1. Outline 11-3A Studying Heredity
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Outline 11-3A
Studying Heredity
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2. I. Monohybrid Crosses
A. This is a cross between individuals that involves ONE PAIR of contrasting traits.
1. Examples:
a. Tall versus short plants
b. Brown versus blue eyes
B. A simple way to figure out the probabilities of the offspring of a cross is to use a diagram called a Punnett Square.
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3. Punnett Square

4. 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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5. Switch to transparencies for the test of 11-3
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6. 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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7. The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y).
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8. The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation.
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9. 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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10. A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW).
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11. 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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12. Beyond Dominant and Recessive Alleles
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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.
AIbino: cc
Chinchilla: cchch, cchcch, or cchc
Himalayan: chc, or chch
Full color: CC, Ccch, Cch, or Cc
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13. 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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