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MENDELIAN GENETICS - UMBC

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					MENDELIAN GENETICS
                 OBJECTIVES
• Understand Mendel’s principles governing genetics

• Understand meaning of relevant vocabulary

• Predict results of mono/dihybrid crosses using Punnett
  square

• Know exceptions to rules
GENETICS                      Gene (Character):

TERMINOLOGY                   Locus (loci= pl.):

Brown   Blue                  Allele (Trait):
Eyes    Eyes



               Black
                              Genotype:
                       Red
               Hair    Hair



                              Phenotype:
              GENETICS
• Genetics:



• Heredity:
                 MENDEL
• Gregor Mendel

  – Interested in basic patterns governing the
    transmission of traits from parent to offspring

  – Mendel’s work precedes an understanding of
    meiosis/genes/chromosomes
        HISTORICAL HYPOTHESES
        REGARDING INHERITANCE
Blending Inheritance Hypothesis:
  – Offspring represent blend of characteristics from two
    parents

Inheritance of Acquired Characteristics Hypothesis
  – Traits in parents are modified, through use, and
    passed on to offspring in modified form
          MENDEL
• Mendel performed crosses between pure bred
  garden peas
     • Began studying inheritance of ONE trait




     • Later, multiple traits studied together
         MENDEL’S CROSSES
• Mendel's plants had two observable “forms”
  (alleles) of each “character” (gene)

  – Green or Yellow seeds
                            • Round or Wrinkled seeds
   Trait                Forms of Trait (Phenotype)


                                 or
Seed shape
              Round                          Wrinkled
                   Round                   Wrinkled


                                 or
Seed color
              Yellow                           Green
                        Yellow              Green


Pod shape                             or
             Inflated                       Constructed
                 Smooth                    Constricted


                                      or
Pod color
                 Green
              Green                         Yellow
                                                Yellow
   Trait       Forms of Trait (Phenotype)


Flower                   or
color
              Purple                   White
                Purple             White

Flower
and                      or
pod
position   Axial (on stem)
           Axial (on stem)        Terminal (at tip)
                               Terminal (attip)



 Stem
 length                   or


                Dwarf                 Tall
                Tall                   Dwarf
          MENDEL
• Looked at traits exhibited by progeny of crosses

  – He studied phenotype


        – Found observable evidence of how parents transmit
          traits to offspring
   MENDEL’S PEA PLANTS
• Pisum sativum

  – Can self fertilize
                                   Male parts = anthers
                                            –Pollen grain contain sperm

SELF FERTILIZATION                 Female parts = pistil
                                            –Ovule = eggs

        Stigma (receives pollen)




                                                       Anthers
                                                       (produce pollen
                                                       grains, which
                                                       contain male
                                                       gametes)




                 Ovules (produce female gametes)
      MENDEL’S PEA PLANTS
Controlled Mating:
  – Cut off reproductive organ

  – Cross fertilize plants in a controlled fashion
        – Plant that contributes pollen

        – Plant that receives the pollen
CROSS-POLLINATION
          1. Remove anthers
          from one plant.




          2. Collect pollen
          from a different plant.



          3. Transfer pollen
          to stigma of the 1st plant
          (plant without anthers)
  MENDEL’S CROSSES
   Round Parent x Wrinkled Parent



                                    All Round
                                    Offspring
                                    (F1)


(Wrinkled trait disappeared!)
        MENDEL’S CROSSES
Next, Mendel crossed F1 (bro-sis mating):
            Round    x   Round   (F1)




                                   Get both types of
                                   offspring (F2)


        Wrinkled trait re-emerges in F2
         MENDEL’S CROSSES
Mendel Determines (Phenotypic) Ratio:

F2: 5474 Round : 1850 Wrinkled

            » Nearly 3          : 1

  – Similar pattern observed in other 6 traits
     MENDEL’S CONCLUSIONS
• There are alternative versions of genes

• For each inherited character, an organism inherits TWO
  alleles
      • Inherit same two alleles
      • Inherit two different alleles


• If the two alleles differ, one allele determines the
  organism’s appearance

• Each gamete carries ONE allele for ea. character
        MENDEL’S SYMBOLS
• Mendel assigned symbols to hereditary
  characters

• Letters represent versions of a gene (alleles):
  – Ex: If Letter “r” represents a gene then:
        – R symbolizes dominant allele
        – r symbolizes recessive allele
           PUNNETT SQUARE
• R.C. Punnett
  – Leading geneticist in early 1900s

  – Invented technique called the “Punnett Square”
        – Place gamete possibilities for 2 parents along axes

        – Internal boxes represent union of gametes

        – Used to predict genotype and phenotype of potential zygote
MENDEL’S CROSS OF PURE LINE PEA
           PLANTS
                   PARENT: R R


                                              R is round
     PARENT: r r


                                              r is wrinkled




   ** hint: gametes along top, “whole people” on inside
     MENDEL’S CROSS OF F1 PLANTS

Crossing the F1s:     Rr   x     Rr



                                               R = round
                                               r = wrinkled




• Phenotypic Ratio:        • Genotypic Ratio
        POSSIBLE GENOTYPES
Three possible genotypes for each gene in a diploid cell:
• Homozygous:

      • Homozygous Dominant:

      • Homozygous Recessive:

• Heterozygous:
PROBABILITY THEORY
         In Diploid Organisms,

         • Each allele has 50% chance of
           being found in a particular
           gamete

         • To calculate likelihood of two
           alleles combining (i.e.
           fertilization):
       MENDEL’S CROSSES
• Dihybrid Crosses:

  – Mendel used pea plants to follow inheritance of
    TWO traits

     • Another pattern began to emerge…
          MENDEL’S SYMBOLS
• Seed Shape:
     • R is dominant allele (Round)
     • r is recessive allele (Wrinkled)


• Seed Color:
     • Y is dominant allele (Yellow)
     • y is recessive allele (Green)
 F2 OFFSPRING FROM DIHYBRID CROSS



F2 generation
phenotype



 Mendel’s       315     101         108        32 = 556
 Numbers

Fraction of
                /16      /16        /16        /16 = 1
progeny


Note: New mixtures of traits are observed in F2!!
PUNNETT SQUARE FOR TWO TRAITS
Do genes segregate into gametes
together or independently?

  – If together, we end up with only 2
    potential phenotypes in the F2
    generation
PUNNETT SQUARE FOR TWO TRAITS
If genes segregate independently
of each other, we end up with
FOUR different phenotypes

  – Some combos of traits in F2 are
    unique; differ from parental
    generation
    MENDEL’S LAW OF
INDEPENDENT ASSORTMENT

• Law of Independent Assortment:
     INDEPENDENT ASSORTMENT
During meiosis I, homologs can line up two different ways before separating.



   Brown eyes                  Blue eyes


                                           OR
    Black hair
                               Red hair




      Brown eyes         Blue eyes              Brown eyes       Blue eyes
      Black hair         Red hair               Red hair         Black hair
           THE TEST CROSS

• To determine genotype of an individual
Homozygous
                  THE TEST CROSS
recessive
parent

             YOUR
  rryy       HOMEWORK!         ?




         All ry

                  1/4    1/4       1/4   1/4
               MENDEL
Why Mendel Was Successful:
    • Unbiased
    • Selected good model organism
    • Used pure breeds as parents
    • Large sample size
    • Quantitative analysis
    • Controlled experiments
    • Studied obvious traits
CONCLUSIONS: MENDELIAN GENETICS
 • Mendel studied traits influenced by a single gene
   (two alleles)

 • Traits exhibited
   complete dominance



 • Autosomal inheritance

				
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posted:6/23/2012
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