Exceptions to Mendelian Genetics

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					Exceptions to Mendelian
        Genetics
        Incomplete Dominance
• phenotype of heterozygote falls between that
  of two homozygotes
  – RR: red flower
  – Rr: pink flower
  – rr: white flower
                Codominance
   Heterozygote expresses both phenotypes simulataneously

   Ex. Blood type


Blood Type          Surface      Plasma          Can Donate to
                    Antigens    Antibodies
    A                  A       Anti B protein       A, AB

    B                  B       Anti A protein       B, AB

   AB               A and B        None               AB

    O                None      Anti A and Anti        all
                                      B
                        Epistasis
• Interaction of two or more genes to control a single
  phenotype.
   – Epistatic gene- gene that determines whether a trait will
     be expressed, can mask other genes
   – Hypostatic gene- gene whose expression is masked by an
     epistatic gene
• Types of Epistasis
   –   Duplicate genes
   –   Complementary Gene Action
   –   Dominant epistasis
   –   Dominant suppression
   –   Recessive epstasis
                Duplicate genes
• Need either A or B
  – wheat kernel color
  – 15:1 ratio
     Genotype        Kernel color    Enzyme activity

       A-B-              Colored      Both A and B

       A-bb              Colored           A

       aaB-              Colored           B

      Aabb               Colorless   Neither a nor B
    Complementary Gene Action
• Need C and P
  – Flower color in sweet pea
  – 9:7 ratio
     Genotype        Flower color   Enzyme Activity
       A-B-            Colored          A and B
       A-bb           Colorless           A
       aaB-           Colorless           B
       aabb           Colorless          none
              Dominant Epistasis
• Color is recessive to no color
  – Fruit color in squash
  – W= white, w= colored, G= yellow, g= green
  – 12:3:1
      Genotype       Fruit Color      Gene Actions

       W-G-            White       Dominant W masks G

       W-gg            White       Dominant W masks G

       wwG-            Yellow         Recessive w

       wwgg            Green          Recessive w
        Dominant Suppression
• Production of the chemical malvidin in the
  plant primula
• both synthesis of malvidin is dominant, K, and
  so is the suppression of it,D.
• 13:3 ratio
              Genotype        Phenotype/Expressoin

                K-D-         No malvidin because of D

                K-dd                Malvidin
                kkD-          No malvidin because D

                kkdd         No malvidin because kk
           Recessive Epistasis
• coat color in mice
• need C to express color A
• 9:3:4 agouti, black, albino

             Genotype           Color
               A-C-             Agouti
               A-cc             Albino
               aaC-             Black
               Aacc             albino
               Lethal Genes
• cause death
• dominant- usually not studied
• recessive- change ratio to out of 3 because 1
  die
  – ratio becomes 1:2
  – example yellow mice
                         Penetrance
• Complete penetrance
   – In a population, 15 individuals carry the dominant allele for
      achondroplasia
      All 15 are achondroplastic dwarves
      Penetrance = 100%
      Complete penetrance
• Incomplete penetrance
   – A population has 20 individuals with the dominant allele for
      neurofibromatosis
      16 individuals show symptoms of the disease
      Penetrance = 80%
• Expressivity :The degree to which a penetrant genotype is phenotypically
  expressed
  Example: neurofibromatosis, differing amount and size of tumors
                More Exceptions
• Pleiotropy- a single gene has multiple effects on phenotype
   – Sickle Cell anemia- physical weakness, slow development,
     hypertrophy of bone marrow, damage to organs, heart
     failure
• Phenocopy- A phenotype caused by the external environment
  which mimics a phenotype that is genetically caused
   – Examples- rickets from vitamin D deficiency, Improper
     development of the limbs, to give a flipper-like
     appearance, Rare dominant allele or taking thalidomide
     during a particular period of pregnancy
           Traits Related to Gender
• Sex-limited traits
   – Genes that affect a particular character that appears in one sex but
      not the other.
   – These genes are usually not on the sex chromosomes (autosomal).
   – Milk yield in cattle, Size and shape of horns in rams, Secondary sex
      characters. Ovary, sperm development.
• Sex-influenced traits
   – Also usually autosomal
   – Appear in both sexes, but in different frequencies or with different
      genotype-phenotype relationship.
   – Pattern baldness is dominant in males and recessive in females. Need
      testosterone to be dominant.
                    Sex Determination
• Sex determination-
     – SRY gene, sex determining region of the Y gene,
       necessary for testis formation
     – Lacking this SRY makes you female.
•   XX- normal female
•   XY- normal male
•   XXY- Kleinfelter’s male, breast development, difficulty reading and writing,
    undersized testes, most sterile
•   XO- Turner’s syndrome, female, short stature, lack of ovarian development,
    cardiovascular problems, infertile,
•   XXX- female, quiet, passive, fertile, but go through menopause earlier than most
•   XYY- male, increased height, more physically active, delayed mental maturation
                       Linkage
• Some genes are so close together that they do
  not cross-over as much
  – Two-Point Test Crosses
     • Should yield 2 recombinants with similar frequencies
     • Should yield parental genotype with similar frequencies
     • (# of recombinants/# of progeny) = recombination
       frequency
     • Recombination frequency of 50% shows the genes are
       unlinked
        Three-Point Test Cross
• Double crossovers
  have the fewest
  progeny
• The only allele
  that changed is in
  the middle
• Gives us overall
  organization and
  distances
              Sex-linked genes
• Y linked are found on the Y chromosome and
  only males can inherit them. Very rare.
• X linked are found on the X chromosome and
  both can inherit them.
  – Dominant-:females more likely to have
     • dwarfism
  – Recessive: males more likely to have
     • hemophilia, red-green colorblindness, duchene
       muscular dystrophy
     • male hemizygous

				
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posted:6/19/2012
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