Chapter 11.1-11.3
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11-1 Work of
Gregor Mendel
11-2 Probability &
Punnett Squares
Introduction to Genetics
11-3 Exploring
Mendelian Genetics
11-3 Exceptions to
Mendelian Genetics
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Genetics Vocabulary
Genetics – the study of
passing on of genes from
parent to offspring
 Gene = part of DNA coding
for trait
 Trait = Characteristic
 Seed shape, seed color,

plant height

Allele = alternate forms of
a gene
Trait/
Gene
Seed
Shape
Seed
Color
Plant
Height
Allele
Round
Wrinkled
Yellow
Green
Tall
Short
 Eye color: blue, green, brown,
hazel, etc.
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Gregor Mendel & Pea Plants

7 traits each with 2 contrasting alleles
 seed shape, seed color, plant height, pod color, pod
shape, seed coat color, flower position

Self-pollination (also known as true-breeding) vs.
Cross-pollination (also known as selective
breeding)
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Mendel’s 1st experiment
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.
5
Mendel’s 1st Conclusion

Law of Dominance
 Dominant allele v. Recessive allele
○ Capital letter = dominant (ex: tall= T)
○ Lowercase letter = recessive (ex: small= t)
 Each individual has 2 alleles (homologous)
○ TT =Tall
○ Tt = Tall
○ tt = small
 Genotype = gene combination (represented by
alphabet letters)
 Phenotype = Physical appearance (represented
by words)
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Mendel’s 2nd Experiment

F1 generation x F1 generation
 Tall (____) x Tall (____)

2nd Experiment Results
7
Mendel’s 2nd Conclusion

Law of segregation
 Alleles separate during gamete formation (meiosis)
○ Homologous chromosomes separate!! (phase?)

Review
 Tall (____) x Tall (____)
 Phenotype:
○ 3 tall & 1 short
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Genetic Terminology

Genotype: Gene make-up

 Alleles inherited
 Homozygous Dominant
 Ex: Height: TT, Tt, tt
 Phenotype: Physical
appearance
○ Ex: Height
○ Genotype= TT
○ Phenotype= Tall
 Homozygous Recessive
 Ex: Height:
○ TT=
Tall
○ Tt=
Tall
○ tt=
Short
Homozygous = Purebred
 Alleles in pair alike
○ Ex: Height
○ Genotype= tt
○ Phenotype= small

Heterozygous = Hybrid
 Alleles in pair different
 Ex: Height
○ Genotype= Tt
○ Phenotype=Tall
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Predicting Monohybrid Crosses: Height

Punnett Square = diagram to predict
offspring
Gametes
t
t
t
t
 Larger # offspring closer to expected
Monohybrid cross = cross of one trait
 Homozygous Dominant x Homozygous
Recessive

T
T
 Step 1: determine parents
○ Height TT x tt
 Step 2: Put parents on punnett square
 Step 3: Predict
 Step 4: Genotypic & Phenotypic ratios
T Tt Tt
○ Genotypic- 0 TT:4 Tt: 0 tt
○ Phenotypic- 4 Tall: 0 Short
T Tt Tt
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Punnett Monohybrid Cross: Height

Homozygous Dominant x Heterozygous
 Step 1: ID parents
 Step 2: add parents to Punnett
 Step 3: Predict
 Step 4: Genotypic & Phenotypic ratios
○ Genotypic ratio = ____________
○ Phenotypic ratio = ____________

Homozygous recessive x Heterozygous
 Genotypic ratio = _______________
 Phenotypic ratio = ______________
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Punnett Monohybrid Cross
• Heterozygous x Heterozygous
Genotypic ratio = ____________
Phenotypic ratio = ____________
• Testcross
– Determine unknown genotype of
phenotypic dominant trait
– Unknown Dominant x recessive
Ratios
OR
Ratios
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Punnett Dihybrid Crosses

Cross 2 pairs of contrasting traits
 Ex: seed texture & seed color at same time

Hom. Round &Yellow x Hom. Wrinkled & Green
 Step 1: determine parents
○ RRYY x rryy
 Step 2: determine possible gametes (* independent
assortment)
○ RRYY RY, RY, RY, RY
○ rryy ry, ry, ry, ry
 Step 3: Set-up Punnett Square
 Step 4: Predict
 Step 5: Phenotypic ratios
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Dihybrid Cross Punnett Square
ry
Phenotypic Ratios
RY
RY
RY
RY
Round Yellow =16
Round Green = 0
RrYy RrYy RrYy RrYy Wrinkled Yellow=0
Wrinkled Green=0
RrYy RrYy RrYy RrYy Ratio 16:0:0:0
ry
RrYy RrYy RrYy RrYy
ry
RrYy RrYy RrYy RrYy
ry
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Dihybrid Cross
Heterozygous x Heterozygous
Phenotypic Ratio:
Round Yellow=
Round Green=
Wrinkled Yellow=
Wrinkled Green=
Ratio:
WHY & HOW?
15
Mendel’s 3rd Conclusion

Law of independent assortment
 Alleles on separate chromosomes separate independently
of each other during meiosis
 AKA homologous chromosomes separate independently of
each other (phase?)
 Accounts for genetic variation!!

Review of possible gametes in dihybrid cross
 RRYY RY, RY, RY, RY
 rryy ry, ry, ry, ry
Due to Law of
Independent assortment
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Mendel Summary
Traits determined by genes & are passed on
2. Most sexually reproducing adults have 2 copies
of each gene (AKA 2 alleles) one from each
parent
3. Law of Dominance vs Recessive
4. Law of Segregation
5. Law of Independent Assortment
1.
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Artificial Selection

Selective Breeding – known as artificial selection, people
select the traits they want to have passed on.

Inbreeding – can cause genetic defects because the DNA is
too similar to each other.

Hybridization – A mix of two
different traits
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