Mendelian Genetics

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Mendelian Genetics
While assigned to teach, he was also assigned to
tend the gardens and grow vegetables for the
monks to eat.
Augustinian Monk at Brno Monastery in Austria
(now Czech Republic)
Not a great teacher but well trained in math,
statistics, probability, physics, and interested
in plants and heredity.
Mountains with short, cool growing season meant
pea (Pisum sativum) was an ideal crop plant.

• Gregor Mendel
• Father of Genetics

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Contributions in 1860s (US Civil War Era)

• Discovered Genes as Particles of Inheritance
• Discovered Patterns of Inheritance
• Discovered Genes Come from Both Parents
• Egg Sperm Zygote
• Nature vs Nurture
• Sperm means Seed (Homunculus)

• Discovered One Form of Gene (Allele) Dominant to
  Another
• Discovered Recessive Allele Expressed in Absence
  of Dominant Allele

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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!

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One Example of Mendels Work
Tall
Dwarf
x
Phenotype
P
TT
tt
Genotype
Homozygous Dominant
Homozygous Recessive
All Tall
Clearly Tall is Inherited What happened to Dwarf?
F1
Tt

• Tall is dominant to Dwarf

Heterozygous
F1 x F1 F2
possible gametes
Punnett Square
d
D
3/4 Tall 1/4 Dwarf
F2
Tall Tt
Tall TT
D
possible gametes
Dwarf tt
Tall TT
d
Dwarf is not missingjust masked as recessive
in a diploid state there IS a female
contribution.
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Mendel as a Scientist
Test Cross
Unknown Tall
Dwarf
x
tt
possible gametes
If Unknown is TT
t
t
Tall Tt
Tall Tt
T
possible gametes
Test Progeny All Tall
Tall Tt
Tall Tt
T
1/3 of F2 Tall are TT 2/3 of F2 Tall are Tt
possible gametes
If Unknown is Tt
t
t
Tall Tt
Tall Tt
T
possible gametes
Test Progeny Half Tall Half Dwarf
Dwarf tt
Dwarf tt
t
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Another Example of Mendels Work
Green
Yellow
x
Phenotype
P
yy
YY
Genotype
Homozygous Recessive
Homozygous Dominant
All Yellow
Clearly Yellow is Inherited What happened to
Green?
F1
Yy

• Yellow is dominant to Green

Heterozygous
F1 x F1 F2
possible gametes
NEVER use G/Y or g/y
Punnett Square
y
Y
3/4 Yellow 1/4 Green
F2
Yellow Yy
Yellow YY
Y
possible gametes
Green yy
Yellow Yy
y
Green is not missingjust masked as recessive
in diploid state
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Mendel as a Scientist
Test Cross
Unknown Yellow
Green
x
yy
possible gametes
If Unknown is YY
y
y
Yellow Yy
Yellow Yy
Y
possible gametes
Test Progeny All Yellow
Yellow Yy
Yellow Yy
Y
1/3 of F2 Yellow are YY 2/3 of F2 Yellow are Yy
possible gametes
If Unknown is Gg
y
y
Yellow Yy
Yellow Yy
Y
possible gametes
Test Progeny Half Yellow Half Green
Green yy
Green yy
y
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Actual Results
Decision
3 Yellow 2 Green
Yy
2 Yellow 3 Green
Yy
1 Yellow 4 Green
Yy
Small families do not follow expected ratios
perfectly!
0 Yellow 5 Green
Yy
Rare, but it can happen!
Yy
4 Yellow 1 Green
It only takes 1 green to be sure the unknown is
Gg!
5 Yellow 0 Green
YY
lt5 chance unknown is Yy
1/2 1/2 1/2 1/2 1/2 1/32
You could be wrong (rarely)!
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After 1900 several scientists tried to replicate
Mendels crosses using other species including
snapdragon.
Genetics After Mendel
Red
Yellow
P
x
PRPR
PYPY
When these alleles go walking, they both do some
talking (codominance)! OK, so we cannot use R/r
nor Y/y so we pick a third letterP for the petal
color gene. Notice we do NOT mix R/Y or r/y!
All Orange
F1
PRPY
F1 x F1 F2
possible gametes
Punnett Square
PY
PR
F2
Orange PRPY
Red PRPR
PR
This F2 will NOT have a 31 ratio of
phenotypes. Instead it shows a 121 ratio! The
exception here proves the rule.
possible gametes
Yellow PYPY
Orange PRPY
PY
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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 PRPY orange PYPY yellow
PRp pink PYp cream 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 HBnHBd sandy brown HBnhR
auburn HBnhbk dark brown
HBdHBd blonde HBdhR strawberry
blonde HBdhbk blonde
hRhR red hRhbk red hbkhbk black
Recessive can be common!
Dominant does NOT mean frequent!
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Another Example of Recessive Being Common Pisum
sativum
Garden Peas green seed, wrinkled seed, dwarf
stature, white flower
yy
rr
tt
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 Intermediat
e Crop Yield AabbCCDdEe Lowest Crop
Yield aabbccddee
Darkest Skin Color AABBCCDDEE Intermediat
e 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.
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Phenotype Genotype Environment
Crop Yield Genotype Minerals Water
Light - Pests etc.
Optimizing these factors determines agricultural
productivitylast 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.
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Who Gets To Mate With Whom? Two Extremes
Inbreeding Depression related parents give same
recessives to children
Hemophilia Queen Victorias Mutation and
Diseased Grandchildren Tay-Sachs Jewish
Populations Bipolar Irish Populations Dog
Diseases German Shepherd hip dysplasia
Incest Taboobut
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!
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Yet Another Example of Mendels Work
Wrinkled
Round
x
Phenotype
P
rr
RR
Genotype
Homozygous Recessive
Homozygous Dominant
All Round

• Round is dominant to Wrinkled

F1
Rr
Heterozygous
F1 x F1 F2
NEVER use W/R or w/r
possible gametes
Punnett Square
r
R
F2
Round Rr
Round RR
R
3/4 Round 1/4 Wrinkled
possible gametes
Wrinkled rr
Round Rr
r
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Mendel as a Scientist
Test Cross
Unknown Round
Wrinkled
x
rr
possible gametes
If Unknown is RR
w
w
Round Rr
Round Rr
R
possible gametes
Test Progeny All Round
Round Rr
Round Rr
R
possible gametes
If Unknown is Rr
r
r
Round Rr
Round Rr
R
possible gametes
Test Progeny Half Round Half Wrinkled
Wrinkled rr
Wrinkled rr
r
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Mendel as a Scientist
Actual Results
Decision
Test Cross
3 Round 2 Wrinkled
Rr
Unknown Round
Wrinkled
x
rr
2 Round 3 Wrinkled
Rr
possible gametes
1 Round 4 Wrinkled
Rr
If Unknown is WW
r
r
Small families do not follow expected ratios
perfectly!
Round Rr
Round Rr
R
0 Round 5 Wrinkled
Rr
possible gametes
Rare, but it can happen!
Round Rr
Round Rr
R
Rr
4 Round 1 Wrinkled
It only takes 1 wrinkled to be sure the unknown
is Rr!
possible gametes
If Unknown is Ww
5 Round 0 Wrinkled
RR
r
r
lt5 chance unknown is Rr
Round Rr
Round Rr
R
possible gametes
1/2 1/2 1/2 1/2 1/2 1/32
Wrinkled rr
Wrinkled r
r
You could be wrong (rarely)!