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Patterns of Inheritance
Chapter 12
Mystery of heredity
• Before the 20th century, 2 concepts were
the basis for ideas about heredity
– Heredity occurs within species
– Traits are transmitted directly from parent to
offspring
• Thought traits were borne through fluid
and blended in offspring
• Paradox – if blending occurs why don’t all
individuals look alike?
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Early work
• Josef Kolreuter – 1760 – crossed tobacco
strains to produce hybrids that differed
from both parents
– Additional variation observed in 2nd generation
offspring contradicts direct transmission
• T.A. Knight – 1823 – crossed 2 varieties of
garden pea, Pisum sativa
– Crossed 2 true-breeding strains
– 1st generation resembled only 1 parent strain
– 2nd generation resembled both
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Gregor Mendel
• Chose to study pea plants because:
1.Other research showed that pea hybrids
could be produced
2.Many pea varieties were available
3.Peas are small plants and easy to grow
4.Peas can self-fertilize or be cross-fertilized
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Mendel’s experimental method
• Usually 3 stages
1.Produce true-breeding strains for each
trait he was studying
2.Cross-fertilize true-breeding strains having
alternate forms of a trait
– Also perform reciprocal crosses
3.Allow the hybrid offspring to self-fertilize
for several generations and count the
number of offspring showing each form of
the trait
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Monohybrid crosses
• Cross to study only 2 variations of a single
trait
• Mendel produced true-breeding pea
strains for 7 different traits
– Each trait had 2 variants
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F1 generation
• First filial generation
• Offspring produced by crossing 2 truebreeding strains
• For every trait Mendel studied, all F1
plants resembled only 1 parent
– Referred to this trait as dominant
– Alternative trait was recessive
• No plants with characteristics intermediate
between the 2 parents were produced
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F2 generation
• Second filial generation
• Offspring resulting from the selffertilization of F1 plants
• Although hidden in the F1 generation, the
recessive trait had reappeared among
some F2 individuals
• Counted proportions of traits
– Always found about 3:1 ratio
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3:1 is 1:2:1
• F2 plants
– ¾ plants with the dominant form
– ¼ plants with the recessive form
– The dominant to recessive ratio was 3:1
• Mendel discovered the ratio is actually:
– 1 true-breeding dominant plant
– 2 not-true-breeding dominant plants
– 1 true-breeding recessive plant
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Conclusions
• His plants did not show intermediate traits
– Each trait is intact, discrete
• For each pair, one trait was dominant, the other
recessive
• Pairs of alternative traits examined were
segregated among the progeny of a particular
cross
• Alternative traits were expressed in the F2
generation in the ratio of ¾ dominant to ¼
recessive
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5 element model
1. Parents transmit discrete factors (genes)
2. Each individual receives one copy of a
gene from each parent
3. Not all copies of a gene are identical
– Allele – alternative form of a gene
– Homozygous – 2 of the same allele
– Heterozygous – different alleles
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4. Alleles remain discrete – no blending
5. Presence of allele does not guarantee
expression
– Dominant allele – expressed
– Recessive allele – hidden by dominant allele
• Genotype – total set of alleles an
individual contains
• Phenotype – physical appearance
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Principle of Segregation
• Two alleles for a gene segregate during
gamete formation and are rejoined at
random, one from each parent, during
fertilization
• Physical basis for allele segregation is the
behavior of chromosomes during meiosis
• Mendel had no knowledge of
chromosomes or meiosis – had not yet
been described
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Punnett square
• Cross purple-flowered plant with white-flowered
plant
• P is dominant allele – purple flowers
• p is recessive allele – white flowers
• True-breeding white-flowered plant is pp
– Homozygous recessive
• True-breeding purple-flowered plant is PP
– Homozygous dominant
• Pp is heterozygote purple-flowered plant
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Human traits
• Some human traits are controlled by a
single gene
– Some of these exhibit dominant and recessive
inheritance
• Pedigree analysis is used to track
inheritance patterns in families
• Dominant pedigree – juvenile glaucoma
– Disease causes degeneration of optic nerve
leading to blindness
– Dominant trait appears in every generation
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• Recessive pedigree – albinism
– Condition in which the pigment melanin is not
produced
– Pedigree for form of albinism due to a
nonfunctional allele of the enzyme tyrosinase
– Males and females affected equally
– Most affected individuals have unaffected
parents
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Dihybrid crosses
• Examination of 2 separate traits in a single
cross
• Produced true-breeding lines for 2 traits
• RR YY x rryy
• The F1 generation of a dihybrid cross
(RrYy) shows only the dominant
phenotypes for each trait
• Allow F1 to self-fertilize to produce F2
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• F1 self-fertilizes
• RrYy x RrYy
• The F2 generation shows all four possible
phenotypes in a set ratio
– 9:3:3:1
– R_Y_:R_yy:rrY_:rryy
– Round yellow:round green:wrinkled
yellow:wrinkled green
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Principle of independent assortment
• In a dihybrid cross, the alleles of each
gene assort independently
• The segregation of different allele pairs is
independent
• Independent alignment of different
homologous chromosome pairs during
metaphase I leads to the independent
segregation of the different allele pairs
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Probability
• Rule of addition
– Probability of 2 mutually exclusive events
occurring simultaneously is the sum of their
individual probabilities
• When crossing Pp x Pp, the probability of
producing Pp offspring is
– probability of obtaining Pp (1/4), PLUS
probability of obtaining pP (1/4)
–¼ + ¼ = ½
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• Rule of multiplication
– Probability of 2 independent events occurring
simultaneously is the product of their individual
probabilities
• When crossing Pp x Pp, the probability of
obtaining pp offspring is
– Probability of obtaining p from father = ½
– Probability of obtaining p from mother = ½
– Probability of pp= ½ x ½ = ¼
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Testcross
• Cross used to determine the genotype of an
individual with dominant phenotype
• Cross the individual with unknown genotype
(e.g. P_) with a homozygous recessive (pp)
• Phenotypic ratios among offspring are
different, depending on the genotype of the
unknown parent
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Extensions to Mendel
• Mendel’s model of inheritance assumes
that
– Each trait is controlled by a single gene
– Each gene has only 2 alleles
– There is a clear dominant-recessive
relationship between the alleles
• Most genes do not meet these criteria
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Polygenic inheritance
• Occurs when multiple genes are involved
in controlling the phenotype of a trait
• The phenotype is an accumulation of
contributions by multiple genes
• These traits show continuous variation and
are referred to as quantitative traits
– For example – human height
– Histogram shows normal distribution
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Pleiotropy
• Refers to an allele which has more than
one effect on the phenotype
• Pleiotropic effects are difficult to predict,
because a gene that affects one trait often
performs other, unknown functions
• This can be seen in human diseases such
as cystic fibrosis or sickle cell anemia
– Multiple symptoms can be traced back to one
defective allele
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Multiple alleles
• May be more than 2 alleles for a gene in a
population
• ABO blood types in humans
– 3 alleles
• Each individual can only have 2 alleles
• Number of alleles possible for any gene is
constrained, but usually more than two
alleles exist for any gene in an
outbreeding population
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• Incomplete dominance
– Heterozygote is intermediate in phenotype
between the 2 homozygotes
– Red flowers x white flowers = pink flowers
• Codominance
– Heterozygote shows some aspect of the
phenotypes of both homozygotes
– Type AB blood
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Human ABO blood group
• The system demonstrates both
– Multiple alleles
• 3 alleles of the I gene (IA, IB, and i)
– Codominance
• IA and IB are dominant to i but codominant to each
other
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Environmental influence
• Coat color in
Himalayan
rabbits and
Siamese cats
– Allele
produces an
enzyme that
allows
pigment
production
only at
temperatures
below 30oC
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Epistasis
• Behavior of gene products can change the
ratio expected by independent assortment,
even if the genes are on different
chromosomes that do exhibit independent
assortment
• R.A. Emerson crossed 2 white varieties of
corn
– F1 was all purple
– F2 was 9 purple:7 white – not expected
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