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Biology
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2. 11-3 Exploring Mendelian Genetics
11–3 Mendel and beyond
11-3 Exploring Mendelian Genetics
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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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4. Independent Assortment
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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6. Beyond Dominant and Recessive Alleles
What inheritance patterns exist aside from simple dominance?
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7. 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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8. Beyond Dominant and Recessive AllelesRR
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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9. Beyond Dominant and Recessive Alleles
Codominance 
In codominance, both alleles contribute to the phenotype.
With codominance a cross between a white flower and a red flower produces seeds that will grow into plants that are red and white, not pink.
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11. 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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12. Beyond Dominant and Recessive Alleles
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.
Full color CC Ccch Cch Cc
Chinchilla: cchch, cchcch, or cchc
Himalayan: chc, or chch
Full color: CC, Ccch, Cch, or Cc
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13. Chinchilla cch cch cch ch cch c
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Himalayan chch chc
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15. Beyond Dominant and Recessive Alleles
Different combinations of alleles result in the colors shown here.
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
Himalayan: chc, or chch
Full color: CC, Ccch, Cch, or Cc
Chinchilla: cchch, cchcch, or cchc
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17. 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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Epistasis
Sometimes one gene can influence the expression of another gene In mice the allele for black fur is dominant and the allele for brown fur is recessive There is another separate gene that determines if the fur will have any color at all. If this gene is homozygous no color (recessive) the fur will be white, no matter what other color genes are present
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20. 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4  BbCc BbCc Sperm Eggs BBCC BbCC BBCc
Fig 
BbCc
BbCc
Sperm
1/4
1/4
1/4
1/4 BC bC Bc bc
Eggs
1/4 BC
BBCC
BbCC
BBCc
BbCc
1/4 bC
BbCC
bbCC
BbCc
bbCc
1/4 Bc
Figure An example of epistasis
BBCc
BbCc
BBcc
Bbcc
1/4 bc
BbCc
bbCc
Bbcc
bbcc 9
: 3
: 4

21. Epistasis and polygenic inheritance can be hard to tell apart
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22. 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.
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23. Applying Mendel's Principles
Mendel’s principles can be used to study inheritance of human traits and to calculate the probability of certain traits appearing in the next generation.
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24. Genetics and the Environment
Characteristics of any organism are determined by the interaction between genes and the environment.
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Sex-Linked Genes
Sex-Linked Genes
The X chromosome and the Y chromosomes determine sex.
Genes located on these chromosomes are called sex-linked genes.
More than 100 sex-linked genetic disorders have now been mapped to the X chromosome.
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Sex-Linked Genes
X Chromosome
Duchenne muscular dystrophy
The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes.
Melanoma
X-inactivation center
X-linked severe combined immunodeficiency (SCID)
Colorblindness
Hemophilia
Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes.
Y Chromosome
Testis-determining
factor
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Sex-Linked Genes
Why are sex-linked disorders more common in males than in females?
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Sex-Linked Genes
For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes.
Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.
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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
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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Sex-Linked Genes
For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes.
Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.
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Sex-Linked Genes
Sex-Linked Genes
The X chromosome and the Y chromosomes determine sex.
Genes located on these chromosomes are called sex-linked genes.
More than 100 sex-linked genetic disorders have now been mapped to the X chromosome.
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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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37. END OF SECTION