112 Chapter 14 Mendelian genetics by m6Dv2G0

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									Mendelian Genetics
Fig. 14-1
• Blending theory versus Particulate theory of
  heredity
                Definitions

• Character = detectable inheritable feature of
  an organism
• Trait = a variant of an inheritable character

• True Breeding = Always producing offspring
  with the same traits as the parents when the
  parents are self-fertilized
                 Definitions

• P generation = parental

• F1 generation = first filial (offspring of P
  generation)

• F2 generation = second filial (offspring of the
  first filial generation)
                 Definitions

• Alleles = alternate forms of a gene

• Monohybrid cross = a mating between
  parents that differ in a single character
• Dihybrid cross = a mating between parental
  varieties that differ in 2 characters
                 Definitions

• Homozygous = 2 identical alleles for a given
  trait (PP or pp)
• Heterozygous = 2 different alleles for a trait
  (Pp)

• Phenotype = An organism’s expressed traits
  (purple or white flowers)
• Genotype = An organism’s genetic makeup
  (PP or Pp or pp)
Fig. 14-2
            TECHNIQUE                         How Mendel
                         1                    made his
                                              crosses

                                  2



            Parental
            generation
            (P)               Stamens
                             Carpel
                              3


                                      4




             RESULTS
            First                         5
            filial
            gener-
            ation
            offspring
            (F1)
Mendel’s Monohybrid Crosses
Fig. 14-3-2
               EXPERIMENT

              P Generation               
              (true-breeding
                  parents)     Purple         White
                               flowers       flowers




              F1 Generation
                 (hybrids)
                                All plants had
                                purple flowers
Fig. 14-3-3
               EXPERIMENT

              P Generation                     
              (true-breeding
                  parents)           Purple         White
                                     flowers       flowers




              F1 Generation
                 (hybrids)
                                       All plants had
                                       purple flowers




              F2 Generation




                          705 purple-flowered 224 white-flowered
                                plants             plants
   F2 generation showed 3:1 ratio (no
                blending)
• From this he reasoned:
   – Alternate forms of genes are responsible for variations in
     inherited characters
   – For each character, an organism inherits two alleles, one
     from each parent
   – If the 2 alleles differ, one is fully expressed (dominant
     allele) and the other is masked, with no noticeable effect on
     the organism’s appearance (recessive allele)
   – The 2 alleles for each character segregate during gamete
     production
Fig. 14-4




                           Allele for purple flowers



                                                   Homologous
            Locus for flower-color gene            pair of
                                                   chromosomes




                            Allele for white flowers
     Law of segregation
• Allele pairs segregate during gamete
  formation and the paired condition is
  restored by the random fusion of gametes at
  fertilization
Fig. 14-5-1
              P Generation


              Appearance:    Purple flowers White flowers
              Genetic makeup:       PP             pp

              Gametes:             P             p
Fig. 14-5-2
              P Generation


              Appearance:    Purple flowers White flowers
              Genetic makeup:       PP             pp

              Gametes:                P                p




              F1 Generation


              Appearance:             Purple flowers
              Genetic makeup:               Pp
              Gametes:          1/
                                  2   P         1/
                                                  2    p
              Punnett Square

• Possible gametes from one parent are listed on
  one side of the square
• Possible gametes from the other parent are
  listed on the other side of the square
Fig. 14-5-3
               P Generation

 Mendel’s
               Appearance:    Purple flowers White flowers
 Law of        Genetic makeup:       PP             pp
 Segregation   Gametes:                 P                   p




               F1 Generation


               Appearance:              Purple flowers
               Genetic makeup:                Pp
               Gametes:           1/
                                    2   P             1/
                                                        2   p

                                                Sperm
               F2 Generation                P          p


                                  P
                                            PP        Pp
                               Eggs

                                  p
                                            Pp        pp


                                        3         1
Fig. 14-6
                Phenotype     Genotype


                 Purple         PP
                                             1
                            (homozygous)




            3    Purple           Pp
                            (heterozygous)

                                             2

                 Purple           Pp
                            (heterozygous)




                  White          pp
            1                                1
                            (homozygous)


                Ratio 3:1    Ratio 1:2:1
Fig. 14-7
                  TECHNIQUE
Testcross:
breeding of an                                   

organism of               Dominant phenotype, Recessive phenotype,
unknown                    unknown genotype:    known genotype:
                               PP or Pp?               pp
genotype with a
homozygous        Predictions
recessive.
                                  If PP                        If Pp
                                                or
                                Sperm                        Sperm
                                p      p                     p       p

                          P                             P
                                Pp    Pp                     Pp     Pp
                   Eggs                         Eggs
                          P                             p
                                Pp    Pp                     pp     pp


                  RESULTS

                                           or
                   All offspring purple         1/2   offspring purple and
                                                      1/2offspring white
Mendel’s Dihybrid Crosses
Fig. 14-8
             EXPERIMENT
            P Generation                    YYRR                         yyrr


                                      Gametes YR                   yr


            F1 Generation
                                                                    YyRr

                                    Hypothesis of                    Hypothesis of
                                     dependent                       independent
            Predictions
                                     assortment                       assortment

                                                               or                                      Sperm
            Predicted                                                             1/            1/           1/          1/
                                                                                    4   YR        4   Yr       4   yR      4    yr
            offspring of                 Sperm
            F2 generation            1/ YR 1/
                                       2     2 yr
                                                                     1/
                                                                       4    YR
                                                                                   YYRR YYRr                  YyRR         YyRr
                         1/
                           2   YR
                                      YYRR         YyRr              1/
                                                                       4     Yr
                  Eggs                                                              YYRr          YYrr                     Yyrr
                                                                                                                  YyRr
                         1/
                           2   yr                               Eggs
                                          YyRr         yyrr          1/
                                                                       4     yR
                                                                                    YyRR          YyRr        yyRR         yyRr
                                    3/           1/
                                      4            4
                                                                     1/
                                                                       4     yr
                               Phenotypic ratio 3:1                                 YyRr          Yyrr            yyRr      yyrr
                                                                           9/            3/                3/            1/
                                                                             16            16                16            16


                                                                                        Phenotypic ratio 9:3:3:1
             RESULTS
            315          108               101            32             Phenotypic ratio approximately 9:3:3:1
    Mendel’s Dihybrid Crosses

• F2 generation showed 9:3:3:1 ratio              (not
  3:1)

• From this he reasoned:
  – Law of independent assortment = each allele
    pair segregates independently of other gene pairs
    during gamete formation.
               Probability

Segregation and independent assortment of
 alleles are random events.

• Random events are independent of each other.
                  Probability
• Rule of multiplication = the probability that
  independent events will occur simultaneously
  is the product of their individual probabilities.

• Rule of Addition = The probability of an
  event that can occur in two or more
  independent ways is the sum of the separate
  probabilities of the different ways.
Fig. 14-9
                    Rr                                                         Rr
             Segregation of                                               Segregation of
            alleles into eggs                                           alleles into sperm




                                                Sperm

                          1/                          1/
                               2        R                  2        r



                                   R                           R
            1/                              R                           r
                 2   R

                                       1/                      1/
                                            4                       4
             Eggs

                                   r                           r
            1/                              R                           r
                 2   r

                                       1/                      1/
                                            4                       4
Variations on Mendelian Genetics
              Dominance

• Incomplete dominance

• Complete dominance

• Codominance
Fig. 14-10-1

               P Generation
                              Red                   White
                              CRCR                  CWCW


                                Gametes   CR   CW
Fig. 14-10-2

               P Generation
                               Red                                     White
                               CRCR                                    CWCW


                                 Gametes    CR             CW




                                                                Pink
               F1 Generation                                    CRCW




                                 Gametes 1/2 CR   1/
                                                       2   CW
Fig. 14-10-3

               P Generation
                               Red                                                      White
                               CRCR                                                     CWCW


                                 Gametes                     CR             CW




                                                                                 Pink
               F1 Generation                                                     CRCW




                                 Gametes 1/2 CR                   1/
                                                                       2    CW

                                                              Sperm
                                                    1/                 1/
                                                         2   CR             2   CW
               F2 Generation
                                      1/
                                           2   CR
                               Eggs                  CRCR                  CRCW

                                      1/
                                           2   CW
                                                    CRCW                   CWCW
          ABO Blood Group

• An example of Codominance and Multiple
  alleles
Fig. 14-11
                            Allele   Carbohydrate
                              IA         A
                             IB          B
                            i            none
             (a) The three alleles for the ABO blood groups
                 and their associated carbohydrates

                            Red blood cell     Phenotype
             Genotype        appearance      (blood group)

             IAIA or IA i                           A


             IBIB or IB i                           B


                 IAIB                               AB


                  ii                                O
             (b) Blood group genotypes and phenotypes
                   Pleiotropy
• Most genes have multiple phenotypic effects, a
  property called pleiotropy

• For example, pleiotropic alleles are responsible for
  the multiple symptoms of certain hereditary
  diseases, such as cystic fibrosis and sickle-cell
  disease
                    Epistasis
• In epistasis, a gene at one locus alters the
  phenotypic expression of a gene at a second locus
• For example, in mice and many other mammals,
  coat color depends on two genes
• One gene determines the pigment color (with alleles
  B for black and b for brown)
• The other gene (with alleles C for color and c for no
  color) determines whether the pigment will be
  deposited in the hair
Fig. 14-12



                                                  
                                     BbCc             BbCc

                         Sperm
                         1/           1/              1/           1/
                              4 BC         4 bC            4 Bc         4 bc
             Eggs
             1/
                  4 BC
                         BBCC          BbCC            BBCc         BbCc
             1/
                  4 bC
                          BbCC         bbCC            BbCc             bbCc
             1/
                  4 Bc
                          BBCc         BbCc            BBcc             Bbcc
             1/
                  4 bc
                             BbCc      bbCc            Bbcc             bbcc


                         9             : 3                   : 4
          Polygenic inheritance

• Additive effect of 2 or more genes determines
  a single phenotypic character

• Quantitative characters = characters that
  vary by degree in a continuous distribution
  rather than by discrete (either/or) qualitative
  differences
Fig. 14-13
                                                                     

                                                 AaBbCc                  AaBbCc

                                   Sperm
                                   1/       1/        1/        1/       1/       1/         1/           1/
                                        8        8         8         8        8        8          8            8

                          1/
                               8
                          1/
                               8
                          1/
                               8
                          1/
                               8
                   Eggs
                          1/
                               8
                          1/
                               8
                          1/
                               8
                          1/
                               8



             Phenotypes:       1/
                                  64
                                            6/
                                                 64
                                                      15/
                                                               64
                                                                     20/
                                                                           64
                                                                                  15/
                                                                                        64
                                                                                                  6/
                                                                                                       64
                                                                                                                   1/
                                                                                                                        64
             Number of
             dark-skin alleles: 0            1             2             3         4                  5             6
  The Environmental Impact on Phenotype

• Another departure from Mendelian genetics arises
  when the phenotype for a character depends on
  environment as well as genotype

• The norm of reaction is the phenotypic range of a
  genotype influenced by the environment

• For example, hydrangea flowers of the same
  genotype range from blue-violet to pink, depending
  on soil acidity
Fig. 14-14




       Norms of reaction are generally broadest for polygenic
         characters
       Such characters are called multifactorial because genetic and
         environmental factors collectively influence phenotype
         Pedigree Analysis

Key
  Male     Affected    Mating
           male
  Female               Offspring, in
           Affected    birth order
           female      (first-born on left)
Fig. 14-15b



         1st generation
         (grandparents)         Ww     ww        ww     Ww



         2nd generation
         (parents, aunts,
         and uncles)         Ww ww ww Ww         Ww     ww

         3rd generation
         (two sisters)

                                            WW   ww
                                            or
                                            Ww


                            Widow’s peak              No widow’s peak
          (a) Is a widow’s peak a dominant or recessive trait?
Fig. 14-15c

          1st generation
          (grandparents)          Ff      Ff        ff     Ff


          2nd generation
          (parents, aunts,
          and uncles)      FF or Ff ff   ff    Ff   Ff     ff

          3rd generation
          (two sisters)

                                               ff   FF
                                                    or
                                                    Ff




                           Attached earlobe               Free earlobe

          (b) Is an attached earlobe a dominant or recessive trait?
Some Human Disorders
    The Behavior of Recessive Alleles
• Recessively inherited disorders show up only in
  individuals homozygous for the allele

• Carriers are heterozygous individuals who carry
  the recessive allele but are phenotypically normal

• Albinism is a recessive condition characterized by a
  lack of pigmentation in skin and hair
Fig. 14-16




                      Parents
             Normal             Normal
               Aa                Aa

             Sperm
                 A               a
Eggs
                                 Aa
                AA
      A                       Normal
              Normal          (carrier)

                 Aa
              Normal            aa
      a                       Albino
              (carrier)
        Some Human Disorders
Recessively inherited disorders
  – Cystic fibrosis

  – Tay-Sachs disease

  – Sickle-cell disease
                Cystic Fibrosis
• Cystic fibrosis is the most common lethal genetic
  disease in the United States,striking one out of
  every 2,500 people of European descent
  – The cystic fibrosis allele results in defective or absent
    chloride transport channels in plasma membranes
  – Symptoms include mucus buildup in some internal
    organs and abnormal absorption of nutrients in the small
    intestine
              Sickle-Cell Disease
• Sickle-cell disease affects one out of 400 African-
  Americans

   – The disease is caused by the substitution of a single
     amino acid in the hemoglobin protein in red blood cells
   – Symptoms include physical weakness, pain, organ
     damage, paralysis, etc, etc.
   Dominantly Inherited Disorders
• Some human disorders are caused by dominant
  alleles
• Dominant alleles that cause a lethal disease are rare
  and arise by mutation

• Achondroplasia is a form of dwarfism caused by a
  rare dominant allele
Fig. 14-17




                      Parents
              Dwarf             Normal
               Dd                dd

             Sperm
                 D               d
Eggs
                Dd             dd
       d
               Dwarf         Normal


                Dd               dd
       d                     Normal
               Dwarf
        Some Human Disorders
Dominantly inherited disorders
  – Huntington’s disease (late-acting lethal
    dominants)


• Huntington’s disease is a degenerative
  disease of the nervous system

• The disease has no obvious phenotypic effects
  until the individual is about 35 to 40 years of
  age
         Multifactorial Disorders
• Many diseases, such as heart disease and cancer,
  have both genetic and environmental components
• Little is understood about the genetic contribution
  to most multifactorial diseases
  Genetic Testing and Counseling
• Genetic counselors can provide information
  to prospective parents concerned about a
  family history for a specific disease
  – Using family histories, genetic counselors help
    couples determine the odds that their children will
    have genetic disorders
  – For a growing number of diseases, tests are
    available that identify carriers and help define the
    odds more accurately
Fig. 14-18

         Amniotic fluid
         withdrawn

   Fetus                         Centrifugation
                                                                       Fetus
                                                                               Suction tube
                                                                               inserted
   Placenta                                                                    through
                                                  Placenta     Chorionic
             Uterus     Cervix                                 villi           cervix

                      Fluid
                                            Bio-
                      Fetal      Several chemical
                      cells      hours     tests                       Fetal
                                                             Several   cells
                                                             hours
                                     Several
                                     weeks




                                 Several                     Several
                                 weeks     Karyotyping       hours

   (a) Amniocentesis                               (b) Chorionic villus sampling (CVS)
Fig. 14-UN2

        Degree of dominance        Description                   Example

        Complete dominance    Heterozygous phenotype
        of one allele         same as that of homo-  PP               Pp
                              zygous dominant


        Incomplete dominance Heterozygous phenotype
        of either allele     intermediate between
                             the two homozygous
                             phenotypes
                                                        C RC R     C RC W C WC W

        Codominance           Heterozygotes: Both
                              phenotypes expressed          IAIB


        Multiple alleles      In the whole population, ABO blood group alleles
                              some genes have more
                              than two alleles          IA , IB , i

        Pleiotropy            One gene is able to      Sickle-cell disease
                              affect multiple
                              phenotypic characters
Fig. 14-UN3
          Relationship among
          genes                   Description               Example

          Epistasis            One gene affects     BbCc               BbCc
                               the expression of
                               another                  BC bC Bc bc
                                                   BC
                                                   bC
                                                   Bc
                                                   bc

                                                        9    :3   :4
          Polygenic            A single phenotypic
          inheritance          character is        AaBbCc         AaBbCc
                               affected by
                               two or more genes

								
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