POST MENDELIAN GENETICS

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POST MENDELIAN GENETICS
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EXTENDING MENDEL

• Thomas Hunt Morgan

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MORGAN Drosophila

• Morgan Revealed
• Extensions of Mendels
• Rules

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MORGANS WORK WITH Drosophila
Red Eyes In Flies Is Normal (wild)
White Eyes Is Rare (mutation)
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MORGAN Drosophila

• To Explore How Eye Color is Inherited in Flies
  Morgan crossed

Red-eyed Female
White-eyed Male
X
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MORGAN Drosophila

• Next, Morgan crossed F1

Red-eyed Female
Red-eyed Male
X
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THE DISCOVERY OF SEX CHROMOSOMES

• Nettie Stevens
• Observed differences in chromosomes
• between male female beetles
• Chromosomes named X and Y
• Half male gametes contain Y, other half X
• All female gametes contain X

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THE DISCOVERY OF SEX CHROMOSOMES

• Nettie Stevens
• Developed hypothesis about sex determination
• Male is formed when
• Female is formed when

X
X
Y
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NORMAL SEX CHROMOSOMES
A Male
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BACK TO MORGAN

• Morgan guessed that
• D. melanogaster (like T.
• molitor) had chromosomes
• that differ between sexes

Gamete Formation In a Male Fly
sperm
sperm
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MORGAN Drosophila

• The X Linked Hypothesis
• Morgan hypothesized that the gene controlling eye
  color

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PRACTICE SEX LINKED PROBLEM

• Cross a heterozygous red-eyed female with a
• red-eyed male
• R red eyes, r white eyes
• Genotype of female parent
• Genotype of male parent

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PRACTICE SEX LINKED PROBLEM
PARENT XR / Xr

• Red eyed
• female
• crossed
• with Red
• eyed male
• R red
• r white

PARENT XR / Y
hint gametes along top, whole people on
inside
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MORGAN Drosophila

• Morgans work on Drosophila provided evidence
  that
• The X chromosome contains genes the Y doesnt
• Inheritance patterns of sex-linked genes vary
  between sexes
• Genes are located on chromosomes

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GENE LINKAGE

• Linkage
• Autosomal Genes
• Reside on the autosomal chromosomes
• In humans genes located on chromosome
• Sex-Linked Genes
• Found on sex chromosomes
• In humans genes found on

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MORGAN LINKED GENES

• First examples of linked genes were found on X
  chromosome of Drosophila
• Morgan established that eye color body color
  are linked traits
• Both found on X chromosome of fruit fly

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MORGAN LINKED GENES

• Morgan re-evaluated Mendels Principle of
  Independent Assortment
• Morgan predicted
• Linked genes should be transmitted together
  during gamete formation

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LINKAGE HYPOTHESIS

• Hypothesis (Morgan)
• When two genes occur on ONE
• chromosome, INDEPENDENT
• ASSORTMENT DOES NOT OCCUR

White eyes
A Female (2X chromo)
Red eyes
Body Color Gray body wild type (G) Yellow body
mutant (g)
R R
r r
Gray body
g g
G G
Yellow body
Eye Color Red wild type (R) White mutant
(r)
Meiosis I
R R
r r
G G
g g
Meiosis II
R
R
r
r
Gametes
g
g
G
G
rG
Rg
ONLY 2 gamete types
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GENETIC RECOMBINATION PRODUCTION OF NEW
COMBINATION OF TRAITS
Linked genes can become unlinked through cross
over
r
R
G g G g
Crossing over during meiosis I
r
r
R
R
G
g
g
G
Meiosis II
R
R
r
r
Gametes
G
g
G
g
rg
RG
rG
Rg
Recombinant chromosomes
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RECOMBINATION PROBABILITY

• The farther apart two genes, the higher the
  probability they will be separated during
  crossover

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RECOMBINATION PROBABILITY

• A D are more likely than B C to become
  separated (unlinked)
• B C more likely to be inherited together (stay
  linked)

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GENE MAPPING

• Maps of genes can be constructed from
  recombination data

Linkage map genetic map based on recombination
frequencies
Crossing Over
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recombinant gametes Reflects distance between
2 loci
GENE MAPPING
Gene 1
Crossing over rarely occurs between adjacent
loci, recombinations are rare.
0 Yellow body
0 Yellow body
0 Yellow body
Gene 2
1.4 White eyes
1.4 White eyes
Gene 3
Gene 4
Map units
Gene 5
Gene 6
Gene 7
20 Cut wings
Crossing over almost always occurs
between distant loci, recombinations are
frequent.
Gene 8
Gene 9
Gene 10
Gene 11
Gene 12
Linkage map
Chromosomes are composed of genes
The physical distance between loci determines
the frequency of crossing over
Frequency of cross over can be used to map
physical between loci
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GENE MAPPING

• If of recombinant gametes is high, 2 genes are
  assumed to be far apart on a chromosome
• Map Unit refers to distance between 2 loci
• Function of recombinants