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Genetics Mendelian Genetics



                            Biology 12
                             A. Allen                        1

                 Mendelian Genetics
Augustinian                                   While assigned to
Monk at Brno                                  teach, he was also
Monastery in                                  assigned to tend
Austria (now                                  the gardens and
Czech Republic)                               grow vegetables
                                              for the monks to
Not a great teacher                           eat.
but well trained in
math, statistics,
physics, and
interested in plants
and heredity.                                 Mountains with
                         Gregor Mendel        short, cool growing
                                              season meant pea
                       “Father of Genetics”   (Pisum sativum)2was
                                              an ideal crop plant.
 Contributions in 1860s (US Civil War Era)
   • Discovered Genes as Particles of Inheritance
   • Discovered Patterns of Inheritance
   • Discovered Genes Come from Both Parents
      Egg + Sperm = Zygote
      Nature vs Nurture

      Sperm means Seed (Homunculus)

   • Discovered One Form of Gene (Allele) Dominant
     to Another
   • Discovered Recessive Allele Expressed in
     Absence of Dominant Allele                             3

Mendel worked with peas (Pisum sativum)
    • Good choice for environment of monastery
    • Network provided unusual varieties for testing
    • Obligate self-pollination reproductive system
         Permits side-by-side genetic barriers
         Cross-pollinations require intentional process

    • Crosses meticulously documented
    • Crosses numerically/statistically analyzed
    • Scientists of 1860s could not understand math
    • Work lost in journals for 50 years!
    • Rediscovered in 1900s independently by 3 scientists
    • Recognized as landmark work!                          4
• Peas naturally self-pollinate
  because the male and female
  flower parts are enclosed by the
  petal arrangement


‘True breeding’ or
‘pure stock’ tall and
short plants

                        Mendel noticed that
      F1                tall peas crossed with
                        short peas yielded all
                        tall peas in the F1
                        generation (first
                        group of offspring)
One Example of Mendel’s Work
            Tall                 x            Dwarf            Phenotype
            TT                                 tt              Genotype
      Homozygous                             Homozygous
       Dominant                               Recessive
                              All Tall          Clearly Tall is Inherited…
F1                                              What happened to Dwarf?
                          Heterozygous           Tall is dominant to Dwarf

                         F1 x F1 = F2
                         possible gametes
     Punnett Square:      T              t      3/     Tall
F2                   T
                         Tall        Tall       1/
                                                     4 Dwarf
          possible       TT           Tt
          gametes                               Dwarf is not missing…just masked as
                         Tall        Dwarf      “recessive” in a diploid state… there
                          Tt          tt        IS a contribution from the short plant
                                                which reappears in F2 generation.

                                                                 Mendel then took
                                                                 the F1 peas and
     F1                                                          crossed them with
                                                                 themselves to
                                                                 produce an F2
                                                                 generation (2nd
                                                                 group of offspring)

Mendel as a Scientist                                              F2               F1 x F1 = F2
                                                                                    possible gametes
Test Cross:                                              Punnett Square:             T             t
   Unknown Tall x                Dwarf                                              Tall          Tall
                                  tt                                           T
                                                               possible             TT             Tt
                                                               gametes              Tall      Dwarf
                                 possible gametes                               t
If Unknown is TT:                                                                    Tt        tt
                                   t             t
                                 Tall           Tall
             possible             Tt             Tt
             gametes                                           Test Progeny All Tall
                                 Tall           Tall
                                  Tt             Tt
                                                                   1/       of F2 Tall are TT
                                 possible gametes                       3
If Unknown is Tt:
                                   t             t                 2/       of F2 Tall are Tt
                                 Tall           Tall
             possible             Tt             Tt
             gametes                                           Test Progeny Half Tall
                                 Dwarf        Dwarf
                         t                                                Half Dwarf
                                  tt           tt                                  11

Another Example of Mendel’s Work
         Green                     x                    Yellow                 Phenotype
P         yy                                             YY                    Genotype
      Homozygous                                       Homozygous
       Recessive                                        Dominant
                         All Yellow                      Clearly Yellow is Inherited…
F1                                                       What happened to Green?
                         Heterozygous                     Yellow is dominant to Green

                         F1 x F1 = F2
                         possible gametes
     Punnett Square:         Y            y
                                                          3/     Yellow
F2                 Y
                        Yellow          Yellow
        possible         YY              Yy                    4 Green
        gametes         Yellow          Green             Green is not missing…just masked
                   y                                      as “recessive” in diploid state
                         Yy              yy                                                  12
Mendel as a Scientist                           F2                   F1 x F1 = F2
                                                                     possible gametes
                                            Punnett Square:
Test Cross:                                                            Y             y
   Unknown Yellow x        Green                              Y
                                                                    Yellow      Yellow
                            yy                 possible              YY          Yy
                                               gametes              Yellow       Green
                         possible gametes                      y
If Unknown is YY:                                                     yy          yy
                          y           y
                        Yellow    Yellow
         possible        Yy        Yy
         gametes                              Test Progeny All Yellow
                        Yellow    Yellow
                         Yy        Yy
                                                1/       of F2 Yellow are YY
                         possible gametes
If Unknown is Yy:                               2/
                          y           y              3   of F2 Yellow are Yy
                        Yellow    Yellow
         possible        Yy        Yy
         gametes                               Test Progeny Half Yellow
                        Green      Green
                    y                                        Half Green
                         yy         yy                             13

Mendel as a Scientist
                                                 Actual Results              Decision
Test Cross:
                                               3 Yellow 2 Green                  Yy
   Unknown Yellow x        Green
            Y?              yy                 2 Yellow 3 Green                  Yy
                         possible gametes      1 Yellow 4 Green                  Yy
If Unknown is YY:
                          y           y          Small families do not follow
                        Yellow    Yellow         expected ratios perfectly!
         possible        Yy        Yy          0 Yellow 5 Green                  Yy
         gametes        Yellow    Yellow         Rare, but it can happen!
                         Yy        Yy          4 Yellow 1 Green                  Yy
                                                 It only takes 1 green to be sure
                         possible gametes        the unknown is Yy!
If Unknown is Yy:
                          y           y        5 Yellow 0 Green                  YY
                        Yellow    Yellow                 <5% chance unknown is Yy
         possible        Yy        Yy           1/       • 1/2 • 1/2 • 1/2 • 1/2 = 1/32
         gametes        Green      Green
                    y                                You could be wrong (rarely)!
                         yy         yy                                          14
Yet Another Example of Mendel’s Work
         Wrinkled              x             Round              Phenotype
          rr                                  RR                Genotype
      Homozygous                            Homozygous
       Recessive                             Dominant
                        All Round             1.        Round is dominant to Wrinkled

      F1 x F1 = F2                                 NEVER use W/R or w/r
                       possible gametes
     Punnett Square:    R            r
F2                 R
                       Round       Round
        possible        RR          Rr              4 Round
        gametes                                1/
                       Round    Wrinkled            4 Wrinkled
                        Rr         rr                                         15

         Vocabulary Challenge
     Gene                                 monohybrid cross
     Homozygous                           genotype
     Allele                               Dominant
     Heterozygous                         dihybrid cross
     Recessive                            Phenotype

            Mendel’s Laws of Heredity
1.   The Law of Segregation:
     The two members of a gene pair
     segregate (separate) from each
     other into the gametes, so that
     one half of the gametes carry
     one member of the pair and the
     other half of the gametes carry
     the other member of the gene
     pair. Each pair of alleles (which
     are located on homologous
     chromosomes) segregates
     during the formation of sex
     Think in terms of meiosis: a
     diploid cell undergoes meiosis
     to produce haploid cells (only
     one allele in a gamete for a
     given gene)


          …Mendel’s Laws of Heredity
     2. Law of Unit Characters: Each parent
        contributes one allele during cross-fertilization.
     • Mendel emphasized this to explain the continued
        presence of shortness in the F1 tall peas and to
        dispel the idea of “Blending Inheritance”
        common in his time.
     • Mendel discovered that the recessive allele
        reappeared in the F2 generation after the F1
        plants were crossed.

              …Mendel’s Laws of Heredity
3.        Law of Dominance: The dominant allele is always expressed
          when the recessive allele is present. Ex: Genotypes TT and Tt both
          result in tall plants. The phenotype of a Tt plant is tall even though
          it has a ‘short’ (t) allele.


              …Mendel’s Laws of Heredity
     4.     Law of independent assortment (which says that an
            organism's individual traits are passed on independently of one

                   Genetics After Mendel

     Incomplete Dominance
     • A Heterozygous genotype that creates an
       intermediate phenotype. In other words,
       when two different alleles are present, the
       phenotypes are blended.


Incomplete Dominance
      Red                Yellow        After 1900 several scientists tried to
P                  x                   replicate Mendel’s crosses using
      PRPR                PYPY         other species including snapdragon.

                                 When these alleles go walking, they both do
                                 some talking !
          All Orange
F1                               OK, so we cannot use R/r nor Y/y so we pick
             PRPY                a third letter…P for the petal color gene.

        F1 x F1 = F2
                         possible gametes
     Punnett Square:     PR           PY
F2                 PR
                        Red        Orange       This F2 will NOT have a 3:1 ratio
                                                of phenotypes.
        possible        PRPR        PRPY
        gametes         Orange     Yellow       Instead it shows a 1:2:1 ratio!
                         PRPY       P YP Y      The exception here proves the rule.
            Make a Prediction
   • Draw a Punnett Square to predict the
     phenotypes of the offspring from a
     Red x Orange cross


In addition to this, there are multiple alleles possible:
      PR = red PY = yellow p = no pigment
The combination of alleles in a diploid determine the flower color:
     PRPR = red                PRp = pink
     PRPY = orange             PYp = cream
     PYPY = yellow             pp = white

Human hair color follows a similar pattern:
   Alleles: HBn = brown HBd = blonde hR = red hbk = black

The combinations of these alleles determine the base hair color:
    HBnHBn = dark brown HBdHBd = blonde            hRhR = red
    H BnHBd = sandy brown HBdhR = strawberry hRhbk = red

    HBnhR = auburn                    blonde
    HBnhbk = dark brown HBdhbk = blonde            hbkhbk = black
                                                  Recessive can
     Dominant does NOT mean frequent!                          25
                                                   be common!
             Genetics After Mendel
    • The situation in which two different alleles for a
      trait are expressed unblended in the phenotype of
      heterozygous individuals. Neither allele is
      dominant or recessive, so that both influence the
      phenotype. Type AB Blood is an example. Such
      traits are said to be codominant.


       Codominance: ABO Blood Type
• There are 4 ABO blood types; A, B, AB, & O      Blood   Allelic
• Your blood type is determined by the               Type    combinations
  presence of antigens on the surface of your
  erythrocytes (red blood cells).                 A       IA IA or IA i
• The three alleles are:
   – IA codes for production of A antigen         B       IB IB or IB i
   – IB codes for production of B antigen
   – i     no antigens produced on                AB      IA IB
• The IA and IB alleles are dominant over the i
  allele, which is always recessive.              O       ii
• Presence of both IA and IB alleles exhibit

          Try this Blood Type
   • Lauren is blood type A. Her dad is type O.
   • Lauren marries her high school sweetheart
     who is type B. Lauren’s father-in-law is
     type O.

   • What are the possible blood types of their


 Another Example of Recessive Being Common: Pisum sativum
  Garden Peas: green seed, wrinkled seed, dwarf stature, white flower
                   yy            rr            tt             aa
                   In other words: a quadruple double-recessive
                     is the most common garden pea on Earth!
 Quantitative Inheritance: multiple genes control trait
              Highest Crop Yield: AABBCCDDEE
         Intermediate Crop Yield: AabbCCDdEe
               Lowest Crop Yield: aabbccddee

                  Darkest Skin Color: AABBCCDDEE
             Intermediate Skin Color: AaBbCcDdEe
                  Lightest Skin Color: aabbccddee
AaBbCcDdEe x AaBbCcDdEe can produce a huge range of colors!
    Yet TV talk show guests argue this point for Maury, etc.30
          Sex-Linked Traits


   What is a Sex-Linked trait?
• Sex-linked traits are due to genes located on
  sex chromosomes.
• Males have XY sex chromosomes
• Females have XX sex chromosomes.
  – The X chromosome contains over 1000 genes
    while the Y chromosome contains as few as 26.
    Therefore, many sex-linked traits are discussed
    in terms of the X-chromosomes.

• Because females have two copies of the X
  chromosome, it is possible to have certain
  traits “hidden” by a dominant copy.
  – However, because males only have one X
    chromosome, the observable phenotype is
    obvious and identifies the genotype.


• When a female contains a recessive allele
  that is hidden by the dominant allele, we
  call them carriers.
  – A carrier maintains the ability to pass on a trait
    even if they do not express/show it.

    Examples of X-linked genes
• Other than determining sex, genes on the X
  chromosome are responsible for traits.
  Some examples are:
    – Hemophilia
    – Red-green color blindness
    – Muscular dystrophy


  …Examples of X-linked genes
Red-green color

• Three types of cones,
  “red’, green’ and ‘blue’
  are receptive different
  wavelengths of light.
• an inability to distinguish
  between red and green,
  is caused by a defect in
  one of the three color-
  sensitive cells in the
      …Examples of X-linked genes
    • Human blood contains special proteins, known as
      clotting factors.
    • Clotting factors help stop bleeding and allow a blood
      vessel to heal after an injury.
    • The last step in the clotting process (also called
      coagulation) is the creation of a "net" that closes the
      torn blood vessel and stops the bleeding. This part of
      the process involves clotting factors VIII and IX.
    • People with hemophilia are deficient in one of those
      factors due to their abnormal genes and, as a result,
      their blood can't clot properly.


      …Examples of X-linked genes
Muscular dystrophy
• incorrect or missing genetic information
  that prevents the body from making the
  proteins needed to build and maintain
  healthy muscles.
• Symptoms may but may include:
• Muscle weakness that slowly gets worse
   – Frequent falls
   – Delayed development of muscle
      motor skills (children)
   – Problems walking
   – Difficulty using one or more muscle
   – Eyelid drooping (ptosis)
   – Drooling
• Hypotonia (low muscle tone)
• Joint contractures (loss of joint
  movement) clubfoot, clawhand, or others                       38
• Scoliosis (curved spine)
             Punnett Practice
• ‘Elephant ears’ (e) is a fictional
   recessive sex-linked trait:
• The woman is a carrier and her
  partner does not have the trait.
    – What % chance do the boys & girls
      have of receiving the trait?
    – What % chance do the boys & girls
      have of having elephant ears?
• If the man has an X-recessive trait
  and his partner does not have it (nor
  is a carrier).
    – What % chance do the boys & girls
      have of receiving the trait?
    – What % chance do the boys & girls
      have of having elephant ears?


               Recessive Lethals
• In some rare instances, recessive traits are
  lethal-- meaning the organism is born very
  weak and sickly or dies not long after birth.
    – When looking at genes on non-sex
      chromosomes (AKA autosomal), we can apply
      standard Punnett square probability.
    – However, when looking at genes on sex
      chromosomes we see that males have a much
      higher tendency for recessive lethals. Why?

Human Pedigree Symbols


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