Chapter 10: Mendel

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• Chapter 10 Mendel Meiosis
• Section 10.1
• Mendels Laws of Heredity

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• The study of heredity actually began in an
  Austrian Monastery in the mid 19th century.
• Mendels Interests and Experiments w/ Pea Plants
• Gregor Mendel carried out the
• first important studies of heredity
• A. General Terms
• 1. Heredity - the passing on of
• characteristics from
• parents to offspring.

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• 2. Genetics - the branch of biology which
  studies heredity.
• 3. Traits - characteristics that are inherited.
• B. Choosing the right subject
• Mendel studied many types of plants before
  deciding on using pea plants.
• 1. Why Peas?
• a) peas reproduce sexually they
• have two distinct sex cells, male
• female

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• b) in pea plants, both male and female gametes
  are on the same flower therefore pollination is
  very easy to manipulate.
• c) peas reproduce quickly, thus the results can
  be seen quickly.
• 2. Gametes - the sex cells egg (ovule) and
  sperm/pollen.
• in plants
• a) female gamete ovule
• b) male gamete pollen grain

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• Parts of a flower

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• 3. Pollination - the transfer of the male pollen
  grain to the pistil of a flower.
• a) self - pollination - a reproductive process
  in which a
• plant/ flower fertilizes itself.
• b) cross - pollination - pollination between
  different
• plants/ flowers of the same species.
• 4. Fertilization - the uniting of male and
  female gametes in plants it occurs when the male
  gamete of the pollen grain meets and fuses with
  the female gamete in the ovule.
• upon fertilization the ovule becomes a seed.

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• C. What Mendel Did The Experiment
• 1. Mendel chose several characteristics of pea
  plants and studied each characteristics
  contrasting traits separately.

Characteristic Dominant Recessive
1) Seed texture Round Wrinkled
2) Seed color Yellow Green
3) Seed coat color Colored White
4) Pod appearance Inflated Constricted
5) Pod color Green Yellow
6) Position of flowers Axial Terminal
7) Stem length Tall Short
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• Mendels Characteristics

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• 2. Mendel grew plants that were pure or
  true-breeders.
• True breeders or pure plants - plants that
  produce
• offspring that are identical to themselves.
• ex) pea plants that are pure for tallness will
  always
• produce offspring that are tall
• when all plants are pure for a specific trait
  , that trait is
• called a strain.
• 3. Mendel produced strains via self-pollination.

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• 4. Mendel was able to get 14 strains, one for
  each contrasting trait. Each of the 14 strains
  were called the Parental or P1 generation. This
  is because these strains serve as the parents or
  parent plants for his experiments.
• 5. The next step Mendel took was to
  cross-pollinate these pure strains w/ the pure
  strains of their contrasting traits.
• ex) P1 Tall X P1 Short
• The offspring of this mating is called the 1st
  Filial
• Generation or F1 Generation.
• Filial sons or daughters

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• 6. Mendel then self-pollinated the F1 generation
  and termed the offspring from this
  self-pollination the Second Filial Generation or
  F2 Generation.
• Summary
• 1) Mendel produced plants pure for each trait
• 2) He self-pollinated them and called them the
• P1 generation
• 3) He cross-pollinated the contrasting P1s to
  get an
• F1 generation P1 --gt F1
• 4) Mendel then self-pollinated the F1s and got
  the
• F2 generation F1 --gt F2

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• Mendels Experiment

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• Mendels Experiment

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• II. Mendels Results and Conclusions
• hybrid - the offspring of parents that have
  different forms of a trait, such as tall and
  short height.
• A. A dry run of Mendels Experiment
• 1. The First Generation
• In one experiment, Mendel crossed a pea plant
  that
• was pure for tallness with a pea plant that
  was pure
• for shortness
• P1 generation --gt Tall Plant x Short Plant

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• the result of this cross was ALL TALL plants
  as if the short plant never existed.
• Therefore
• P1 --gt Tall x Short
• F1 ALL TALL
• Mendel did not stop here, however, if he did he
  would have proposed that Tall X Short will always
  Tall but this is not actually the case.
• 2. The Second Generation
• Mendel decided to self-pollinate the F1
  Generation and
• was surprised by his findings.

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• The F2 Generation was
• 3/4 Tall
• 1/4 Short (31 ratio)
• the short trait that had been lost now
  reappeared!!!!!!
• Summary
• P1 Tall X Short (cross-pollination)
• F1 ALL TALL (self-pollinated)
• F2 3/4 Tall 1/4 Short

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• Tall vs. Short Experiment

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• Purple vs. White Flowers

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• a) this pattern occurred in thousands of
  pollinations. Mendel termed the plants in the F1
  generation Dominant.
• Dominant - the trait that occurs the most
  frequently.
• b) the trait that was lost, but reappeared in
  the F2 generation was termed recessive.
• B. Mendels Three Principles/ Laws
• from his results, Mendel concluded that 3
  principles or laws governed inheritance
• 1) The Principle or Law of Dominance
  Recessiveness
• 2) The Principle or Law of Segregation
• 3) The Principle or Law of Independent Assortment

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• 1. The Principle or Law of Dominance and
  Recessiveness
• a) Mendel declared that some factor was
  responsible
• for controlling which characteristics would be
  expressed
• b) Each characteristic has 2 traits
• ex) height - tall or short
• c) Mendel concluded that each characteristic is
  the
• result of the interaction of the pair of the
  two traits.
• d) Mendel proposed that one factor in a pair
  may mask
• other factor, thus preventing it from having an
  effect or
• being able to be seen or observed.

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• e) Recessive - the factor which was masked or
  could not be observed or physically seen.
• f) Dominant - the factor which masked the other
  factor and thus was readily observed, or
  physically seen.
• 2. The Principle or Law of Segregation
• a) Mendel wanted to know why some traits could
  disappear in the F1 generation, but then reappear
  in the F2 generation.
• b) He figured that if each parent had 2
  factors, then each offspring must have 2 as
  well. Each parent could not pass BOTH of their
  factors to the offspring or else the offspring
  would result in 4 factors

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• Parent X Parent
• (2 factors) (2 factors)
• Offspring
• (2 factors)
• NOT
• Parent X Parent
• (2 factors) (2 factors)
• Offspring
• (4 factors)

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• c) Mendel proposed that each reproductive cell
  received only one factor for each characteristic,
  so if the parent cell had the following
• Assuming that the parents are heterozygous (
  each one has one of each factor), then the
  reproductive cell would receive only ONE factor
  for each characteristic.
• Therefore
• for pod color, the reproductive cell will have
  
• either the green factor OR the yellow factor
• (not both), and for stem length the cell will
  get
• either the tall OR the short factor, not both.

Characteristic Dominant Factor Recessive Factor
Pod color Green Yellow
Stem length Tall Short
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• d) Mendel defined this as the Principle of
  Segregation - so that the two factors for a
  characteristic segregate or separate during the
  formation of eggs and sperm.
• 3. Principle or Law of Independent Assortment
• a) As part of his experiment, Mendel crossed
  plants w/ 2 different characteristics to see if
  one would have an effect on the other.
• ex) he used height and seed color
• Characteristic (D) Traits ( R)
• Height Tall or Short
• Seed Color Yellow or Green

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• b) What these crosses illustrated was that all
  dominant factors did not necessarily appear
  together, therefore, you could have a tall plant
  w/ green seeds or a short yellow seeded one.
• These traits acted Independently of each other.
• c) This served as the basis for Mendels
  Principle or Law of Independent Assortment -
  where factors for different characteristics are
  distributed to reproductive cells independently
  of one another.
• III. Chromosomes Genes
• 1. For the most part, Mendels findings agree w/
  what is known today about genes and chromosomes.

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• Unfortunately for Mendel, he was considered a
  FREAK! He had no solid basis to explain his
  findings and he had no knowledge of what genes or
  chromosomes were.
• 2. Mendel would not receive credit for his
  findings until Walter Sutton proposed the
  Chromosome Theory.
• 3. Today, Mendels characteristics or traits
  have come to be known as GENES.
• genes - a segment of DNA on a chromosome which
• controls a particular hereditary trait.
• 4. Because chromosomes are paired, genes are
  often paired as well.

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• 5. Mendels factors have come to be known as
  ALLELES.
• Alleles - an alternative form of a gene
  represented by
• letters
• Dominant traits CAPITALS
• Recessive traits lowercase
• IV. Genotype Phenotype
• 1. Phenotype - the external appearance or
  physical appearance of an organism.
• 2. Genotype - the genetic makeup of an organism
  combination of the genes.

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• The genotype consists of two alleles
• ex) the dominant tall pea plant would be
• TT or Tt
• the recessive short plant would be tt
• 3. Homozygous - if both alleles of pair are the
  same, the organism is described as homozygous.
• ex) TT homozygous dominant
• tt homozygous recessive
• 4. Heterozygous - if the alleles of the pair are
  NOT the same, the organism is said to be
  heterozygous.
• ex) Tt heterozygous

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• V. Probability, Monohybrid Crosses, and Dihybrid
  Crosses
• A. Probability
• 1. Probability - the likelihood that a certain
  event will occur.
• 2. Equation -
• Probability is determined by the following
  equation
• Probability of one kind of event or item
• of total events or items
• ex) given yellow 6,022
• green 2,001
• total 8,023

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• The probability for yellow would be
• P of one kind yellow (6,022)
  0.75 or 75
• Total all (8,023)
• What would be the Probability of getting green?
• How would you set up the equation?
• P of one kind green (2,001) 0.25
  or 25
• Total all (8,023)

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• B. Monohybrid Crosses
• 1. Monohybrid Crosses - a cross between
  individuals that involves ONE (1) pair of
  contrasting traits.
• ex) yellow vs. green seed color
• 2. Punnett Square - grid type diagram used to
  help biologists in determining probabilities.
  Devised by Reginald Punnett in 1905.

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• Punnett Squares
• Example 1) Homozygous X Homozygous
• (dominant) (recessive)
• TT (Tall) tt (short)
• Steps
• 1) draw a square and divide it into 4 sections

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• 2) Represent the alleles by placing one of the
  parents on top of the boxes, and the other parent
  to the left of the boxes. Be sure that each
  letter gets its own box.
• T T
• t
• t

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• 3) Fill in each box with the letter that is
  above it and the letter that is aside of it.
  Each box should have two letters.
• What are the Phenotypes?
• What are the Genotypes?

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• Example 2) Guinea Pig fur texture
• R dominant rough fur
• r recessive smooth fur
• Homozygous X Heterozygous
• (dominant) RR Rr
• RR X Rr
• What are the phenotypes?
• What are the Genotypes?
• What is the Ratio?

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• R R
• R
• r
• Phenotypes all will have rough fur (4/4)
• Genotypes RR - homozygous dominant
• Rr - heterozygous
• Ratios Phenotype 4 Rough or 4/4 Rough
• Genotype 2RR 2Rr

RR rough RR rough
Rr rough Rr rough
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• Example 3) Fur color in Rabbits
• B Black (dominant)
• b brown (recessive)
• Heterozygous X Heterozygous
• What is the Phenotype ratio?
• What is the Genotype Ratio?

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• B b
• B
• b
• Phenotype Ratio 3 Black 1 brown
• Genotype Ratio 1 BB 2 Bb 1 bb

BB Black Bb Black
Bb Black bb brown
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• What would you get if
• BB x bb
• Phenotypes ?
• Genotypes ?
• What if
• BB x Bb ?

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• The Test Cross
• Test Cross - this is a procedure in which an
  individual of an unknown genotype is crossed w/ a
  homozygous recessive individual. It can
  determine the genotype of any individual whose
  phenotype is dominant.
• ex) BB Bb both represent rabbits w/ black
  coats, how would you know if a parent was BB or
  Bb?
• Need to cross the unknown with a homozygous
  recessive
• brown bunny.

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• B? X bb
• If the following is the result
• B ?
• b
• b
• Then the unknown parent must be Heterozygous Bb,
  that is the only way it could have produced brown
  bunnies.

Bb Black bb brown
Bb Black bb brown
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• If, however, you get the following results
• B B
• b
• b
• Then the unknown parent must have been BB,
  because that would have resulted in all black
  baby bunnies.

Bb Black Bb Black
Bb Black Bb Black
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• C. Dihybrid Crossess
• 1. Dihybrid Cross - a cross between individuals
  that involves TWO PAIRS of contrasting traits.
  You look at 2 different traits at the same time.
• ex) seed color (yellow vs. green)
• height (tall vs. short)
• Doing a Punnett Square for a Dihybrid Cross
• Homozygous X Homozygous
• Predict the results for a pea plant pure for
  round and yellow seeds crossed with a pea plant
  that is pure for wrinkled and green seeds.

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• Dominant Recessive
• R - Round r - wrinkled
• Y - Yellow y - green
• Cross Round Yellow X wrinkled green
• (RRYY) (rryy)
• Steps
• 1) need to figure out the gametes
• 2) use foil method to determine the gametes
• 3) make a grid with 16 boxes
• 4) place each gamete into its corresponding box
• along the top and side.

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• FOIL Method
• Remember, each egg and each sperm can get only
  ONE of each letter. Therefore, each egg should
  have an R and a Y and each sperm should
  receive one R and one Y.
• Once you have figured out the gametes, now you
  can do the punnett square.

FOIL R R Y Y Gametes r r y y Gametes

First 2 R Y RY r y ry
Outer 2 R Y RY r y ry
Inner 2 R Y RY r y ry
Last 2 R Y RY r y ry
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• Setting up and filling in the Punnett square
• Place one set of gametes across the top, and one
  set down the side
• All are heterozygous - Round Yellow

ry ry ry ry
RY RrYy RrYy RrYy RrYy
RY RrYy RrYy RrYy RrYy
RY RrYy RrYy RrYy RrYy
RY RrYy RrYy RrYy RrYy
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• What happens when Mendel self-pollinates these F1
  offspring?
• RrYy x RrYy
• Do the foil method - what are the gametes?
• 1st parent - RY, Ry, rY, ry
• 2nd parent - RY, Ry, rY, ry
• What would the punnett square look like?

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• The F2 results

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• Review