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Genes
• Units of information about specific traits
• Passed from parents to offspring
• Each has a specific location (locus) on a chromosome
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Alleles
• Different molecular forms of a gene found on homologous chromosomes
• Arise by mutation
• Dominant allele masks a recessive allele that is paired with it
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Allele Combinations
• Homozygous – having two identical alleles – Homozygous dominant, AA – Homozygous recessive, aa
• Heterozygous – having two different alleles– Aa
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fertilization produces heterozygous offspring
meiosis II
meiosis I
(chromosomes duplicated
before meiosis)
homozygous dominant parent
homozygous recessive parent
(gametes) (gametes)
Fig. 8-4, p.114
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Average F2 dominant-to-recessive ratio for all of the traits studied:
Dominant Form Recessive Form
FLOWER COLOR
FLOWER POSITION
STEM LENGTH
3.15:1
3:1
2.84:1787 tall
705 purple 224 white
227 dwarf
651 along stem 207 at tip 3.14:1
Fig. 8-5, p.115
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female gametes
mal
e g
amet
es
aA
a aa
A
aA
aaAa
A
a
aA
aaAa
AaA
a
aA
aaAa
AaAAA
a
Fig. 8-6a, p.115
Probability and the Punnett Square
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Monohybrid Cross
Experimental intercross between
two F1 heterozygotes
AA X aa Aa (F1 monohybrids)
Aa X Aa ?
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AMonohybrid
Cross
True-breedinghomozygous recessiveparent plant
True-breedinghomozygous dominantparent plant
An F1 plantself-fertilizesand producesgametes:
F1 PHENOTYPES
F2 PHENOTYPES
aa
Aa
AA
aaAa
Aa
Aa Aa
Aa Aa
Aa Aa
Aa Aa
Aa
Aa
AA
aa
A
A
A
A
a a
a
a
AA
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Mendel’s Theory of Segregation
• Individual inherits a unit of information (allele) for a trait from each parent
• During gamete formation, the alleles segregate from each other
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Dihybrid Cross
AB X ab
Experimental cross between individuals that are heterozygous for different versions of two traits
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Dihybrid Cross: F1 Results
AABB aabbx
AaBb
AB AB ab ab
TRUE-BREEDING PARENTS:
GAMETES:
F1 HYBRID OFFSPRING:
purple flowers, tall
white flowers,dwarf
all purple-flowered, tall
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AB ab
1AABBpurple-
flowered,tall parent
(homozygousdominant)
2aabb
white-flowered,
dwarf parent(homozygous
recessive)
3 F1 OUTCOME: All of the F1 plants are purple-flowered, tall(AaBb heterozygotes)
X
Fig. 8-7, p.116
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AaBb AaBb
meiosis,gamete formation
meiosis,gamete formation
Fig. 8-7, p.116
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1/16aaBB
1/16aaBb
1/16aaBb
1/16Aabb
1/16Aabb
1/16AAbb
1/16AABB
1/16AABb
1/16AaBB
1/16AaBb
1/16AABb
1/16AaBb
1/16AaBB
1/16AaBb
1/16AaBb
1/4 AB 1/4 Ab 1/4 aB 1/4 ab
1/16aabb
1/4 AB
1/4 Ab
1/4 aB
1/4 ab
AaBb AaBbX
1/16 white-flowered, dwarf
3/16 white-flowered, tall
3/16 purple-flowered, dwarf
9/16 purple-flowered, tall
Dihybrid Cross: F2 Results
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Independent Assortment
• “Units” for one trait were assorted into gametes independently of the “units” for the other trait
• Members of each pair of homologous chromosomes are randomly sorted into gametes during meiosis
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Independent Assortment
Metaphase I:
Metaphase II:
Gametes:
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
A A A A
A A A A
AAAA
B B
B B
BB
B B
BBBB
a a a a
aa aa
aaaa
bb b b
bb b b
b b b b
OR
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Alleles
• Different molecular forms of a gene
• Arise through mutation
• Diploid cell has a pair of alleles at each
locus
• Alleles on homologous chromosomes
may be same or different
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The Y Chromosome
• Small, with few genes
• Master gene for male sex determination – SRY gene (sex-determining region of Y)
• SRY present, testes form
• SRY absent, ovaries form
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The X Chromosome
• Carries more than 2,000 genes
• Most genes deal with nonsexual traits
• Genes on X chromosome can be expressed in both males and females
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Dominance Relations
Complete dominance
Incomplete dominance
Codominance
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Recessive traits
• Recessive inheritance–Two recessive alleles are
needed to show disease
–Heterozygous parents are carriers of the disease-causing allele
–Probability of inheritance increases with inbreeding, mating between close relatives
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Dominant Traits
• Dominant inheritance–One dominant allele is needed to
show disease–Dominant lethal alleles are
usually eliminated from the population
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Codominance
Codominance• Neither allele is dominant over the
other
• Expression of both alleles is observed as a distinct phenotype in the heterozygous individual
• Observed for type AB blood
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Incomplete dominance
Incomplete dominance• Neither allele is dominant over the
other• Expression of both alleles is
observed as an intermediate phenotype in the heterozygous individual
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Autosomal Recessive Inheritance Patterns
• If parents are
both
heterozygous,
child will have a
25% chance of
being affected
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X-Linked Recessive Inheritance
• Males show disorder more than females
• Son cannot inherit disorder from his father