Mendel’s Laws of Heredity

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Mendels Laws of Heredity

• Section 10.1
• Page 253

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Mendel

• Gregor Mendel (1822 1884)
• Austrian Augustinian Monk that
• taught high school science
• Known as the Father of Modern
• Genetics
• Discovered heredity was passed to successive
  generations but it was 34 years before other
  scientists caught up with his ideas
• The practical result of Mendel's research is that
  it not only changed the way we perceive the
  world, but also the way we live in it

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Why Mendel Succeeded

• What traits do you have that you inherited from
  your parents?
• Eye color
• Hair color
• Shape of your nose
• Shape of your lips
• Shape of your face
• ? ? ?

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Why Mendel Succeeded

• Mendel studied heredity
• Heredity is the passing on of characteristics
  from parents to offspring
• Inherited characteristics are called traits
• Mendel was the first person that predicted how
  traits were transferred from one generation to
  the next
• His studies founded genetics study of heredity

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Why Mendel Succeeded

• Mendel studied garden peas
• Sexual reproduction
• Gametes sex cells
• Male gamete forms in pollen grain
• Female gamete forms in ovary
• Fertilization occurs when male and female gametes
  unite and a zygote is produced
• In a plant fertilization occurs when pollen
  grains from a male reproductive organ is
  transferred to a female reproductive organ and
  this is called pollination

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Pea Plants

• Both male and female organs are located on the
  same flower tightly enclosed by petals
• For this reason pea plants usually self-pollinate
  male and female gametes come from the same
  plant
• This allowed Mendel to remove the male organs
  from one plant and cross-pollinate with another
  plant by dusting the female organ with pollen
  from a DIFFERENT plant

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Controlled Experiments

• Mendel carefully controlled his experiments
• He studied only one trait at a time, controlling
  the variables
• He mathematically analyzed his data
• He used true breeding plants plants from
  populations that have always produced a specific
  trait (true breeding tall plants or true breeding
  short plants)
• True breeding plants are also called purebred

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Mendels Monohybrid Crosses

• The First Generation
• Mendel crossed two different kinds of true
  breeding pea plants to make hybrids
• Hybrid is offspring of two parents with different
  forms of a trait (example tall x short)
• Monohybrid crosses are crossing different forms
  of ONE trait
• Mendel crossed true breeding tall plants with
  true breeding short plants and their offspring
  (the first generation) were all TALL!!

P1 parental generation F1 filal
generation/first generation offspring
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Mendels Monohybrid Crosses

• The Second Generation
• Mendel let the first generation plants
  self-pollinate and when the seeds were planted ¾
  of the plants were tall the other ¼ were short
  (75 tall and 25 short)
• Tall and short plants occurred at a ratio of 31
• Where did the short trait come from?

F2 second generation offspring
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Mendels Monohybrid Crosses

• Mendel found each plant had 2 factors that
  controlled each trait
• These are genes
• Genes have alternate forms called alleles
• Mendels peas had two forms of the height gene
  tall and short

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Mendels Rule of Dominance

• Dominant trait is the trait you observe (see)
• Recessive trait is the trait that is hidden
• When Mendel crossed tall peas with short peas,
  the offspring were tall, this would be the
  dominant trait because you saw tall plants the
  short trait (allele) was hidden and recessive
• Plants with 2 alleles for tallness were tall and
  those with 2 alleles for shortness were short

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Mendels Rule of Dominance

• To write results of crosses letters are used
• The same letter is used for the different forms
  of one gene
• The dominant allele is always written as a
  CAPITAL letter
• The recessive allele is always written as a lower
  case letter
• What alleles would the F1 generation have?

Tall plants were about 6 tall and short plants
were about 2 tall
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Mendels Rule of Dominance

• To write results of crosses letters are used
• The same letter is used for the different forms
  of one gene
• The dominant allele is always written as a
  CAPITAL letter
• The recessive allele is always written as a lower
  case letter
• What alleles would the F1 generation have?

Tall plants were about 6 tall and short plants
were about 2 tall
Tt
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Law of Segregation

• Mendels first law of heredity
• Every individual has two alleles of each gene and
  when gametes are produced, each gamete receives
  one of these alleles
• During fertilization, the gametes randomly pair
  producing four combinations of alleles

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Phenotypes and Genotypes

• Mendels crosses showed that two plants can look
  alike (both be 6 tall) but they can have
  different allele combinations
• The way an organism looks and behaves is called
  its phenotype (tall plants are tall) pheno
  physical
• The allele combination an organism has is its
  genotype (tall plants genotypes are TT or Tt)
  geno genetics
• An organism is homozygous for a trait if both
  alleles are the same (TT or tt) Homo same
• An organism is heterozygous for a trait if both
  alleles are different (Tt) Hetero different
• Therefore Medels pea plants were either
  homozygous tall (TT) or heterozygous tall (Tt) or
  homozygous short (tt)

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Mendels Dihybrid Crosses

• Mendel crossed plants that differed in 2 traits
  dihybrid
• He crossed true breeding plants with round yellow
  seeds (RRYY) with true breeding plants with
  wrinkled green seeds (rryy)
• All the offspring in the F1 generation had round
  yellow seeds
• What alleles are dominant?

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Dihybrid Second Generation

• Mendel then let the F1 generation cross pollinate
• The F2 generation had plants with round yellow
  seeds, round green seeds wrinkled yellow seeds,
  and wrinkled green seeds The phenotypes were in a
  9331 ratio
• This ratio lead Mendel to his second law

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Law of Independent Assortment

• Alleles for different traits are inherited
  independently of each other they are distributed
  to gametes independently of each other
• When a pea plant with alleles RrYy produces
  alleles the R and r separate from each other (law
  of segregation) and the alleles for seed shape (R
  or r) and seed color (Y or y) separate from each
  other (law of independent assortment) and the
  alleles can recombine in four different ways

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Punnett Squares Monohybrid Crosses

• In 1905, Ronald Punnett created a way to find the
  expected proportions of genotypes of the
  offspring in test crosses using Mendels laws
• Phenotypes can be determined after the genotypes
  are found

The Punnett Squares are 2 boxes tall And 2 boxes
wide because each parent Can produce 2 kinds of
gametes
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Punnett Squares Dihybrid Crosses

• Mendels cross for two traits had four possible
  outcomes therefore a Punnett Square for a
  dihybrid cross needs to be 4 squares tall by 4
  squares high
• Separating gametes is more difficult for dihybrid
  crosses
• Use the FOIL method from algebra to separate
  gametes from RrYy parents F RY, O Ry I
  rY and L ry

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Probability

• Punnett squares are good at showing all the
  possibilities of gamete combination and the
  probability that each can occur
• In reality you dont always get the exact ratio
  predicted because genetics follows the rules of
  chance
• The results of probability predicted by Punnett
  squares is more likely to be the outcome with
  large numbers of offspring

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Probability

• Knowing the parents genotypes we can predict the
  probable offspring genotypes
• What is the probability of having RR offspring?
• What is the probabilty of having Rr offspring?
• What is the probabilty of having rr offspring?

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Probability

• What is the probability of having RR offspring?
• 14 or ¼ or 25
• What is the probability of having Rr offspring?
  24 or 2/4 or 50
• What is the probability of having rr offspring?
• 14 or ¼ or 25