1. 11-2 Probability and Punnett Squares
photo credit: W. Perry Conway/CORBIS
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2. Genetics and Probability
The principles of probability can be used to predict the outcomes of genetic crosses.
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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.
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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Probability
Probability = number of ways a specific event can occur number of total possible outcomes
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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5. 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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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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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.
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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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
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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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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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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Punnett Squares
The plants have different genotypes (TT and Tt), but they have the same phenotype (tall).
Although these plants have different genotypes (TT and Tt), they have the same phenotype (tall). TT
Homozygous Tt
Heterozygous
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12. 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
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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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
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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Punnett Squares
A testcross is a cross between an organism with an unknown genotype and an organism with the recessive phenotype.
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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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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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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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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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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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.
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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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.
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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11-2
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11-2
Probability can be used to predict
average outcome of many events.
precise outcome of any event.
how many offspring a cross will produce.
which organisms will mate with each other.
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11-2
Compared to 4 flips of a coin, 400 flips of the coin is
more likely to produce about 50% heads and 50% tails.
less likely to produce about 50% heads and 50% tails.
guaranteed to produce exactly 50% heads and 50% tails.
equally likely to produce about 50% heads and 50% tails.
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11-2
Organisms that have two different alleles for a particular trait are said to be
hybrid.
heterozygous.
homozygous.
recessive.
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11-2
Two F1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall?
100%
50% 0%
25%
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11-2
The Punnett square allows you to predict
only the phenotypes of the offspring from a cross.
only the genotypes of the offspring from a cross.
both the genotypes and the phenotypes from a cross.
neither the genotypes nor the phenotypes from a cross.
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