Download 11-2 Probability and Punnett Squares

11-2 Applying Mendel’s
Principles
11-2 Probability and Punnett Squares
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When Mendel crossed 2 hybrid pea plants,
he always found that about ¾ of the
offspring showed the dominant trait and ¼
showed the recessive one.
He realized that the principles of
probability could be used to predict the
outcomes of genetic crosses.
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11-2 Probability and Punnett Squares
Genetics and Probability
Genetics and Probability
The likelihood that a particular event will occur
is called probability.
For example, if you flip a coin, there are 2
possible outcomes, heads or tails. The
chance or probability that the coin will land
heads up is 1 out of 2 (or ½ or 50%). Each
coin flip is an independent event so each toss
has a 50% chance of landing heads up.
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11-2 Probability and Punnett Squares
Using Segregation to Predict Outcomes
The way in which alleles segregate during gamete
formation is every bit as random as a coin flip.
Therefore, the principles of probability can be used
to predict the outcomes of genetic crosses.
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11-2 Probability and Punnett Squares
Punnett Squares
Punnett Squares
The gene combinations that might result
from a genetic cross can be determined by
drawing a diagram known as a Punnett
square.
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
A capital letter (T)
represents the
dominant allele for tall.
A lowercase letter (t)
represents the
recessive allele for
short.
Gametes produced by
each parent are shown
along the top and left
side.
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11-2 Probability and Punnett Squares
Punnett Squares
Possible gene
combinations for the F2
offspring appear in the
four boxes.
The phenotypes
(physical
characteristics) and
genotypes (genetic
makeup) of the offspring
can then be analyzed.
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11-2 Probability and Punnett Squares
Punnett Squares
Phenotypes: ¾ (75%)
of the offspring are
tall; ¼ (25%) are
short.
Genotypes: ¼ (25%)
of the offspring are
TT; 2/4 (50%) are Tt;
¼ (25%) are tt.
Note that not all of the
tall plants have the
same genotype.
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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. The
TT and tt genotypes are homozygous.
Organisms that have two different alleles for the
same trait are heterozygous. The Tt genotype is
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
Probability and
Segregation
Probability and
Segregation
Mendel predicted, because
the allele for tallness (T) is
dominant over the allele for
shortness (t), that 3/4 of the
F2 plants would be tall.
He observed that the ratio of
tall plants (TT or Tt) to short
(tt) plants in the cross was
3:1.
This showed that segregation did occur.
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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
Independent Assortment
After showing that alleles segregate
during gamete formation, Mendel
wondered if the segregation of one pair
of alleles affected the segregation of
another pair?
For example, do the genes for seed
shape in peas have anything to do with
those for seed color?
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11-2 Probability and Punnett Squares 2 Factor Crosses – F1
To determine if the segregation of one pair of
alleles affects the segregation of another pair of
alleles, Mendel performed a two-factor cross.
Mendel crossed true-breeding plants that produced
round yellow peas (genotype RRYY) with truebreeding plants that produced wrinkled green peas
(genotype rryy).
Recall that 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 2 Factor Crosses – F1
All of the F1 offspring produced
round yellow peas (RrYy).
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11-2 Probability and Punnett Squares Independent Assortment
The Two-Factor Cross: F2
Mendel then 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
2 Factor Cross – F2
If the alleles DID segregate
independently of one another, what
possible gametes would be produced?
What would the genotypes and
phenotypes of the F2 generation be?
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11-2 Probability and Punnett Squares Independent Assortment
The Punnett square predicts a
9 : 3 : 3 :1 phenotype ratio in the F2
generation.
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11-2 Probability and Punnett Squares Independent Assortment
In Mendel’s experiment, the F2 generation
produced the following:
9/16 seeds were round and yellow
3/16 seeds were wrinkled and green
3/16 seeds were round and green
1/16 seeds were wrinkled and yellow
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11-2 Probability and Punnett Squares Independent Assortment
This proved that the alleles for seed
shape segregated independently of those
for seed color. This principle is known as
independent assortment.
Genes that segregate independently do
not influence each other's inheritance.
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11-2 Probability and Punnett Squares
Independent Assortment
The principle of independent assortment
states that genes for different traits can
segregate independently during the
formation of gametes.
Independent assortment helps account for the
many genetic variations observed in plants,
animals, and other organisms.
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11-2 Probability and Punnett Squares A Summary of Mendel's
Principles
A Summary of Mendel's Principles
• Traits are determined by genes and genes
are passed from parents to their offspring.
• If 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.
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11-2 Probability and Punnett Squares A Summary of Mendel's
Principles
•In most sexually reproducing organisms, each
adult has two copies of each gene in body cells.
These genes are segregated from each other
when gametes are formed.
•The alleles for different genes usually
segregate independently of one another.
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11-2 Probability and Punnett Squares Applying Mendel's
Principles
Applying Mendel's Principles
Thomas Hunt Morgan used fruit flies to advance
the study of genetics.
Morgan and others tested Mendel’s principles and
learned that they applied to other organisms as
well as plants.
The basic principles of Mendelian genetics can
also be used to study the inheritance of human
traits.
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