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Objectives – chap 11.3
What is the principle of independent assortment?
What are dominant and recessive alleles and how is heredity determined?
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2. 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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3. 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).
All of the F1 offspring produced round yellow peas (RrYy).
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The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y).
When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F2 generation.
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5. 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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The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation.
When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F2 generation.
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7. Independent Assortment
In Mendel’s experiment, the F2 generation produced the following:
some seeds that were round and yellow
some seeds that were wrinkled and green
some seeds that were round and green
some seeds that were wrinkled and yellow
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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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Mendel's experimental results were very close to the 9 : 3 : 3 : 1 ratio predicted by the Punnett square.
Mendel had discovered the principle of independent assortment.
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A Summary of Mendel's Principles
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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In most sexually reproducing organisms, each adult has two copies of each gene. 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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12. Beyond Dominant and Recessive Alleles
What inheritance patterns exist aside from simple dominance?
Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.
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Incomplete Dominance 
When one allele is not completely dominant over another it is called incomplete dominance.
In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes.
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14. Beyond Dominant and Recessive Alleles
A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). WW
Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring.
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15. Beyond Dominant and Recessive Alleles
Codominance 
In codominance, both alleles contribute to the phenotype.
In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.
Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately.
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16. Beyond Dominant and Recessive Alleles
Multiple Alleles 
Genes that are controlled by more than two alleles are said to have multiple alleles.
An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population.
A rabbit's coat color is determined by a single gene that has at least four different alleles.
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Different combinations of alleles result in the colors shown here.
KEY
C = full color; dominant
to all other alleles
cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles
ch = Himalayan; color in certain parts of the
body; dominant to
c allele
c = albino; no color;
recessive to all other
alleles
Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here. photo credits: 1. ©John Gerlach/Visuals Unlimited 2.Animals Animals/©Richard Kolar 3. ©Jane Burton/Bruce Coleman, Inc. 4. ©Hans Reinhard/Bruce Coleman, Inc.
AIbino: cc
Chinchilla: cchch, cchcch, or cchc
Himalayan: chc, or chch
Full color: CC, Ccch, Cch, or Cc
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Polygenic Traits  
Traits controlled by two or more genes are said to be polygenic traits.
Skin color in humans is a polygenic trait controlled by more than four different genes.
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In a cross involving two pea plant traits, observation of a 9 : 3 : 3 : 1 ratio in the F2 generation is evidence for
the two traits being inherited together.
an outcome that depends on the sex of the parent plants.
the two traits being inherited independently of each other.
multiple genes being responsible for each trait.
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11–3
Traits controlled by two or more genes are called
multiple-allele traits.
polygenic traits.
codominant traits.
hybrid traits.
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11–3
In four o'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o'clock plants have
pink flowers.
white flowers.
half white flowers and half red flowers.
red flowers.
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11–3
A white male horse and a tan female horse produce an offspring that has large areas of white coat and large areas of tan coat. This is an example of
incomplete dominance.
multiple alleles.
codominance.
a polygenic trait.
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11–3
Mendel's principles apply to
pea plants only.
fruit flies only.
all organisms.
only plants and animals.
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