PowerPoint Presentation - WELCOME TO BIOLOGY 2002

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Figure 10.7
R Dominant allele for seed shape (round) r
Recessive allele for seed shape (wrinkled)
Round-seeded phenotype Wrinkled-seeded
phenotype
Mother
Rr
Female gametes
r
R
R
R
Father
Rr
RR
Male gametes
Rr
r
Rr
rr
Resulting genotypes 1/4 RR 1/2 Rr 1/4 rr
Resulting phenotypes 3/4
1/4
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Crosses Between Plants that Differ in One Trait
Identifies A Fundamental Pattern in Genetics

• Mendel develops Principle of Segregation
• The two alleles of each gene must segregate into
  different gamete cells during the formation of
  eggs and sperm in the parent.

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Figure 10.11a
Principle of segregation
Dominant allele for seed shape (round)
R
r
Recessive allele for seed shape (wrinkled)
Chromosomes replicate
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Figure 10.8a
Cross peas that differ in two traits
R Dominant allele for seed shape (round)
r Recessive allele for seed shape (wrinkled)
Y Dominant allele for seed color (yellow)
y Recessive allele for seed color (green)
Father
Mother
Parental generation
Meiosis
Gametes
Fertilization
F1 generation
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Figure 10.8b
Self-fertilize F1 peas and count F2 offspring
F2 generation phenotype
Number
315
101
108
32 556
Fraction of progeny
9/16
3/16
3/16
1/16 1
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Figure 10.8c
How does the 9 3 3 1 ratio observed for two
traits relate to the 3 1 ratio observed for one
trait?
Round seeds Wrinkled seeds
315 108 101 32
423 133
3 1
Yellow seeds Green seeds
315 101 108 32
416 140
3 1
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Figure 10.9
R Dominant allele for seed shape (round) r
Recessive allele for seed shape (wrinkled) Y
Dominant allele for seed color (yellow) y
Recessive allele for seed color (green)
Parental generation
rryy
RRYY
RrYy
F1 generation
ALL
Female gametes
RrYy
1/4 RY
1/4 Ry
1/4 rY
1/4 ry
1/4 RY
RRYY
RrYy
RrYY
RRYy
1/4 Ry
RrYy
Rryy
RRyy
RRYy
RrYy
Male gametes
1/4 rY
rrYy
RrYY
RrYy
rrYY
1/4 ry
rryy
rrYy
Rryy
RrYy
Resulting genotypes
9/16R-Y-
3/16R-yy
3/16rrY-
1/16rryy
9/16
3/16
3/16
1/16
Resulting phenotypes
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Crosses Between Plants that Differ in Two Traits
Identifies A Second Fundamental Pattern in
Genetics

• Mendel develops Principle of Independent
  Assortment
• Each pair of alleles segregates into gametes
  separately from every other pair of alleles.
• A testcross to a homozygous recessive confirms
  independent assortment of alleles. (Fig. 10.10)
  

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Figure 10.10
Homozygous recessive parent
F1 parent
RrYy
rryy
Ry
ry
1/4
1/4
1/4
RY
rY
1/4
ry
All
1/4 RrYy
1/4 Rryy
1/4 rrYy
1/4 rryy
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Chromosome Theory of Inheritance

• Mendel's rules of inheritance can be explained
  by independent assortment of chromosomes during
  meiosis. (Fig. 10.11b)

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Figure 10.11b
Principle of independent assortment
R
r
R
r
Alleles for seed shape
Alleles for seed color
y
y
Y
Y
R
r
r
R
Meiosis I
Meiosis I
y
y
Y
Y
Meiosis II
Meiosis II
r
r
R
R
Gametes
Y
y
y
Y
1/4 RY
1/4 ry
1/4 Ry
1/4 rY
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Figure 10.12
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Thomas Hunt Morgan
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Thomas Hunt Morgan discovered a male fly with
white eyes in one of his Drosophila cultures

• Mated true-breeding red-eyed flies with the
  white-eyed mutant male
• Bred P flies to produce an F1 generation
• F1 generation all had red eyes
• Concluded gene for white eyes is recessive
• Bred F1 flies to produce an F2 generation
• Got ratio of 31 red-eyed flies to white-eyed
  flies
• However, all of the females had red eyes and half
  of the males had white eyes

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• To explore this unexpected result, Morgan next
  bred some of the F1 females (red-eyed) with
  white-eyed males
• Some of the female offspring had white eyes
• Now Morgan could do the reciprocal cross

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• Cross white-eyed females with wild type
  (red-eyed) males
• All of the female offspring had red eyes
• All of the male offspring had white eyes!!!
• Clearly, the reciprocal cross did not produce the
  same results as the male white-eyed x female
  red-eyed cross