Transcript Training - Tistory

Chapter 2
Transmission Genetics:
Heritage from Mendel
Gregor Mendel
• G. Mendel carried out his experiments from 1856 to
1863 in a small garden plot nestled in a corner of the
St. Thomas monastery in the town of Brno
• He published the results and his interpretation in its
scientific journal in 1866
• Mendel’s paper contains the first clear exposition of
the statistical rules governing the transmission of
hereditary elements from generation to generation
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Mendel’s Genetic Hypothesis
• Each parent contributes to its progeny distinct
elements of heredity = factors = genes
• Factors remain unchanged as they pass through
generations
• Mendel thought in quantitative, numerical terms.
He looked for statistical regularities in the
outcome from his crosses
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Mendel’s Experiments
• Experimental organism: garden pea, Pisum
sativum
• Advantages: many known varieties with different
alternative traits, self-fertilization, easy artificial
fertilization
• True-breading varieties = self-fertilized plants
produce only progeny like themselves
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Figure 2.1: Crossing pea plants
Figure 2.2: Reciprocal crosses of truebreeding pea plants
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Figure 2.3: The seven character differences in peas studied by Mendel
Mendel’s Experiments
• True-breading plants with different forms of a trait,
such as round vs. wrinkled seeds
• All of the F1 progeny exhibited only one parental
trait (round seeds)
• In F2 generation obtained by self-fertilization of F1
plants, the observed ratio of visible traits was 3
round : 1 wrinkled
• Outcome of cross was independent of whether the
trait came from the male or female parent:
reciprocal crosses produced the same result
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Figure 2.5: Expression of Mendel’s traits in plants and seeds
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Table 2.1 Results of Mendel’s monohybrid experiments
Mendel’s Hypothesis
• Each true-breading parent has two identical copies
of the genetic information specifying the trait =
homozygous
• Each gamete contains only one copy of a hereditary
factor specifying each trait
• Random fertilization unites two copies of the gene
in the progeny
• F1 progeny contains different variants (alleles) of
the gene = heterozygous
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Mendel’s Hypothesis
• The genetic constitution of an organism =
genotype
• The observable properties of an organism =
phenotype
• In the cross between round and wrinkled seed pea
plants:
– Round seed parent has two identical copies of
genetic information = its genotype = AA
– The genotype of a wrinkled seed parent = aa
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Dominance
• Round seed parent contributes “A” gamete to
offspring
• Wrinkled seed parent contributes “a” gamete to
offspring
• Offspring genotype = A + a = Aa contains one copy of
“A” and one copy of “a”
• All offspring produce round seeds although their
genotype is “Aa” because “A” is dominant and
“a” is recessive
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Round vs. Wrinkled: Modern Context
• The gene that determines the shape of a seed
encodes an enzyme, starch-branching enzyme I
(SBEI), required to synthesize a branched-chain
form of starch known as amylopectin
• Round (W) seeds contain amylopectin and shrink
uniformly as they dry
• Wrinkled (w) seeds lack amylopectin and shrink
irregularly
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Round vs. Wrinkled: Modern Context
• Wrinkled peas have an inborn error in starch
metabolism
• The molecular basis of the wrinkled (w) mutation =
SBEI gene is interrupted by the insertion of a DNA
sequence called a transposable element
• Transposable elements = DNA sequences capable
of moving (transposition) from one location to
another
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Round vs. Wrinkled: Modern Context
• A procedure called gel electrophoresis is used
to separate DNA molecules of different sizes
• DNA fragment corresponding to the W form of
the SBEI gene moves farther than the w
fragment, because the w fragment is larger
(owing to the insertion of the transposable
element)
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Figure 2.4: Banding as a result of distinct sizes of DNA molecules
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Round vs. Wrinkled: Modern Context
• Classical geneticists studied primarily morphological
traits = the shape of a seed is manifestly round or
wrinkled
• Modern geneticists study morphological traits, too,
but they supplement this with molecular traits = the
pattern of bands in a gel
• Morphological traits are frequently dominant or
recessive, but this is not necessarily true of molecular
traits
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Round vs. Wrinkled: Modern Context
• When alternative forms of a gene (W and w) can both
be detected when they are present in the cell, we say
that the forms of the gene are codominant
• Molecular traits are often (but by no means always)
codominant
• Dominance is not an intrinsic feature of a gene; it
rather depends on the method we chose to examine it
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Figure 2.6: A diagrammatic explanation of the 3 : 1 ratio of
dominant : recessive visible traits observed
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Figure 20: Three attributes of phenotype affected by Mendel’s alleles W
and w
Segregation
• When an F1 plant is self-fertilized, the A and a
determinants segregate from one another and are
included in the gametes in equal numbers
• The gametes produced by segregation come
together in pairs at random to yield the progeny of
the next generation
• In the F2 generation, the ratio of the progeny with
dominant trait to the progeny with recessive trait is
3:1. In case of round and wrinkle seeds, 3/4 round
and 1/4 wrinkled offspring
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The Principle of Segregation
• The Principle of Segregation:
• In the formation of gametes, the paired hereditary
determinants (genes) segregate in such a way that
each gamete is equally likely to contain either
member of the pair
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Monohybrid Genetic Cross
• Genetic cross : Aa X Aa produces A and a
gametes from each parent
• Punnett square shows four possible outcomes =
AA, Aa, aA, and aa
• Three combinations = AA, Aa, and aA produce
plants with round seeds and display a round
phenotype
• Fourth combination = aa displays wrinkled
phenotype
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Figure 2.7: In the F2 generation, the ratio of WW : Ww : ww is 1 : 2 : 1.
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Monohybrid Genetic Cross
Parents: Aa X Aa
Each parent produces A and a gametes and
contributes one gamete at fertilization
1/4
AA
round
dominant
1/2
Aa
round
dominant
1/4
aa
wrinkled
recessive
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Figure 2.8: Mendel’s results of self-fertilization of the F2 progeny
Testcross Analysis
• Testcross = a cross between an organism of
dominant phenotype (genotype unknown) and an
organism of recessive phenotype (genotype
known to be homozygous recessive)
• In a testcross, the relative proportion of the
different gametes produced by the heterozygous
parent can be observed directly in the proportion
of phenotypes of the progeny, because the
recessive parent contributes only recessive
alleles
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Testcross Results
• AA + aa = Aa – testcross produces dominant
progeny only: parent homozygous
• Aa + aa = 1/2 Aa + 1/2 aa – testcross produces 1/2
dominant and 1/2 recessive individuals: parent
heterozygous
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Figure 2.9: A testcross shows the result of segregation directly in the
phenotypes of the progeny
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Table 2.2 Results of Mendel’s testcross experiments
Dihybrid Cross
• Mendel studied inheritance of two different traits,
such as seed color (yellow vs. green) and seed
shape (round vs. wrinkled) in the same cross =
dihybrid cross
• The F1 progeny were hybrid for both characteristics,
and the phenotype of the seeds was round
(dominant to wrinkled) and yellow (dominant to
green)
• In the F2 progeny, he observed the 9 round yellow : 3
wrinkled yellow : 3 round green : 1 wrinkled green
ratio
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Dihybrid Cross
• Mendel carried out similar experiments with
other combinations of traits. For each pair of
traits, he consistently observed the 9:3:3:1
ratio
• He also deduced the biological reason for the
observation:
• In the F2 progeny, if the 3:1 ratio of round:
wrinkled is combined at random with the 3:1
ratio of yellow: green, it yields the 9:3:3:1 ratio
of a dihybrid cross
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Figure 2.10: 9 : 3 : 3 : 1 ratio that Mendel observed in the
F2 progeny of the dihybrid cross
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Independent Segregation
• The Principle of
Independent Assortment:
• Segregation of the
members of any pair of
alleles is independent of
the segregation of other
pairs in the formation of
reproductive cells.
Figure 2.11: Independent segregation of the Ww and Gg allele pairs
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Figure 2.12: Diagram showing the basis for the 9 : 3 : 3 : 1 ratio of F2
phenotypes resulting from a cross
Figure 2.13: The ratio of homozygous dominant, heterozygous, and
homozygous recessive genotypes
Dihybrid Testcross
• The progeny of testcrosses show the result of independent
assortment
• The double heterozygotes produce four types of gametes in
equal proportions, the ww gg plants produce one type
• The progeny phenotypes are expected to consist of round
yellow, round green, wrinkled yellow, and wrinkled green in a
ratio of 1:1:1:1
This observation confirmed Mendel’s assumption that the gametes
of a double heterozygote included all possible genotypes in
approximately equal proportions
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Figure 2.14: Genotypes and phenotypes resulting from a testcross of a Ww
Gg double heterozygote
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Trihybrid Genetic Cross
• Trihybrid cross = three pairs of elements that
assort independently, such as WwGgPp
• For any pair phenotypic ratio = 3:1
• For any two pairs ratio = 9:3:3:1
• Trihybrid cross pattern of segregation and
independent assortment is identical to dihybrid
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Probabilities
• Mendelian patterns of inheritance follow laws of
probability
• Addition Rule: The probability of the realization
of one or the other of two mutually exclusive
events, A or B, is the sum of their separate
probabilities
• Prob {WW or Ww} = Prob {WW} + Prob{Ww} =
0.25 + 0.50 = 0.75
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Figure 2.15: The use of the addition and multiplication rules to determine
the probabilities of genotypes and phenotypes
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Probabilities
• Multiplication Rule:
The probability of two
independent events, A
and B, being realized
simultaneously is
given by the product
of their separate
probabilities
•
Prob {WG} = Prob {W}
x Prob{G} = 0.5 x 0.5 =
0.25
Figure 2.16: Two important types
of independence
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Pedigree Analysis
• In humans, pedigree analysis is used to determine
individual genotypes and to predict the mode of
transmission of single gene traits
Figure 2.17: Conventional symbols used in depicting human pedigrees
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Autosomal Dominant
• Huntington disease is a progressive nerve
degeneration, usually beginning about middle age,
that results in severe physical and mental disability
and ultimately in death
• The trait affects both sexes
• Every affected person has an affected parent
• ~1/2 the offspring of an affected individual are affected
Figure 2.18: Pedigree of a human family showing the inheritance of the
dominant gene for Huntington disease
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Autosomal Recessive
• Albinism = absence of pigment in the skin, hair,
and iris of the eyes
• The trait affects both sexes
• Most affected persons have parents who are not
themselves affected; the parents are heterozygous
for the recessive allele and are called carriers
• Approximately 1/4 of the children of carriers are
affected
• The parents of affected individuals are often
relatives
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Figure 2.19: Pedigree of albinism, a recessive genetic disorder
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Incomplete Dominance
• Incomplete dominance = the
phenotype of the
heterozygous genotype is
intermediate between the
phenotypes of the
homozygous genotypes
• Incomplete dominance is
often observed when the
phenotype is quantitative
rather than discrete
Figure 2.21: Incomplete dominance in the inheritance
of flower color in snapdragons
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Multiple Alleles/Codominance
• Codominance means that the heterozygous genotype
exhibits the traits associated with both homozygous
genotypes
• Codominance is more frequent for molecular traits
than for morphological traits
• Multiple alleles = presence in a population of more
than two alleles of a gene
• ABO blood groups are specified by three alleles IA, IB
and IO
• IA and IB codominant, both IA and IB are dominant to IO
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Multiple Alleles/Codominance
• People of:
– blood type O make both anti-A and anti-B antibodies
– blood type A make anti-B antibodies
– blood type B make anti-A antibodies
– blood type AB make neither type of antibody
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Figure 2.22: The ABO antigens on the surface of human red blood cells are
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carbohydrates
Table 2.3 Genetic control of the Human ABO Blood Groups
Figure 2.23: Antibodies against blood type antigens
Expressivity and Penetrance
• A mutant gene is not always expressed in exactly in
the same way
• Variation in the phenotypic expression of a
particular genotype may happen because other
genes modify the phenotype or because the
biological processes that produce the phenotype
are sensitive to environment
• Variable expressivity refers to genes that are
expressed to different degrees in different
organism
• Penetrance refers to the proportion of organisms
whose phenotype matches their genotype for a
given trait. A genotype that is always expressed
has a penetrance of 100 percent
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Epistasis
• Epistasis refers to any type of gene
interaction that results in the F2
dihybrid ratio of 9:3:3:1 being
modified into some other ratio
• In a more general sense, it means
that one gene is masking the
expression of the other
• Flower color in peas: formation of
the purple pigment requires the
dominant allele of both the C and P
genes: the F2 ratio is modified to 9
purple:7 white
Figure 2.24: A cross showing epistasis in the determination
of flower color in peas
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Epistasis
• There are nine possible dihybrid ratios when both
genes show complete dominance
Examples:
• 9:7 occurs when a homozygous recessive mutation in
either or both of two different genes produces the
same phenotype
• 12:3:1 results when a dominant allele of one gene
masks the genotype of a different gene
• 9:3:4 is observed when homozygosity for a recessive
allele masks the expression of a different gene
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Figure 2.25: Modified F2 dihybrid ratios. In each row, different colors
indicate different phenotypes
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