Chapter 14 Mendel

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Reebop Ratios
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Part 1 IntroductionPart 2 Mendelian Genetics
and Beyond
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Part 1 Introduction
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Remember your baby Reebops?

• Lets think about the tail trait.
• What were the possible phenotypes?
• curly tail
• straight tail
• What were the possible genotypes?
• TT curly tail
• Tt curly tail
• tt straight tail

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Important Words

• Homozygous Having two alleles (forms of a gene)
  that are identical (TT or tt).
• Heterozygous Having two alleles (forms of the
  gene) that are different (Tt).
• Dominant The dominant allele is expressed when
  homozygous (TT) and when heterozygous (Tt).
• Recessive The recessive allele is only expressed
  when homozygous (tt).

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• Which of the genotypes below are heterozygous?
  Homozygous?
• Tt TT tt
• Based on these genotypes and phenotypes, which
  allele is dominant, T or t? How do you know?
• TT curly tail
• Tt curly tail
• tt straight tail

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Quick-Think Time

• If your original Reebop parents had 100
  offspring, how many of them would you expect to
  have a curly tail?
• How many of them would you expect to have a
  straight tail?
• Why do you think so?
• Why would someone want to predict the numbers of
  offspring to have a particular trait?

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Punnetts Square

• We can find out the expected number of
  curly-tailed and straight-tailed babies with a
  tool called Punnetts Square.
• We will call it Punnetts Fertilization
  Probability Square.

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Who was Punnettand what is his square?

• REGINALD CRUNDALL PUNNETT (1875-1967)was among
  the first English geneticists. He created the
  Punnett Square to work out what the possible
  offspring of two parents will be.

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Curly-tailed Dad
possible gametes
t
T
T
t
T
Tt
TT
possible gametes
tt
Tt
t
t
T
Curly-tailed Mom
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Quick-Think Time
T
T
T
t
t
T
Tt
TT
T
tt
Tt
t
t
t
How many out of 100?
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Expected Genotype
25
T
T
T
t
t
T
50
Tt
TT
T
tt
Tt
t
t
t
25
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Expected Phenotype
T
T
T
t
t
T
50 25 75 curly-tailed
Tt
TT
T
tt
Tt
t
t
t
25 straight-tailed
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From Percentages to Ratios
25 TT to 50 Tt to 25 tt
Genotypic ratio of 1 to 2 to 1, or 121
75 curly-tailed to 25 straight-tailed
Phenotypic ratio of 3 to 1 or 31
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How can we test this prediction?
Expected genotypic ratio 121
Expected phenotypic ratio 31
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STOP to complete Reebop Ratios activity
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• Part 2 Mendilian Genetics and Beyond

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You and Gregor Mendel

• In the first Reebop activity, you looked at the
  variety of offspring produced by two parents.
• You looked at 7 traits, the gene for each
  residing on a different chromosome.
• You learned about how meiosis and fertilization
  contribute to variation in organisms.

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You and Gregor Mendel

• In the second Reebop activity, you looked at the
  expected and experimental ratios of offspring
  with curly and straight tails.
• You, and a nineteenth-century monk named Gregor
  Mendel have a lot in common.
• Lets find out why.

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Mendel the Garden Pea

• Gregor Mendel, Austrian Monk
• In 1866, Mendel performed breeding experiments
  with garden pea plants and observed consistent,
  predictable patterns of inheritance.
• From his observations, Mendel developed a number
  of principles, today referred to as Mendel's Laws
  of Inheritance.

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Before Mendel

• In the 19th century, when both Gregor Mendel and
  Charles Darwin were alive, blending inheritance
  was a popular idea.
• According to blending inheritance, traits from
  parents blended in offspring producing
  intermediate traits.
• Example The offspring of one short parent and
  one tall parent would be of medium height. This
  offspring would pass the new medium sized trait
  to its own offspring.

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Quick-Think Time

• Imagine that blending inheritance occurs in
  Reebops.
• What type of tail would the offspring of
  curly-tailed and straight-tailed parents have?
• What would happen to variation in this population
  over time?

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Darwin and Mendel
18091882
18221884
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Darwin and Mendel

• Darwin observed that variation, in the form of
  individual differences, exists in every species,
  and is not reduced from one generation to the
  next.
• He proposed that natural selection acts on this
  variation.
• Darwin knew that in order for his idea to be
  valid, a mechanism for preserving and inheriting
  variation had to exist.

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Darwin and Mendel

• THE BIG IDEA
• Mendels work answered Darwins difficult
  question.
• Mendel showed that traits from parents do not
  blend. The traits remain distinct and can be
  passed on to later generations. Genetic variation
  is preserved.
• Mendel also proposed a mechanism for how this
  variation is inherited and preserved.

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Mendel summarized his findings in three theories

• Mendels first law, or the principle of
  segregation
• During the formation of eggs and sperm, paired
  factors (now known as alleles, which reside on
  chromosomes) segregate, or separate.

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Quick-Think Time

• Explain how Mendels Principle of Segregation was
  demonstrated in the Reebop activities.

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Mendel summarized his findings in three theories

• Mendels second law, or the principle of
  independent assortment
• Each pair of factors (now known as alleles)
  separate and recombine into gametes independently
  of each other.

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Quick-Think Time

• Explain how Mendels Principle of Independent
  Assortment was demonstrated in the Reebop
  activities.

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Mendel summarized his findings in three theories.

• Mendels third law, or the law of dominance
• Each characteristic is determined by the
  interaction of two hereditary factors (now called
  alleles), one from each parent. Mendel found that
  one allele was always dominant over the other.

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Quick-Think Time

• Explain how Mendels Law of Dominance was
  demonstrated in the Reebop activities.

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Mendel Probability

• coin (heads/tails)
• probability of heads or tails ½
• probability of getting heads 3 times in a row
• multiplication rule
• (½ x ½ x ½ 1/8)

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Extensions of Mendelian Genetics

• Many researchers have encountered exceptions to
  Mendelian Principles was he wrong?

• Majority of heritable characters not as simple as
  peas
• Phenotypes affected by many genes, sequential,
  jointly, environment, etc.

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Extensions of Mendel

• Incomplete Dominance appearance between
  dominant and recessive

F2 generation 1 2 1 CRCRCRCWCWCW
F1 Generation All CRCW
Eggs
CR
CRCR
Sperm
Cw
121 phenotypic ratio 121 genotypic ratio
CWCW
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Another example of Incomplete Dominance
Chestnut, CC
Palomino, CCcr
Cremello, CcrCcr
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Extensions of Mendel

• Codominance both alleles affect phenotype in
  separate ways
• Only observed at the cellular or molecular level

• e.g., M N bloodgroups (RBC surface marker)
• MM
• NN
• MN (both equally expressed)

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Extensions of Mendel

• Multiple alleles of a single gene ABO blood
  groups

• A, B, AB, O
• Combinations of 3 different alleles
• IA, IB, I
• A and B dominant to O, but not to each other
• A and B are codominant

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Extensions of Mendel

• Multiple alleles of a single gene ABO blood
  groups

• I enzyme that adds sugar molecules to lipids on
  surface of RBC recognized by our immune system
• IB adds galactose (IBIB or IBi) B
• IA adds galactosamine (IAIA or IAi) A
• i does not add a sugar (ii) O
• IA IB adds both sugars AB

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Multiple AllelesABO Blood Groups
Possible alleles from female
Possible alleles from male
Blood Types
A
AB
B
O
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Extensions of Mendel

• Epistasis one gene influences a second gene
  (gene interaction)

• For example in dogs
• B gene determines whether pigment (melanin) is
  brown or black.
• E geneenables or prevents melanin from being
  deposited in the coat.

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Epistatic Ineractions on Coat Color
ee
E_ No dark pigment in fur
Dark pigment in fur Yellow
Lab E_bb E_B_
eebb eeB_ Chocolate Lab Black
Lab Yellow fur, Yellow fur, Brown fur,
Black fur, brown nose, black
nose, nose, lips, nose, lips, lips, eye
rims lips, eye rims eye rims eye
rims
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Pedigree Analysis

• Display of family relationships to help keep
  track of relationships and traits
• phenotypes
• genotypes
• e.g., X-linked clotting disorder
• XX (normal) XY (normal)
• XXh (carrier) XhY (hemophilia)
• XhXh (hemophilia)

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Hemophilia in European Royalty
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Click and choose the button
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Genetic Testing

• fetal testing

• amniocentesis needle inserted in uterus to
  remove 10 mL of fluid from amniotic sac (14-16th
  week)
• chromosomal analysis

• chorionic villus sampling (CVS)
• tube inserted through cervix into uterus removing
  sample of fetal tissue from placenta