Heredity
Chapter 10
Patterns of Inheritance
Observations?
10.1 Curiosity about Inheritance
• The “Blending” Hypothesis
– Early 1800s thought…
• If you mix red + yellow = orange
• Same idea for hereditary information?
• BUT?!
– How do you explain getting “reds” and “yellows”
– Second generation offspring not all orange
*Did not explain how traits that
disappear in one generation can
reappear in later ones!
Gregor Mendel
• First to apply an experimental approach to
inheritance questions
• Gregor’s experiment
– 7 years bred pea plants and recorded
inheritance patterns of offspring
• Formed the Particulate Hypothesis
– Parents pass on to their offspring separate and distinct
factors (?) that are responsible for inherited traits
Mendel’s Experiment
• 1- Identify true-breeding plants
• White plants ONLY Purple plants ONLY
• 2-Cross 2 different true-breeding plants
• 3- Those offspring become parents for next
generation breeding (cross fertilization)
• 4- Observed the patterns of inheritance
– What colors were in the next generation?
10.2 Inheritance’s Rules of
Chance
Terminology:
– Parents = P generation
– Hybrid offspring = F1 generation
• Offspring of 2 different true-breeding
varieties are “hybrids”
– F1 plants self-fertilize or fertilize each
other their offspring = F2 generation
Generations…
What Mendel
Saw….
Inheritance’s Rules of Chance
• Monohybrid Cross (he did this w/o calling it this)
– 1 trait (color)
• Each produced same pattern
• Mendel Saw
1 of the 2 traits disappears in F1 and then
reappeared in ¼ of F2
Caused Mendel to form 4 hypotheses…
Mendel’s Hypotheses
• 1. Alternating forms of genes called alleles
Genotype =
letters (AA,
Aa, aa)
Phenotype
= observe
(purple,
white)
Mendel’s Hypotheses
2. For each trait organism has 2 alleles
– Same 2 alleles = homozygous (AA, aa)
– Different alleles = heterozygous (Aa)
Mendel’s Hypotheses
3. When only 1 of the 2 traits in a heterozygous
organisms is visible the allele is dominant (R) the
non visible is recessive (r)
Mendel’s Hypotheses
4. The 2 alleles for a character separate during meiosis
(gametes) Principle of segregation!!
Monohybrid crosses
P = Purple
(dominant)
p = white
(recessive)
Punnett Squares
• Calculate probabilities/outcomes of a genetic
cross
– Genotype genetic makeup
– Phenotype observable trait
• Manipulate genotypes that then show the
phenotypes
– Phenotypic ration 3:1
– Genotypic ratio 1PP: 2Pp: 1pp
Cross H. Dominant X Heterozygous
RR
Rr
X
R
R
If
R= Hairy
R
RR
RR
r
Rr
Rr
r = smooth
What will the
offspring look
like? (phenotype)
What are the
genotypes?
Cross H. Recessive X Heterozygous
If
R= Hairy
r = smooth
What will the
offspring look
like? (phenotype)
What are the
genotypes?
Cross Heterozygous X Heterozygous
If
R= Hairy
r = smooth
What will the
offspring look
like? (phenotype)
What are the
genotypes?
Cross H. Dominant X H. Recessive
If
R= Hairy
r = smooth
What will the
offspring look
like? (phenotype)
What are the
genotypes?
Question
If Delani has brown eyes and Mihir has
brown eyes what are their genotypes?
A = brown and a = blue
Test Cross
• Possible genotype for
– Purple PP Pp
– White pp
How do you know if you have PP or Pp?
– Test Cross!!!!
– Cross your unknown with homozygous
recessive
Testcross
When an individual with a dominant phenotype whose
genotype is unknown breeds with a homozygous recessive
individual.
X
BB or Bb?
bb
X
b
b
If BB, then:
If Bb, then
B
B
B
Bb
Bb
Bb
Bb
All black eyes
b
b
Bb
bb
b
Bb
bb
50%
50%
Test Cross
• If the unknown
individual is
Homozygous
dominant all the
offspring will
show the
dominant
characteristic
X
BB
b
b
X
bb
B
B
Bb
Bb
Bb
Bb
All black eyes
Test Cross
• If the unknown
individual is
X
heterozygous
dominant half the
offspring will show
the dominant
characteristic and
half will show the
recessive
characteristic.
Bb
b
b
X
bb
B
b
Bb
bb
Bb
bb
50%
50%
Questions
• Otis Oompah has an orange face and is
married to Ona Oompah who has a blue
face. They have 60 children, 31 of them
have orange faces. What are the
genotypes of the parents.
• Oompahs generally have a blue face
which is caused by a dominant gene. The
recessive condition results in an orange
face.
• A blue faced Oompah (homozygous) is
married to an orange faced Oompah. How
many children will have blue faces?
• Cross a Hetero for tall with Hetero for Tall
• T = tall
• t = short
• Tt X Tt 
In fat orange cats being
striped is dominant to
being solid orange.
What could you do to
determine if Garfield is
heterozygous or
homozygous for his strips
if you did not know???
Dihybrid Crosses
• Crossing individuals differing in 2
characteristics
Sample DiHybrid
Janet has a homozygous recessive short, heterozygous
seeded plant. Alice has a heterozygous for height,
homozygous recessive wrinkled-seeded plant. If
they were to breed their plants together what would
be the outcome of the offspring? What are the
offspring genotypes, phenotypes and ratios?
(T=tall, t=short
R=rounded seed, r = wrinkled seed)
Janet : ttRr
Alice: Ttrr starting genotypes
Janet’s Genotype
t
T
t
r
r
t
R
r
Key to Punnett Squares
• LABEL EVERYTHING
– CHECK YOUR RATIOS
• Homozygous Dominant = AA
• Heterozygous = Aa
• Homozygous Recessive = aa
• Letters = genotype (AA, Aa, aa)
• What you see = phenotype ( color, height)