Mendelelian Genetics

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Mendelelian Genetics
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Gregor Mendel(1822-1884)
Responsible for the Laws governing Inheritance of
Traits
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Gregor Johann Mendel

• Austrian monk
• Studied the inheritance of traits in pea plants
• Developed the laws of inheritance
• Mendel's work was not recognized until the turn
  of the 20th century

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Gregor Johann Mendel

• Between 1856 and 1863, Mendel cultivated and
  tested some 28,000 pea plants
• He found that the plants' offspring retained
  traits of the parents
• Called the Father of Genetics"

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Site of Gregor Mendels experimental garden in
the Czech Republic
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Particulate Inheritance

• Mendel stated that physical traits are inherited
  as particles
• Mendel did not know that the particles were
  actually Chromosomes DNA

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Genetic Terminology

• Trait - any characteristic that can be passed
  from parent to offspring
• Heredity - passing of traits from parent to
  offspring
• Genetics - study of heredity

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Types of Genetic Crosses

• Monohybrid cross - cross involving a single
  traite.g. flower color
• Dihybrid cross - cross involving two traits e.g.
  flower color plant height

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Punnett Square

• Used to help solve genetics problems

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Designer Genes

• Alleles - two forms of a gene (dominant
  recessive)
• Dominant - stronger of two genes expressed in the
  hybrid represented by a capital letter (R)
• Recessive - gene that shows up less often in a
  cross represented by a lowercase letter (r)

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More Terminology

• Genotype - gene combination for a trait (e.g. RR,
  Rr, rr)
• Phenotype - the physical feature resulting from a
  genotype (e.g. red, white)

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Genotype Phenotype in Flowers
Genotype of allelesR red flowerr yellow
flower All genes occur in pairs, so 2 alleles
affect a characteristic Possible combinations
are
Genotypes RR Rr rr Phenotypes RED RED
YELLOW
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Genotypes

• Homozygous genotype - gene combination involving
  2 dominant or 2 recessive genes (e.g. RR or rr)
  also called pure 
• Heterozygous genotype - gene combination of one
  dominant one recessive allele    (e.g. Rr)
  also called hybrid

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Genes and Environment Determine Characteristics
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Mendels Pea Plant Experiments
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Why peas, Pisum sativum?

• Can be grown in a small area
• Produce lots of offspring
• Produce pure plants when allowed to
  self-pollinate several generations
• Can be artificially cross-pollinated

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Reproduction in Flowering Plants

• Pollen contains sperm
• Produced by the stamen
• Ovary contains eggs
• Found inside the flower

Pollen carries sperm to the eggs for
fertilization Self-fertilization can occur in the
same flower Cross-fertilization can occur between
flowers
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Mendels Experimental Methods

• Mendel hand-pollinated flowers using a paintbrush
• He could snip the stamens to prevent
  self-pollination
• Covered each flower with a cloth bag
• He traced traits through the several generations

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How Mendel Began
Mendel produced pure strains by allowing the
plants to self-pollinate for several generations
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Eight Pea Plant Traits

• Seed shape --- Round (R) or Wrinkled (r)
• Seed Color ---- Yellow (Y) or  Green (y)
• Pod Shape --- Smooth (S) or wrinkled (s)
• Pod Color ---  Green (G) or Yellow (g)
• Seed Coat Color ---Gray (G) or White (g)
• Flower position---Axial (A) or Terminal (a)
• Plant Height --- Tall (T) or Short (t)
• Flower color --- Purple (P) or white (p)

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Mendels Experimental Results
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• Did the observed ratio match the theoretical
  ratio?

The theoretical or expected ratio of plants
producing round or wrinkled seeds is 3 round 1
wrinkled Mendels observed ratio was 2.961 The
discrepancy is due to statistical error The
larger the sample the more nearly the results
approximate to the theoretical ratio
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Generation Gap

• Parental P1 Generation the parental generation
  in a breeding experiment.
• F1 generation the first-generation offspring in
  a breeding experiment. (1st filial generation)
• From breeding individuals from the P1 generation
• F2 generation the second-generation offspring
  in a breeding experiment. (2nd filial
  generation)
• From breeding individuals from the F1 generation

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Following the Generations
Cross 2 Pure PlantsTT x tt
Results in all HybridsTt
Cross 2 Hybridsget3 Tall 1 ShortTT, Tt, tt
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Monohybrid Crosses
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P1 Monohybrid Cross

• Trait Seed Shape
• Alleles R Round r Wrinkled
• Cross Round seeds x Wrinkled seeds
• RR x rr

Genotype Rr Phenotype Round GenotypicRatio
100 Rr PhenotypicRatio 100 round
r
r
Rr
Rr
R
R
Rr
Rr
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P1 Monohybrid Cross Review

• Homozygous dominant x Homozygous recessive
• Offspring all Heterozygous (hybrids)
• Offspring called F1 generation
• Genotypic Phenotypic ratio is ALL ALIKE

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F1 Monohybrid Cross

• Trait Seed Shape
• Alleles R Round r Wrinkled
• Cross Round seeds x Round seeds
• Rr x Rr

G.Ratio 121 RRRrrr P.Ratio
31 roundwrinkled
r
R
RR
Rr
R
r
rr
Rr
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F1 Monohybrid Cross Review

• Heterozygous x heterozygous
• Offspring25 Homozygous dominant RR50
  Heterozygous Rr25 Homozygous Recessive rr
• Offspring called F2 generation
• Genotypic ratio is 121
• Phenotypic Ratio is 31

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What Do the Peas Look Like?
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And Now the Test Cross

• In order to find the genotype of a
  dominant-showing trait, the organism must be
  crossed with a homozygous recessive organism.
• There are two possible testcrossesHomozygous
  dominant x homozygous recessiveHeterozygous x
  Homozygous recessive

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Test Cross (1st)

• Trait Seed Shape
• Alleles R Round r Wrinkled
• Cross Round seeds x wrinkled seeds
• RR x rr

Genotype RR, Rr Phenotype Round GenotypicRatio
100 Rr PhenotypicRatio 100 round
r
r
Rr
Rr
R
R
Rr
Rr
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Test Cross (2nd)

• Trait Seed Shape
• Alleles R Round r Wrinkled
• Cross Wrinkled seeds x Round seeds
• rr x Rr

r
R
Genotype Rr, rr Phenotype Round Wrinkled G.
Ratio 11 P.Ratio 11
Rr
rr
r
r
rr
Rr
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Test Cross Results

• So, if Offspring are50 wrinkled rr50 round
  Rr
• Shows that unknown parent is Heterozygous Rr
• But, if offspring are
• 100 round Rr
• Shows that unknown parent is
• Homozygous dominant RR

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• A black male rabbit is mated with a brown female
  rabbit (bb). Their offspring are black and brown
  in a 11 ratio. Use a Punnett square to
  determine the genotype of the black male parent.

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Mendels Laws
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Results of Monohybrid Crosses

• Inheritable factors or genes are responsible for
  all heritable characteristics
• Phenotype is based on Genotype
• Each trait is based on two genes, one from the
  mother and the other from the father
• True-breeding individuals are homozygous ( both
  alleles) are the same

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Law of Dominance
One allele overshadows the other allele for a
trait. The allele that is overshadowed is called
the recessive allele. The one that does the
overshadowing is the dominant allele.

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Law of Dominance
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Law of Segregation

• During the formation of gametes (eggs or sperm),
  the two alleles responsible for a trait separate
  from each other.
• Alleles for a trait are then "recombined" at
  fertilization, producing the genotype for the
  traits of the offspring.

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Applying the Law of Segregation
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Law of Independent Assortment

• Alleles for different traits are distributed to
  sex cells ( offspring) independently of one
  another.
• This law is illustrated by the meiosis babies
  activity and dihybrid crosses.

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Dihybrid Cross

• A breeding experiment that tracks the inheritance
  of two traits.
• Mendels Law of Independent Assortment
• a. Each pair of alleles segregates independently
  during gamete formation
• b. Formula 2n (n of heterozygotes)

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QuestionHow many gametes will be produced for
the following allele arrangements?

• Remember 2n (n of heterozygotes)
• 1. RrYy
• 2. AaBbCCDd
• 3. MmNnOoPPQQRrssTtQq

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Answer
1. RrYy 2n 22 4 gametes RY Ry rY
ry 2. AaBbCCDd 2n 23 8 gametes ABCD
ABCd AbCD AbCd aBCD aBCd abCD abCd
3. MmNnOoPPQQRrssTtQq 2n 26 64 gametes
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Dihybrid Cross

• Traits Seed shape Seed color
• Alleles R round r wrinkled Y
  yellow y green

RrYy x RrYy
RY Ry rY ry
RY Ry rY ry
All possible gamete combinations
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Dihybrid Cross
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Dihybrid Cross
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Dihybrid Cross
Round/Yellow 9Round/green
3wrinkled/Yellow 3wrinkled/green 1 9331
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Practice

• Cross a pea plant with homozygous for round seeds
  and heterozygous for yellow seeds with a pea
  plant heterozygous for round seeds and homozygous
  for yellow seeds.

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Practice
RRYY
RRYy
RRYY
RRYy
RrYY
RrYy
RrYy
RrYY
RRYY
RRYy
RRYY
RRYy
RrYY
RrYy
RrYY
RrYy
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Practice Results

• Phenotypic Ratios
• Round/yellow 100
• Why different than the previous cross? (9331)
• Could this Punnett square be shortened?

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Test Cross

• A mating between an individual of unknown
  genotype and a homozygous recessive individual.
• Example bbC__ x bbcc
• BB brown eyes
• Bb brown eyes
• bb blue eyes
• CC curly hair
• Cc curly hair
• cc straight hair

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Test Cross

• Possible results

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Summary of Mendels laws