for the gene in question, and can pass one or the
           other, but not both, to their offspring.

    heterozygous parent                gametes
                          gene


      A      a                    A           a




 Homologous chromosomes
The genotypes can be represented with letters, which
symbolize the alleles: capital for dominant alleles, small
                   case for recessive.
        purple parent



   PP                                P      +       P

                                    all P sperm and eggs


        white parent


   pp                                p      +      p

                                    all p sperm and eggs
When the gametes join to produce the F1 generation, all
 offspring of homozygous dominant and homozygous
         recessive parents are heterozygous.

                                          F1
     gametes of parents
                                      offspring
   sperm           eggs

     P       +      p                             Pp



            or


     p       +      P                             Pp
gametes from         F2
F1 plants (Pp)   offspring         The heterozygous F1
sperm eggs                           individuals can put
  P   +   P                  PP   either a dominant OR a
                                  recessive allele in each
                                      of their gametes.
  P   +   p                  Pp



  p   +   P                  Pp



  p   +   p                  pp
A Punnet square is one
                                                   Pp
   way to predict the                         self-fertilize
 outcome of a cross by
                                      1/2 P      eggs 1/2 p
showing all the possible
 combinations of all the
   possible gametes.       1/2 P

                                       1/4 PP            1/4 Pp


                              sperm
                           1/2 p

                                       1/4 pP            1/4 pp
    Solving single-gene (monohybrid) crosses with
     Mendelian (dominant-recessive) inheritance.


Tomato fruit color can be red
or yellow.
a. A red tomato plant is
crossed with a yellow tomato
plant, and all the offspring
have red tomatoes. Which trait
is dominant?
b. If two of the resulting hybrid
red tomato plants are crossed,
what will be the ratio of
phenotypes in the offspring?
    Solving single-gene (monohybrid) crosses with
     Mendelian (dominant-recessive) inheritance.


Tomato fruit color can be red
or yellow.
a. A red tomato plant is
crossed with a yellow tomato
plant, and all the offspring
have red tomatoes. Which trait
is dominant?
b. If two of the resulting hybrid
red tomato plants are crossed,
what will be the ratio of
phenotypes in the offspring?
Law of Independent Assortment


  • When genetic factors segregate in the
    gametes, they segregate independently
    of one another. A dominant allele for one
    trait does not guarantee inheritance of a
    dominant allele for a different trait.
                                   Trait       Dominant form     Recessive form
                                   Seed
                                   shape
                                              smooth           wrinkled
 All organisms have multiple       Seed
                                   color      yellow           green
inheritable traits controlled by   Pod
                                   shape
            genes.                            inflated         constricted
                                   Pod
                                   color      green            yellow

    Each trait is inherited        Flower
                                   color
 independently of the others.
                                              purple           white
A pea plant may, for example,
                                   Flower
     have yellow seeds             location
                                              at leaf          at tips of
                                              junctions        branches
(dominant) but white flowers
         (recessive).              Plant
                                   size
                                              tall             dwarf
                                              (1.8 to          (0.2 to 0.4
                                              2 meters)        meters)
                                                                S
                                                                     s               pairs of alleles on homologous
                                                                                     chromosomes in diploid cells
                                                                 Y
                                                                         y

                                                            chromosomes
                                                               replicate

    Traits carried on               S       Y
                                                        replicated homologues
                                                        pair during metaphase
                                                        of meiosis I,
                                                                                             S   y

        separate                                        orienting like this
                                                              or like this
                                        s   y                                                s   Y
   chromosomes sort                                             meiosis I
 independently of one   S       Y               s       y                    S       y                   s       Y
another during gamete   S       Y               s       y                    S       y                   s       Y
       formation.                                               meiosis II

                        S           S           s           s            S               S           s           s
                            Y           Y           y           y                y           y               Y       Y

                                    independent assortment produces four equally
                                    likely allele combinations during meiosis


   Notice that each gamete receives ONE s-bearing and
    ONE y-bearing chromosome from the original cell.
 Now consider this in terms of                                           S
                                                                              s
        genotypes:
                                                                          Y
                                                                                  y
  Genotype of this                                                   chromosomes
                                                                        replicate
parent (for these two                                            replicated homologues
                                                     Y           pair during metaphase                S   y
                                             S
   traits) is SsYy                                               of meiosis I,
                                                                 orienting like this
                                                                       or like this
                                                 s   y                                                s   Y
                                                                         meiosis I
 Meiosis puts ONE S-
                                  S      Y               s       y                    S       y                   s       Y
  bearing and one Y-              S      Y               s       y                    S       y                   s       Y
bearing chromosome in                                                    meiosis II
     each gamete.                S           S           s           s            S               S           s           s
                                     Y           Y           y           y                y           y               Y       Y

                             independent assortment produces four equally
    Genotypes of the         likely allele combinations during meiosis

 gametes that this parent SY              sy                Sy          sY
    can produce are:
                                                                  SsYy
This Punnet square shows a                                     self-fertilize

  cross between two pea                                           eggs
                                                  1/4 SY    1/4 Sy 1/4 sY       1/4 sy
      plants which are
                                         1/4 SY
heterozygous for two traits.                      1/16 SSYY 1/16 SSYy 1/16SsYY 1/16 SsYy


                                         1/4 Sy




                                 sperm
                                                  1/16 SSyY 1/16 SSyy 1/16 SsyY 1/16 Ssyy
 Again, the Punnet square
                                         1/4 sY
   represents all possible                        1/16 sSYY 1/16 sSYy 1/16 ssYY 1/16 ssYy

     combinations of the                 1/4 sy
                                                  1/16 sSyY 1/16 sSyy 1/16 ssyY 1/16 ssyy
gametes that the plants can
                                            seed shape      seed color   phenotypic ratio
  donate to their offspring.                                                 (9:3:3:1)
                                           3/4 smooth       3/4 yellow = 9/16 smooth yellow
They must put one copy of                  3/4 smooth       1/4 green = 3/16 smooth green

a gene for each trait in their             1/4 wrinkled     3/4 yellow = 3/16 wrinkled green
                                           1/4 wrinkled     1/4 green = 1/16 wrinkled yellow
         gametes.
 Solving dihybrid crosses with Mendelian (dominant-
               recessive) inheritance.

Pea plants can be tall (T) or
short (t) and produce purple
(R) or white (r) blossoms.
a. A pure-breeding tall plant
with purple flowers (TTRR) is
crossed with a pure-breeding
short plant with white flowers
(ttrr). What will the offspring
look like?
b. If two of the hybrid (F1)
plants are crossed, what
offspring can they produce?
 Solving dihybrid crosses with Mendelian (dominant-
               recessive) inheritance.

Pea plants can be tall (T) or
short (t) and produce purple
(R) or white (r) blossoms.
a. A pure-breeding tall plant
with purple flowers (TTRR) is
crossed with a pure-breeding
short plant with white flowers
(ttrr). What will the offspring
look like?
b. If two of the hybrid (F1)
plants are crossed, what
offspring can they produce?
Laws: “proven” forever?
• Mendel’s Laws were good descriptions
  of what he observed in the peas and
  other plants he worked with.
• New knowledge accumulated since
  Mendel’s time has refined his ideas.
  While his laws still hold true in some
  instances, there are many exceptions
  that we will explore in the next
  presentations.
              Recap
• Genes may have multiple alleles, such
  as dominant and recessive alleles.
• Chromosomes, which carry genes,
  separate from one another during
  gamete formation.
• Chromosomes sort independently of one
  another during gamete formation, but
  each gamete gets ONE of each kind of
  chromosome.

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:17
posted:8/7/2011
language:English
pages:34