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Mendelian Genetics
Augustinian While assigned to
Monk at Brno teach, he was also
Monastery in assigned to tend
Austria (now the gardens and
Czech Republic) grow vegetables
for the monks to
Not a great teacher eat.
but well trained in
math, statistics, Mountains with
probability, short, cool growing
physics, and season meant pea
interested in plants (Pisum sativum)
and heredity. Gregor Mendel was an ideal crop
“Father of Genetics” plant.
Contributions in 1860s (US Civil War Era)
• Discovered Genes as Particles of Inheritance
• Discovered Patterns of Inheritance
• Discovered Genes Come from Both Parents
http://forensicservices.utah.gov/biology/pictures/sperm.jpg
Egg + Sperm = Zygote
QuickTime™ and a
TIFF (Uncompressed) decompressor
Nature vs Nurture are needed to see this picture.
Sperm means Seed (Homunculus)
http://academic.evergreen.edu/v/vivianoc/homunculus.gif
• Discovered One Form of Gene (Allele) Dominant
to Another
• Discovered Recessive Allele Expressed in
Absence of Dominant Allele
Mendel worked with peas (Pisum sativum)
• Good choice for environment of monastery
• Network provided unusual varieties for testing
• Obligate self-pollination reproductive system
Permits side-by-side genetic barriers
Cross-pollinations require intentional process
• Crosses meticulously documented
• Crosses numerically/statistically analyzed
• Scientists of 1860s could not understand math
• Work lost in journals for 50 years!
• Rediscovered in 1900s independently by 3 scientists
• Recognized as landmark work!
One Example of Mendel’s Work
Tall x Dwarf Phenotype
P
DD dd Genotype
Homozygous Homozygous
Dominant Recessive
All Tall Clearly Tall is Inherited…
F1 What happened to Dwarf?
Dd
Heterozygous 1. Tall is dominant to Dwarf
2. Use D/d rather than T/t for
symbolic logic
F1 x F1 = F2
possible gametes
Punnett Square: D d
3/ Tall
F2 D
Tall Tall 4
1/ Dwarf
possible DD Dd 4
gametes Tall Dwarf Dwarf is not missing…just masked
d as “recessive” in a diploid state…
Dd dd
there IS a female contribution.
Mendel as a Scientist F2 F1 x F1 = F2
possible gametes
Punnett Square:
Test Cross: D d
Unknown Tall x Dwarf D
Tall Tall
dd possible DD Dd
gametes Tall Dwarf
possible gametes d
If Unknown is DD: Dd dd
d d
Tall Tall
D
possible Dd Dd
gametes Test Progeny All Tall
Tall Tall
D
Dd Dd
1/ of F2 Tall are DD
3
possible gametes
If Unknown is Dd: 2/
d d 3 of F2 Tall are Dd
Tall Tall
D
possible Dd Dd
gametes Test Progeny Half Tall
Dwarf Dwarf
d Half Dwarf
dd dd
Another Example of Mendel’s Work
Green x Yellow Phenotype
P
gg GG Genotype
Homozygous Homozygous
Recessive Dominant
All Yellow Clearly Yellow is Inherited…
F1 What happened to Green?
Gg
Heterozygous 1. Yellow is dominant to Green
2. Use G/g rather than Y/y for
symbolic logic
F1 x F1 = F2
possible gametes NEVER use G/Y or g/y
Punnett Square: G g
3/ Yellow
F2 G
Yellow Yellow 4
1/ Green
possible GG Gg 4
gametes Yellow Green Green is not missing…just masked
g as “recessive” in diploid state
Gg gg
Mendel as a Scientist F2 F1 x F1 = F2
possible gametes
Punnett Square:
Test Cross: G g
Unknown Yellow x Green G
Yellow Yellow
gg possible GG Gg
gametes Yellow Green
possible gametes g
If Unknown is GG: Gg gg
g g
Yellow Yellow
G
possible Gg Gg
gametes Test Progeny All Yellow
Yellow Yellow
G
Gg Gg
1/ of F2 Yellow are GG
3
possible gametes
If Unknown is Gg: 2/ of F2 Yellow are Gg
g g 3
Yellow Yellow
G
possible Gg Gg
gametes Test Progeny Half Yellow
Green Green
g Half Green
gg gg
Mendel as a Scientist
Actual Results Decision
Test Cross:
3 Yellow 2 Green Gg
Unknown Yellow x Green
gg 2 Yellow 3 Green Gg
possible gametes 1 Yellow 4 Green Gg
If Unknown is GG:
g g Small families do not follow
Yellow Yellow expected ratios perfectly!
G
possible Gg Gg 0 Yellow 5 Green Gg
gametes Yellow Yellow Rare, but it can happen!
G
Gg Gg 4 Yellow 1 Green Gg
It only takes 1 green to be sure
possible gametes the unknown is Gg!
If Unknown is Gg:
g g 5 Yellow 0 Green GG
Yellow Yellow <5% chance unknown is Gg
G
possible Gg Gg 1/ • 1/2 • 1/2 • 1/2 • 1/2 = 1/32
2
gametes Green Green
g You could be wrong (rarely)!
gg gg
After 1900 several scientists tried to
Genetics After Mendel replicate Mendel’s crosses using other
species including snapdragon.
Red Yellow
P x
PRPR PYPY
When these alleles go walking, they both do
some talking (codominance)!
All Orange
F1 OK, so we cannot use R/r nor Y/y so we pick
PRPY a third letter…P for the petal color gene.
Notice: we do NOT mix R/Y or r/y!
F1 x F1 = F2
possible gametes
Punnett Square: PR PY
F2 PR
Red Orange This F2 will NOT have a 3:1 ratio
of phenotypes.
possible PRPR PRPY
gametes Orange Yellow Instead it shows a 1:2:1 ratio!
PY
PRPY P YP Y The exception here proves the rule.
In addition to this, there are multiple alleles possible:
PR = red PY = yellow p = no pigment
The combination of alleles in a diploid determine the flower color:
PRPR = red PRp = pink
PRPY = orange PYp = cream
PYPY = yellow pp = white
Human hair color follows a similar pattern:
Alleles: HBn = brown HBd = blonde hR = red hbk = black
The combinations of these alleles determine the base hair color:
HBnHBn = dark brown HBdHBd = blonde hRhR = red
HBnHBd = sandy brown HBdhR = strawberry hRhbk = red
HBnhR = auburn blonde
HBnhbk = dark brown HBdhbk = blonde hbkhbk = black
Recessive can
Dominant does NOT mean frequent!
be common!
Another Example of Recessive Being Common: Pisum sativum
Garden Peas: green seed, wrinkled seed, dwarf stature, white flower
gg ww dd aa
In other words: a quadruple double-recessive
is the most common garden pea on Earth!
Quantitative Inheritance: multiple genes control trait
Highest Crop Yield: AABBCCDDEE
Intermediate Crop Yield: AabbCCDdEe
Lowest Crop Yield: aabbccddee
Darkest Skin Color: AABBCCDDEE
Intermediate Skin Color: AaBbCcDdEe
Lightest Skin Color: aabbccddee
AaBbCcDdEe x AaBbCcDdEe can produce a huge range of colors!
Yet TV talk show guests argue this point for Maury, etc.
Phenotype = Genotype + Environment
Crop Yield = Genotype
+ Minerals Optimizing these
+ Water factors determines
+ Light agricultural
- Pests productivity…last
etc. part of our course!
Human Skin Color = Genotype
+ Sun (UV) Exposure
- Aging Factors
The sun exposure effect is most obvious in
people of intermediate skin base color
but everyone can have “tan lines.”
Who Gets To Mate With Whom? …Two Extremes
Inbreeding Depression: related parents give same recessives to children
Hemophilia: Queen Victoria’s Mutation and Diseased Grandchildren
Tay-Sachs: Jewish Populations Incest Taboo…but
Bipolar: Irish Populations
Dog Diseases: German Shepherd hip dysplasia
Hybrid Vigor: recessives of one family are “covered” by dominant of other family
Wild Corn A x Wild Corn B
“Half-breed” is better
High Yield Hybrid Corn!
Are Human Cultures of “Great Melting Pots” Superior?
“Mutt” is best dog!
Mechanisms Preventing Inbreeding in Plants
Evolution of Unisexuality: Dioecious Holly and Kiwi
Common Gardener Error at Nursery Pickup
Timing Separation of Bisexuality
• Protandry: Male First, Then Female (Alstromeria, Campanula)
• Protogyny: Female First (Oenothera)
• Sequence in Cucumis: Male, Bisexual, Female, Parthenocarpic
Self-Incompatibility: Sweet Cherry, Almond, Filbert
• pollen germination prevented
• pollen tube growth halted
• synergid action (sperm release) prevented
• sperm deactivated
Going Outside the Box of a Species: Bread Wheat
AA (diploid wheat) x DD (diploid grass)
Triticum urartu Aegilops tauschii
AD (sterile diploid) colchicine AADD
similar to mule! or spontaneous (fertile tetraploid)
BB (diploid grass)
Aegilops speltoides
BBAADD
colchicine
BAD (sterile triploid) Fertile hexaploid
or spontaneous
Bread Wheat
Triticum aestivum
Bread Wheat Created 7500 BC with spontaneous doubling of DNA
Levy A. A. and M. Feldman. 2002. The impact of polyploidy on grass genome evolution. Plant Physiol. 130: 1587-1593.
Yet Another Example of Mendel’s Work
Wrinkled x Round Phenotype
P
ww WW Genotype
Homozygous Homozygous
Recessive Dominant
All Round 1. Round is dominant to Wrinkled
F1
Ww 2. Use W/w rather than R/r for
Heterozygous symbolic logic: in handwriting
make it legible! Ww
F1 x F1 = F2 NEVER use W/R or w/r
possible gametes
Punnett Square: W w
F2 W
Round Round
3/
possible WW Ww Round
4
gametes Round Wrinkled 1/ Wrinkled
w 4
Ww ww
Mendel as a Scientist F2 F1 x F1 = F2
possible gametes
Punnett Square:
Test Cross: W w
Unknown Round x Wrinkled W
Round Round
ww possible WW Ww
gametes Round Wrinkled
possible gametes w
If Unknown is WW: Ww ww
w w
Round Round
W
possible Ww Ww
gametes Test Progeny All Round
Round Round
W
Ww Ww
possible gametes
If Unknown is Ww:
w w
Round Round
W
possible Ww Ww
gametes Test Progeny Half Round
Wrinkled Wrinkled
w Half Wrinkled
ww ww
Mendel as a Scientist
Actual Results Decision
Test Cross:
3 Round 2 Wrinkled Ww
Unknown Round x Wrinkled
ww 2 Round 3 Wrinkled Ww
possible gametes 1 Round 4 Wrinkled Ww
If Unknown is WW:
w w Small families do not follow
Round Round expected ratios perfectly!
W
possible Ww Ww 0 Round 5 Wrinkled Ww
gametes Round Round Rare, but it can happen!
W
Ww Ww 4 Round 1 Wrinkled Ww
It only takes 1 wrinkled to be
possible gametes sure the unknown is Ww!
If Unknown is Ww:
w w 5 Round 0 Wrinkled WW
Round Round <5% chance unknown is Ww
W
possible Ww Ww 1/ • 1/2 • 1/2 • 1/2 • 1/2 = 1/32
2
gametes Wrinkled Wrinkled
w You could be wrong (rarely)!
ww ww
