Mendel and his Peas

1
Mendel and his Peas

• Chapter 9

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State Objectives

• CLE 3210.4.5 Recognize how meiosis and sexual
  reproduction contribute to genetic variation in a
  population.
• CLE 3210.4.3 Predict the outcome of monohybrid
  and dihybrid crosses.
• SPI 3210.4.4 Determine the probability of a
  particular trait in an offspring based on the
  genotype of the parents and the particular mode
  of inheritance.
• CLE 3210.4.4 Compare different modes of
  inheritance sex linkage, codominance, incomplete
  dominance, multiple alleles, and polygenic traits.

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Sub-Objectives

• Explain the experiments of Gregor Mendel
• Explain how genes and alleles are related to
  genotypes and phenotypes
• Use a Punnett square to predict genotypes and
  phenotypes

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Gregor Mendel

• A monk
• Worked in the garden at the monestary
• Wrote Experiments in Plant Hybridization in
  1866.
•   Experiments unnoticed until 1900.

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Gregor Mendel

• Wanted to know how traits are passed from one
  generation to the next
• How some traits seem to skip a generation and
  show up in the next
• Chose the pea plant to study

6
Why Peas?

• Grow quickly
• Self pollinating
• Each flower contained both male and female parts

Image from http//www.jic.bbsrc.ac.uk/germplas/pi
sum/zgs4f.htm
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Sexual Reproduction in Peas

• Pollen from the anther of one plant is
  transferred to the stigma of another. Pollen
  travels down to to egg cell

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Image from http//anthro.palomar.edu/mendel/mende
l_1.htm
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Mendels Peas

• Mendel began by studying one trait at a time.
• That way, he could understand the results
• Some of the traits he observed
• Plant height, seed shape, flower color

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How did he start?

• He crossed pure-bred tall plants with pure bred
  tall plants
• Results All tall plants
• He crossed pure-bred short plants with pure bred
  short plants
• Results All short plants

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Cross-pollination

• Anthers of one plant are removed so it can not
  self pollinate
• Pollen from another plant is used to pollinate
  the flower

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Results from cross-pollination

• When the peas were cross pollinated, they
  produced offspring.
• These offspring are the first generation
• In the case of tall and short plants, all the
  offspring came out tall

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Dominant and recessive traits

• The trait that appeared in that first generation
  was called the dominant trait
• Dominant trait masks the presence of other
  traits
• The trait that did not show up he called the
  recessive trait

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Mendels second experiment

• Mendel crossed the individuals from the first
  generation with each other
• The next offspring are called the 2nd generation
• In the second generation, the recessive trait
  reappeared

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Counting the offspring

• Mendel counted the offspring in the second
  generation with each trait to determine the ratio
  of individuals with the dominant trait to those
  with the recessive trait.

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Mendels peas
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Results of Mendels experiment
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Results of Mendels experiment
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Of Genes and Alleles

• Mendel looked at the math and decided for each
  trait offspring had to have two factors one
  from their mother and one from their father
• These factors that coded for the same trait are
  called genes

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So whats a trait?

• A specific characteristic that varies from one
  individual to another
• Mendel looked at seven
• Seed shape, seed color, seed coat color, pod
  shape, pod color, flower position and plant height

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Of Genes and Alleles

• For each gene, there may be more than one form
• These different forms of genes are called alleles

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Terms

•       Gene a unit of heredity on a chromosome.
•       Allele alternate state of a gene.
•       Dominant an allele that masks the
  expression of other alleles.
•       Recessive an allele whose expression is
  masked by dominant alleles.

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Linking with Meiosis

• How does the information of two alleles for each
  gene compare with what we know from meiosis?
• What does each zygote get when sperm and egg
  fertilize?

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What we know from meiosis

• Principle of segregation
• Alleles on homologous chromosomes separate during
  the process of meiosis
• Only one allele from each parent is passed to the
  offspring

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Segregation
Image from http//anthro.palomar.edu/mendel/mende
l_1.htm
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Probability

• The mathematical chance that an event will occur
• If you flip a coin, whats the chance it will
  come up heads?
• What the chance of three tails in a row?
• Biologists use probability to predict the outcome
  of genetic crosses

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Punnett Square

• To understand Mendels conclusions, we use a
  diagram called a Punnett square
• Dominant alleles are symbolized with capital
  letters
• Recessive alleles are symbolized with lower case
  letters

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Remember for each trait there are two alleles

• So a cross from a true breeding tall plant will
  produce a tall offspring whose alleles are
  written, TT
• A true breeding short plant would be tt

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Genotype
TT, tt, or Tt

• The actual letters represent the alleles this
  combination of alleles is called the genotype
• Genotype alleles present in the organism

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Back to our pea plants
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Genotype

• Two possibilities
• Homozygous contains identical alleles (TT or tt)
• Heterozygous contains different alleles (Tt)

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Phenotype

• An organisms appearance, what the gene looks like
  is the phenotype
• Phenotype the physical appearance of the trait
• what it looks like

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Making a Punnett Square
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Baby Steps to a Punnett square

• 1. determine the genotypes of the parent
  organisms 2. write down your "cross" (mating)
  3. draw a p-square 4. "split" the letters of
  the genotype for each parent put them "outside"
  the p-square

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Baby Steps to a Punnett square

• 5. determine the possible genotypes of the
  offspring by filling in the p-square 6.
  summarize results (genotypes phenotypes of
  offspring) 7. bask in the glow of your
  accomplishment !

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Making gametes

• Remember the principle of segregation, especially
  with dihybrid crosses, Each gamete only gets one
  allele for each trait!!!!

Image from http//arbl.cvmbs.colostate.edu/hbooks
/pathphys/reprod/fert/gametes.html
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Are all wrinkled peas yellow??

• Once Mendel found how traits are passed, he
  wanted to know if the segregation of one pair of
  alleles had anything to do with the segregation
  of another pair
• In other words, are all wrinkled peas yellow???

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Dihybrid cross

• Use a Punnett square to track two traits at once
• Works like a monohybrid cross, but you have to
  take care in forming your gametes

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Principle of Independent Assortment

• Genes for different traits do not affect each
  other in segregation
• Works for most traits unless they are linked
  close together on the same chromosome

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Other exceptions to Mendel

• Incomplete dominance
• Codominance
• Multiple Alleles
• Polygenic traits

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Incomplete Dominance

• With incomplete dominance, a cross between
  organisms with two different phenotypes produces
  offspring with a third phenotype that is a
  blending of the parental traits.

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Incomplete dominance

• Neither allele is completely dominant over the
  other
• Example White and red four oclocks
• WhiteW
• Red R

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A Classic Example Snapdragons

• R allele for red flowers W allele for white
  flowers
• red x white ---gt pink RR x WW ---gt 100 RW

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Recognizing Incomplete Dominance

• Two steps 1) Notice that the offspring is
  showing a 3rd phenotype.  Not shown in the
  parents 2) Notice that the trait in the
  offspring is a blend (mixing) of the parental
  traits.

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Codominance

• In Codominance, traits appear together in the
  phenotype of hybrid organisms.
• red x white ---gt red white spotted

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Another classic example Cows

• In cows if you cross a pure bred red cow with a
  pure bred white cow, the offspring are roan
• The color difference in their coats is because
  they have both red and white hairs together

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Practice problems

• Try problems three through five on your practice
  problems sheet

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Multiple alleles

• Genes that have more than two alleles for a trait
• Each individual can only have two, but in the
  population more than two exist

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Another classic example Blood Type

• Humans have three alleles for blood type
• IA Type A
• IB Type B
• I Type O
• A and B are both dominant over O, but are
  codominant with each other

Image from http//www-micro.msb.le.ac.uk/MBChB/bl
oodmap/Blood.html
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Polygenic traits

• Some traits are determined by the interaction of
  many traits
• Examples
• Height Hair color
• Skin color

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Studying genetics

• Thomas Hunt Morgan
• Uses fruit flies Drosophila melanogaster
• Breed a new generation every 14 days
• Used because short generation time allows
  production of many generations

Image from http//www.ceolas.org/fly/intro.html
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Environmental Influences

• Our genes arent all of what we are
• Our environment has influences as well
• Example Heart disease
• People with poor diets have higher incidences of
  heart disease