Mendel

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Mendels Genetics

• Monohybrid Cross

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Gregor Mendel

• As discussed last class, he was an Austrian monk
• He worked at St. Thomas Monastery
• He studied both mathematics and botany

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Why did he actually Succeed?

• He chose the appropriate organism to study
• He designed and performed his experiments
  correctly
• Analysis of the data was done properly

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The Patient

• The common pea plant also known as Pisum sativium
  
• It was chosen for 4 reasons
• It was commercially available
• Easy to grow and mature
• The sex organs are entirely enclosed in the
  flower
• Different varieties had different traits

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Purebred

• What is a purebred organism?
• It is an organism that has descended from
  ancestors of a distinct type or breed
• Why was this important to Mendel and his pea
  plants?
• This is because he used purebred plants to
  conduct all of his experiments

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How did he make sure?

• How did Mendel make sure that his organisms (pea
  plant) were pure bred?
• He bred them as everyone else bred plants, and he
  looked at one trait at a time
• He selected from the progeny, only the plants
  that had the trait that he was looking for
• He then bred them together (F1 x F1) and then he
  continued this for several generations until no
  more of the other trait showed up

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What happened Next?

• He repeated this for all the other traits until
  he had pure bred plants with each trait.

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The first of the Crosses

• So what did he end up doing next?
• He started to cross breed
• He designated the parents as the p generations
  and crossed a true-breeding tall pea plant, with
  a true-breeding short pea plant
• The offspring were of course the F1 generation
  and he also denoted them as Hybrids

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That F1 Generation

• What do you think he saw?
• What was the phenotype of the plants?
• All of the plants were tall
• What is this type of crossing is called?

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Cross?

• Well what is the Greek word for a single unit?
• Well, think back to the first unit, what did we
  call a single sugar
• MONOSACCARIDE
• So now what is the Greek word for a single unit?
• MONO (Greek word for single or alone)

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Creatively

• Creatively the name given to this type of cross
  is Monohybrid
• It is a single cross between two purebred plants
  giving us a hybrid species

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What did his experiment look like?
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Thats the Phenotype

• We have now seen the phenotype
• All of the F1 generations plants were tall
• What do you think that the genotype must be of
  the offspring?
• First we have to learn some more terms

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Terms

• Homozygous Dominant (TT) Two alleles for a
  trait that are the same as a result of pure
  breeding
• Heterozygous (Tt) Having two alleles for a
  trait that are different
• Homozygous Recessive (tt) - Two alleles for a
  trait that are the same as a result of pure
  breeding

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The First Cross P?F1
Tall
Short
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Lets back track

• Yesterday we looked at the different combinations
  of alleles that an individual can have
• We also talked about dominance and recessiveness
• What did we say the dominant allele was?
• What about the recessive allele

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What were the parents?

• The parents that was tall had a double dominant
  allele
• The parent that was short had a double recessive
  allele
• Both of these parents were homozygous

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When he Crossed

• When the parents were crossed, the F1 generation
  was completely heterozygous
• They all expressed the dominant trait, but why?
• This was because the dominant allele trumps the
  recessive allele

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So the First cross is done, what about the second?
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Second Cross Genotype
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Mendelian Ratio

• As Mendel completed the experiment, he found that
  75 of the offspring of the F1 cross were the
  dominant (tall) and 25 were the recessive
  (short)
• The Ratio of 31 is known as the Mendelian ratio

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Based on observations, Why did this happen?

• Each parent in his F1 Generation starts with two
  hereditary factors. One factor is dominant and
  the other recessive
• The factors separate out in the and only one of
  the two factors contribute to the phenotype of
  the offspring

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Why did this happen?

• The offspring inherit one factor from mom and one
  from dad. If the dominant factor is present, it
  will be expressed, even if the recessive one is
  present
• The recessive factors will be express only if the
  recessive factors are present

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The first Law of heredity

• His results gave rise to his first law
• The law of Segregation

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The law of segregation

• His law of segregation states that inherited
  traits are determined by pairs of factors
• These factors segregate in the gametes (after
  meiosis)
• Mendel did not know that his factors were
  actually genes, we know this today

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What do we call this?
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Punnett Squares
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What do they do for us?

• Well, they are used by geneticists so that they
  can calculate the expected ratio (probability)
  and to suggest possible combinations of alleles
  in the offspring
• They also tell us something about the phenotype
  (the appearance of a trait in an organism)

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So we saw TT x tt

• And for that cross we ended up having a ratio of?
• All were Heterozygous (100 - 0)
• What about when we cross the F1 x F1 generations?

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F1 X F1

• Whats the ratio when we are talking about
  phenotype?
• 31 as well, three of the offspring will present
  the dominant gene while the fourth will present
  the recessive gene
• What about the genotype ratio?
• 121 (one homozygous dominant, two heterozygous,
  and one homozygous recessive)

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Example
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Class Work

• Read Pages 123 129
• Complete Questions P135, 1-6, 8-16