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Biology
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11-3 Exploring Mendelian Genetics11–3 Exploring Mendelian Genetics
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11–3 Exploring Mendelian Genetics
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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.
Independent Assortment
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11–3 Exploring Mendelian Genetics
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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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11–3 Exploring Mendelian Genetics
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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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11–3 Exploring Mendelian Genetics
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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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Independent Assortment
The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation.
Represents:
Independent Assortment
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11–3 Exploring Mendelian Genetics
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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.
Independent Assortment
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11–3 Exploring Mendelian Genetics
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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.
Independent Assortment
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11–3 Exploring Mendelian Genetics
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A Summary of Mendel's Principles
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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11–3 Exploring Mendelian Genetics
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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.
A Summary of Mendel's Principles
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Beyond Dominant and Recessive Alleles
Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.
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Beyond Dominant and Recessive Alleles
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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11–3 Exploring Mendelian Genetics
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A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW).
Beyond Dominant and Recessive Alleles
WW
RR
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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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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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Beyond Dominant and Recessive Alleles
Different combinations of alleles result in the colors shown here.
Full color: CC, Ccch, Cch, or CcChinchilla: cchch, cchcch, or cchcHimalayan: chc, or chchAIbino: cc
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
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11–3 Exploring Mendelian Genetics
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Beyond Dominant and Recessive Alleles
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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LE 14-12
aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC
AaBbCcAaBbCc
20/64
15/64
6/64
1/64
Fra
ctio
n o
f p
rog
eny
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11–3
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11–3
In a cross involving two pea plant traits, observation of a 9 : 3 : 3 : 1 ratio in the F2 generation is evidence for
a. the two traits being inherited together.
b. an outcome that depends on the sex of the parent plants.
c. the two traits being inherited independently of each other.
d. multiple genes being responsible for each trait.
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11–3
Traits controlled by two or more genes are called
a. multiple-allele traits.
b. polygenic traits.
c. codominant traits.
d. 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
a. pink flowers.
b. white flowers.
c. half white flowers and half red flowers.
d. 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
a. incomplete dominance.
b. multiple alleles.
c. codominance.
d. a polygenic trait.
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11–3
Mendel's principles apply to
a. pea plants only.
b. fruit flies only.
c. all organisms.
d. only plants and animals.
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