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Section 11.2
Applying Mendel’s Principles
Probability
• Mendel realized that the principle of probability
can be used to explain why ¾ of his F2 showed
the dominant trait while ¼ showed the recessive
trait.
– Probability The likelihood that a specific event will
occur
– Expressed as a decimal, percentage, or a fraction.
Probability= # of times an event is expected to happen
# of opportunities for an event to happen
Probability
Homozygous
• Organisms that have
two identical alleles
for a gene
– Homozygous
dominant- PP
– Homozygous
recessive- pp
Heterozygous
• When the 2 alleles in
a pair are different ex.
Bb
GENOTYPE
• Genetic makeup of an
organism
• Ex. 2 recessive
alleles for white
flowers could be pp;
the genotype for the
purple flowers could
be PP or Pp—Where
P= purple and
p=white)
Phenotype
• Physical appearance
of an organism as a
result of its genotype.
– Ex. PP or Pp = purple;
pp= white
• You can alter your
phenotype but not
your genotype (hair
dye or plastic
surgery).
– You can change the
way you look but not
your genes.
Punnett squares
• Uses mathematical
probability to predict
the phenotype and
genotype of
combination in
genetic crosses.
Predicting Results of Monohybrid Crosses
Monohybrid crosses
• a cross between
individuals that
involves one pair of
contrasting traits
– Ex: T= tall; t =short
Example 1: Homozygous x Homozygous
BB X bb
KEY:
B- black coat
b- brown coat
B
b
b
B
Example 1: Homozygous x Homozygous
BB X bb
KEY:
B- black coat
b- brown coat
• Genotypic Ratio:
4Bb:0
• Phenotypic ratio:
4 black: 0 brown
B
B
b
Bb
Bb
b
Bb
Bb
Example 2: Homozygous x Heterozygous
BB X Bb
KEY:
B- black coat
b- brown coat
B
B
b
B
Example 2: Homozygous x Heterozygous
BB X Bb
KEY:
B- black coat
b- brown coat
• Genotypic Ratio:
2BB:2Bb
• Phenotypic ratio:
4 black:0
B
B
B
BB
BB
b
Bb
Bb
Example 3: Heterozygous x Heterozygous
Bb X Bb
KEY:
B- black coat
b- brown coat
B
B
b
b
Example 3: Heterozygous x Heterozygous
Bb X Bb
KEY:
B- black coat
b- brown coat
• Genotypic Ratio:
1BB:2Bb:1bb
• Phenotypic ratio:
3 black :1brown
B
b
B
BB
Bb
b
Bb
bb
Example 4: Testcross
• BB & Bb are black coat.
Q. How do we know if a guinea pig is
homozygous black or heterozygous black?
Example 4: Testcross
• BB & Bb are black coat.
Q. How do we know if a guinea pig is
homozygous black or heterozygous black?
A. Perform a testcross by crossing the unknown
genotype with a homozygous recessive individual.
Ex-4a: BB x bb
B
b
b
B
Ex-4a: BB x bb
B
B
b
Bb
Bb
b
Bb
Bb
Ex-4b: Bb x bb
B
b
b
b
Ex-4b: Bb x bb
B
b
b
Bb
bb
b
Bb
bb
Two-factor cross
or Dihybrid cross
• Mendel wondered how
alleles segregated
(separated) when more
than one gene was
involved
• To answer this question,
he came up with the twofactor or dihybrid cross
Predicting Results of Dihybrid Crosses
• Dihybrid cross:- is a cross between individuals
that involves 2 pairs of contrasting traits.
Key
R= round; r= wrinkled; Y= yellow; y= green
Two-factor or Dihybrid cross
• Use the concept of FOIL (First Outer,
Inner Last) to get the possible
combinations.
P Generation
RRYY
X
rryy
RY RY RY RY
ry ry ry ry
Mendel crossed pure bred round, yellow peas (RRYY) with
pure bred wrinkled, green seeds (rryy)
Homozygous x Homozygous
(RRYY x rryy)
Use the concept of F.O.I.L (First Outer Inner Last)
RRYYx r r y y
RY RY RY RY x ry ry ry ry
Homozygous x Homozygous
(RRYY x rryy)
RY
ry
ry
ry
ry
RY
RY
RY
Homozygous x Homozygous
(RRYY x rryy)
RY
RY
RY
RY
ry
RrYy RrYy RrYy RrYy
ry
RrYy RrYy RrYy RrYy
ry
RrYy RrYy RrYy RrYy
ry
RrYy RrYy RrYy RrYy
Heterozygous x Heterozygous
(RrYy x RrYy)
Use the concept of F.O.I.L
RrYy x
RrYy
RY Ry rY ry x RY Ry rY ry
Heterozygous x Heterozygous
(RrYy x RrYy)
RY
RY
Ry
rY
ry
Ry
rY
ry
Heterozygous x Heterozygous
(RrYy x RrYy)
RY
Ry
rY
ry
RY
RRYY RRYy RrYY RrYy
Ry
RRYy RRyy RrYy
Rryy
rY
RrYY RrYy
rrYY
rrYy
ry
RrYy
rrYy
rryy
Rryy
• In the F2 generation, the results were
always 9:3:3:1
• The results of the dihybrid cross lead to
the Principle of Independent Assortment
Principle of Independent Assortment:
• states that genes for
different traits
segregate
independently during
the formation of
gametes
• Independent
assortment accounts
for the many genetic
variations observed in
plants animals, &
other organisms.
A Summary of Mendel’s Principles
- Mendel’s principle of
genetics can be applied
to all eukaryotic
organisms based on his
work with the pea plant
and Thomas Hunt
Morgan’s work on the fruit
fly.
Lesson Overview
Applying Mendel’s Principles
A Summary of Mendel’s Principles
1. The inheritance of biological characteristics is determined by individual
units called genes, which are passed from parents to offspring.
2. Where two or more forms (alleles) of the gene for a single trait exist, some
forms of the gene may be dominant and others may be recessive.
Lesson Overview
Applying Mendel’s Principles
A Summary of Mendel’s Principles
3. In most sexually reproducing
organisms, each adult has two
copies of each gene—one from
each parent. These genes
segregate from each other
when gametes are formed.
4. Alleles for different genes
usually segregate
independently of each other.