11-2 Probability and Punnett
Squares
11-2 Probability and Punnett Squares
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11-2 Probability and Punnett Squares
Genetics and Probability
Genetics and Probability
The principles of probability can be used to
predict the outcomes of genetic crosses.
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11-2 Probability and Punnett Squares
Probability
•Heredity patterns can be calculated with
probability.
•Probability is the likelihood that something will
happen.
•Probability predicts an average number of
occurrences, not an exact number of occurrences.
•Probability applies to random events such as
meiosis and fertilization.
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11-2 Probability and Punnett Squares
Probability
•Probability =
number of ways a specific event can occur
number of total possible outcomes
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11-2 Probability and Punnett Squares
Probabilities Predict
Averages
Probabilities Predict Averages
Probabilities predict the average outcome of a large
number of events.
Probability cannot predict the precise outcome of an
individual event.
In genetics, the larger the number of offspring, the closer
the resulting numbers will get to expected values.
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11-2 Probability and Punnett Squares
Punnett Squares
Punnett Squares
Punnett squares can be used to predict
and compare the genetic variations that
will result from a cross.
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11-2 Probability and Punnett Squares
Punnett Squares
•The Punnett square is a
grid system for predicting
all possible genotypes
resulting from a cross.
– The axes represent
the possible
gametes
of each parent.
– The boxes show the
possible genotypes
of the offspring.
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11-2 Probability and Punnett Squares
Punnett Squares
A capital letter
represents the
dominant allele for tall.
A lowercase letter
represents the
recessive allele for
short.
In this example,
T = tall
t = short
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11-2 Probability and Punnett Squares
Punnett Squares
Organisms that have two identical alleles for a
particular trait are said to be homozygous.
Organisms that have two different alleles for the
same trait are heterozygous.
Homozygous organisms are true-breeding for a
particular trait.
Heterozygous organisms are hybrid for a particular
trait.
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11-2 Probability and Punnett Squares
Punnett Squares
All of the tall plants have the same phenotype, or
physical characteristics.
The tall plants do not have the same genotype, or
genetic makeup.
One third of the tall plants are TT, while two thirds of
the tall plants are Tt.
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11-2 Probability and Punnett Squares
Punnett Squares
The plants have
different genotypes
(TT and Tt), but they
have the same
phenotype (tall).
TT
Homozygous
Tt
Heterozygous
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11-2 Probability and Punnett Squares
Probability and
Segregation
Probability and
Segregation
One fourth (1/4) of the F2
plants have two alleles for
tallness (TT).
2/4 or 1/2 have one allele
for tall (T), and one for
short (t).
One fourth (1/4) of the F2
have two alleles for short (tt).
The genotypic ratio is 1:2:1
The phenotypic ratios is 3:1
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11-2 Probability and Punnett Squares
Punnett Squares
•A monohybrid cross
involves one trait.
•Monohybrid crosses
examine the inheritance
of only one specific trait.
– homozygous
dominant x
homozygous
recessive=all
heterozygous, all
dominant
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11-2 Probability and Punnett Squares
Punnett Squares
•A testcross is a
cross between an
organism with an
unknown genotype
and an organism with
the recessive
phenotype.
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11-2 Probability and Punnett Squares
Independent Assortment
Independent Assortment
To determine if the segregation of one pair of
alleles affects the segregation of another pair of
alleles, Mendel performed a two-factor cross.
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11-2 Probability and Punnett Squares
Independent Assortment
The Two-Factor Cross: F1
Mendel crossed true-breeding plants that
produced round yellow peas (genotype RRYY)
with true-breeding plants that produced wrinkled
green peas (genotype rryy).
RRYY
x
rryy
All of the F1 offspring produced round yellow
peas (RrYy).
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11-2 Probability and Punnett Squares
Independent Assortment
The alleles for round (R) and yellow (Y) are
dominant over the alleles for wrinkled (r) and
green (y).
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11-2 Probability and Punnett Squares
Independent Assortment
The Two-Factor Cross: F2
Mendel crossed the heterozygous F1 plants (RrYy)
with each other to determine if the alleles would
segregate from each other in the F2 generation.
RrYy × RrYy
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11-2 Probability and Punnett Squares
Punnett Squares
•A dihybrid cross
involves two traits.
•Predicted genotypic and
phenotypic ratios can be
determined from a Punnett
square.
•Specific ratios will vary
based on the example
Punnett Square used.
•Example, Mendel’s
dihybrid crosses with
heterozygous plants
yielded a 9:3:3:1
phenotypic ratio.
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11-2 Probability and Punnett Squares
Punnett Squares
•Mendel’s dihybrid crosses led to
his second law,
the law of independent
assortment.
•The law of independent
assortment states that allele
pairs separate independently of
each other during meiosis.
•Independent assortment helps
account for the many genetic
variations observed in plants,
animals, and other organisms.
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Probability can be used to predict
a. average outcome of many events.
b. precise outcome of any event.
c. how many offspring a cross will produce.
d. which organisms will mate with each other.
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Compared to 4 flips of a coin, 400 flips of the
coin is
a. more likely to produce about 50% heads and
50% tails.
b. less likely to produce about 50% heads and
50% tails.
c. guaranteed to produce exactly 50% heads
and 50% tails.
d. equally likely to produce about 50% heads
and 50% tails.
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Organisms that have two different alleles for a
particular trait are said to be
a. hybrid.
b. heterozygous.
c. homozygous.
d. recessive.
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Two F1 plants that are homozygous for
shortness are crossed. What percentage of the
offspring will be tall?
a. 100%
b. 50%
c. 0%
d. 25%
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11-2
The Punnett square allows you to predict
a. only the phenotypes of the offspring from a
cross.
b. only the genotypes of the offspring from a
cross.
c. both the genotypes and the phenotypes
from a cross.
d. neither the genotypes nor the phenotypes
from a cross.
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