Outline for today

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Outline for todays lecture (Ch. 14, Part I)

• Ploidy vs. DNA content
• The basis of heredity ca. 1850s
• Mendels Experiments and Theory
• Law of Segregation
• Law of Independent Assortment
• Introduction to Probability

2
Reminder Homologous chromosomes

• Pair at meiosis
• (all pairs)
• Same sequence (except
• sex chromosomes)

3
Ploidy vs. DNA content in Meiosis
Diploid
Haploid
Diploid Contains two chromosomes from a
homologous pair one from each parent Haploid
Contains only one chromosome from a homologous
pair
4
The Nature of Heredity, ca. 1859

• Observation Offspring generally intermediate in
  phenotype (trait value) between those of
  parents
• Obvious example Human children with one African
  and one Nothern European parent
• Proposed explanation Blending Inheritance
• Genetic material miscible, like paint
• Black White Gray
• Tall Short Medium
• Etc.

5
Blending Inheritance A logical difficulty

• Variation reduced every generation
• Ultimate consequence is a homogeneous population
• At odds with reality
• How to explain variation?
• Sports (Mutation in modern parlance)

6
Gregor Mendel The Origin of Genetics

• Austrian farm boy, entered Augustinian monastary
  in 1843
• Attended University of Vienna in early 1850s
• Learned two things about science
• Do experiments
• Analyze your data (i.e., mathematically)
• 1857, began an experimental program to
  investigate the basis of inheritance (i.e.,
  heredity) with peas

7
Mendels Experiments

• Peas were a fortuitous study organism for several
  reasons
• Many variable characters (e.g., flower color,
  seed shape, seed color, etc.)
• Many varieties that bred true for particular
  traits (e.g., purple flowers, round seeds, etc.)
• Easy to do controlled crosses, both self and
  outcross

8
Mendels Experiments Choice of characters

• Used only discrete characters, i.e., either-or,
  not continuous

Height
9
Mendels Experiments Breeding design

1. Start with lines that breed true for different
   traits, e.g., purple and white flowers
2. First generation of a cross is called P
   (parental)
3. Offspring are F1 (filial)
4. Grand-offspring are F2

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Mendels Experiments Breeding design

1. Cross two true-breeding lines (purple, white)
2. Self F1s
3. Observe phenotypes of MANY F2 offspring and COUNT
   THEM

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Mendels Experiments Results

• Cross two true-breeding lines (purple, white)
• F1s ALL PURPLE
• What do we expect if blending inheritance?

P
X
F1
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Mendels Experiments Breeding design

• Cross two true-breeding lines (purple, white)
• F1s ALL PURPLE
• Self F1s
• F2s are NOT all purple
• 705 purple
• 224 white
• i.e., 31 purple white

13
Mendels Experiments Conclusions

• Heritable Factor (i.e., gene) for white flowers
  did not disappear in the F1, but only the purple
  factor affected flower color.
• "Particulate" inheritance
• Purple is dominant and white is recessive

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Mendels Experiments Important Points

• Followed the pattern of inheritance for multiple
  generations (i.e., gt 1)
• What if the experiment terminated after F1?
• Quantitative Analysis
• Many 19th century botanists would have said some
  white flowers reappeared in F2
• Mendel was lucky!

X
15
Mendels Experiments in modern genetic terms

• Alternative versions of genes account for
  variation in inherited characters
• Alternative versions of genes are alleles
• Alleles reside at the SAME genetic locus
• Relationship between alleles, chromosomes, and
  DNA
• DNA at a locus varies in sequence
• Sequence variants cause different phenotypes
  (e.g., purple and white flowers)

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Mendels Experiments in modern genetic terms
Flower-color locus

• Diploid individuals have homologous pairs of
  chromosomes, one from each parent
• An individual inherits one allele from each
  parent
• Alleles may be same or different
• If different, the dominant allele determines the
  organisms phenotype

Purple allele
White allele
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Mendels Experiments in modern genetic terms

• The two alleles at a locus segregate during
  gamete production
• Each gamete gets only one of the two alleles
  present in somatic cells
• Segregation corresponds to the different gametes
  in meiosis (I or II?)

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Recall Meiosis I Metaphase I

• What about crossing-over?

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Mendels Law of Segregation

• If an individual has identical alleles at a locus
  (i.e., is true-breeding), that allele is present
  in all its gametes
• If an individual has two different alleles at a
  locus, half its gametes receive one allele, half
  receive the other allele

All
half
half
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Genetic Terminology

• If a diploid individual has two copies of the
  SAME ALLELE at a locus (i.e., it got the same
  allele from mom and dad), it is a HOMOZYGOTE
• If it has two different ALLELES at a locus (got a
  different allele from mom than from dad) it is a
  HETEROZYGOTE
• The genetic makeup at a locus (or loci) is the
  individuals GENOTYPE
• An organism's Traits comprise its PHENOTYPE

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Mendels Law of segregation a test

• Genotype PP x pp
• Gametes P p
• Genotype Pp
• Gametes 1/2 P, 1/2 p
• Genotype 1/4 PP, 1/2 Pp, 1/4 pp
• Phenotype 3 purple, 1 white

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Mendels Law of segregation a test
F1
X

• Phenotype
• Genotype Pp Pp
• Ova (female gametes) 1/2 P, 1/2
  p
• Sperm (male gametes) 1/2 P, 1/2 p
• Half of male gametes will be P. Of those, half
  will unite with an ovum that is P.
• Thus, the frequency of PP in the F2 is (1/2)(1/2
  ) 1/4
• Frequency of pp (1/2)(1/2) 1/4,
• Frequency of Pp 2(1/2)(1/2) 1/2

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The Punnett Square
Male Parent
Sperm genotype

• Gamete genotypes of one parent given as columns
• Gamete genotypes of other parent given as rows
• Offspring genotypes given as cells in the table
• Each cell has equal frequency (here 1/4)

Female Parent
Egg genotype
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The Punnett Square
Male Parent
Sperm genotype

• Note that in this cross there are TWO ways to get
  a heterozygote
• P from mom, p from Dad
• p from mom, P from Dad
• Frequency of heterozygotes 1/4 Pp 1/4 pP
  1/2

Female Parent
Egg genotype
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The Testcross
Male Parent
Sperm genotype

• Individuals homozygous for a dominant allele have
  the same phenotype as heterozygotes
• To determine the genotype of an individual, cross
  it to a known homozygous recessive
• What is the phenotypic ratio among these
  offspring?
• What is the genotype of the unknown individual?

Female Parent
pp
p
p
P-
P
Egg genotype
?
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The Law of Independent Assortment or Why Mendel
was so Lucky

• Mendel's next step was to cross plants that bred
  true for each of TWO traits, e.g....
• seed shape (Round or wrinkled, Round dominant
  R/r)
• seed color (Yellow or green, yellow dominant
  Y/y)
• Parental cross RRYY x rryy
• F1 are Round, Yellow (RrYy)
• Self F1s...

X
P
F1
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The "Dihybrid Cross" - Dependent Assortment

• Hypothesis Loci ("Traits" to Mendel) assort
  together ("dependently")
• If a gamete has an R allele, it also has a Y
  allele (recall P generation was RRYY, rryy)
• If a gamete has an r allele it also has a y
  allele
• What are the expected frequencies of F2
  phenotypes?

RY
ry
RY
Female F1 parent RrYy
ry
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The "Dihybrid Cross" - Dependent Assortment

• Hypothesis Loci assort together ("dependently")
• If a gamete has an R allele, it also has a Y
  allele (recall parents were RRYY, rryy)
• If a gamete has an r allele it also has a y
  allele
• What are the expected frequencies of F2
  phenotypes?

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The "Dihybrid Cross" - Dependent Assortment

• Predict 3/4 round, yellow, 1/4 wrinkled, green
• NOT WHAT MENDEL OBSERVED!

RY
ry
RRYY
RrYy
RY
rRYy
rryy
ry
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The "Dihybrid Cross" - Independent Assortment
Male F1 parent RrYy

• Four combinations of alleles in gametes
• All are equally likely
• Expect traits in 9331 ratio
• THIS IS WHAT MENDEL OBSERVED

RY
Ry
rY
ry
RY
Ry
Female F1 parent RrYy
rY
ry
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Mendel's Laws

• The Law of Segregation - ONE LOCUS
• If the locus is heterozygous, half the gametes
  get one allele, half the gametes get the other
  allele
• The Law of Independent Assortment - MULTIPLE LOCI
• Alleles at each locus segregate independently of
  alleles at other loci
• (When is this not true? or Why was Mendel so
  lucky?)

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Introduction to Probability Theory

• Independent Events - if the outcome of one event
  does not depend on the outcome of some other
  event
• e.g., rolls of a die, flips of a coin,
  segregation of loci on different chromosomes
• The probability of BOTH of two events happening
  is the product of the probability of each event
  happening independently.
• Formally, Pr(A and B) Pr(A) x Pr(B)
• e.g., Pr(two heads on two flips) Pr(1st flip
  heads) x Pr(2nd flip heads)

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Introduction to Probability Theory

• Probability of EITHER of two events happening is
  the sum of the probability of each event
  happening independently
• Formally, Pr(A or B) Pr(A) Pr(B)
• e.g., Pr(one head on two flips) Pr(head,tail or
  tail,head)
• Pr(1st flip tails)Pr(2nd flip heads)
  (1/2)(1/2) 1/4
• Pr(1st flip heads)Pr(2nd flip tails) 1/4
• 1/4 1/4 ½
• Pr(1-locus heterozygote) Pr(Aa) Pr(aA) ¼
  ¼ ½

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For tomorrow...

• Mendelian Genetics, continued
• Pedigree analysis
• Read the rest of Ch. 14