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Chapter 11
Mendelian Genetics
Copyright © 2010 Pearson Education Inc.
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Genetics is the study of the structure and
function of genes.
◦ a. Early genetics focused on how traits are passed from
parents to offspring (transmission genetics).
◦ b.Advances in biochemistry and molecular methods
allow the study of the structure and function of genes at
the molecular level (molecular genetics).
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Gregor Mendel (1822–1884) laid the foundation
for our current understanding of heredity.
Mendel did not know about chromosomes or
genes, which were discovered after his lifetime
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Hereditary traits are under the control of genes
(Mendel called them factors).
Genotype is the genetic makeup of an organism, a
description of the genes it contains.
Phenotype is the characteristics that can be
observed in an organism.
Phenotype is determined by
interaction of genes and
environment. Genes
provide potential, but
environment determines
whether that potential is
realized
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Mendel began his work in 1854 with the
garden pea, Pisum sativum, by
crossbreeding plants with different
characteristics.
◦ He reported his in 1865, but its significance was
not realized until several decades later.
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He focused on well-defined traits one at a
time, quantifying the offspring and
analyzing the results mathematically.
Garden peas are excellent for this type of
research:
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they grow easily,
produce large numbers of seeds quickly,
routinely self-fertilize.
experimental cross-fertilization is possible.
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First strains of peas were allowed to self-fertilize to be certain
that the traits are unchanged in subsequent generations (truebreeding or pure-breeding strains).
Inheritance of traits with only two distinct possibilities for
phenotype
The traits are:
◦ a. Flower/seed coat color (one gene controls both): *grey/purple vs.
white/white.
◦ b. Seed color: *yellow vs. green.
◦ c. Seed shape: *green vs. yellow.
◦ d. Pod color: *green vs. yellow.
◦ e. Pod shape: *inflated vs. pinched.
◦ f. Stem height: *tall vs. short.
◦ g. Flower position: *axial vs. terminal.
 Dominant trait is with *
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a. Parental generation is the P generation.
b. Progeny of P generation is the first filial
generation, designated F1.
c. When F1 interbreed, the second filial generation,
F2, is produced.
d. Subsequent interbreeding produces F3, F4, and
F5 generations.
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A monohybrid cross involves
true-breeding strains that differ
in a single trait.
To determine whether both
parents contribute equally to the
phenotype of a particular trait in
offspring, a set of reciprocal
crosses is performed. By
convention, the female parent is
given first.
In Mendelian genetics, offspring
of a monohybrid cross will exactly
resemble only one of the parents.
This is the principle of uniformity
in F1.
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Traits that disappear in the
F1 reappear in the F2.
The F2 has a ratio of about
one individual with the
“reappearing” phenotype to
three individuals with the
phenotype of the F1.
Mendel reasoned that
information to create the
trait was present in the F1 in
the form of “factors,” now
called genes.
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Each gene exists in alternative forms (alleles) that
control a specific trait.
◦ True-breeding strains contain identical genes. The F1 contain
one of each, but since the trait is just like one of the parents
rather than a mix, one (dominant) allele has masked
expression of the other (recessive) one.
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By convention, letters may be used to designate
alleles, with the dominant a capital letter (S) and the
recessive in lowercase (s).
◦ Individuals with identical alleles
(e.g., genotypes SS and ss) are
homozygous for that gene.
◦ Individuals with different alleles
(e.g., Ss) are heterozygous, because
1⁄ of their gametes will contain one
2
allele, and 1⁄2 the other.
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Diagrams of a smooth 3 wrinkled cross.
The Punnett square is a diagram showing all possible
gamete combinations of each parent.
3:1 ratio in the F2 generation.
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After Mendel’s experiments for the seven different traits
in garden peas he made these conclusions:
◦ a. Results of reciprocal crosses are always the same.
◦ b. The F1 resembled only one of the parents.
◦ c. The trait missing in the F1 reappeared in about 1⁄4 of the F2
individuals.
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The first Mendelian law, the principle of segregation,
states:
◦ “Recessive characters, which are masked in the F1 from a
cross between two true-breeding strains, reappear in a
specific proportion in the F2.”
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This is because alleles segregate during anaphase I of
meiosis, and progeny are then produced by random
combination of the gametes.
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Mendel observed that plants with the
recessive phenotype are all true-breeding.
When plants with the dominant phenotype are
selfed, 1⁄3 are true-breeding, and 2⁄3 produce
progeny with both phenotypes.
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Better approach to
homozygous or
heterozygous
determination is testcross
by crossing the individual
with one that is
homozygous recessive.
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Analyses of dihybrid cross (two pairs of traits)
resulted in Mendel’s second law, the principle of
independent assortment:
◦ factors for different traits assort independently of one
another. This allows for new combinations of the traits in
the offspring.
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If alleles assort independently, all possible
phenotypes will be represented in the F2, in a ratio
of 9:3:3:1.
◦ If the F1 are testcrossed, all types of offspring in a ratio of
1:1:1:1 will be produced.
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In the F2 of a dihybrid cross
there will be four phenotypic
classes and nine genotypic
classes.
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Mendel’s work was published in 1866 with little
attention from the scientific community until about
1900, when Correns, deVries, and von Tschermark
independently conducted experiments with similar
results.
In 1902 William Bateson, experimenting with fowl,
showed that Mendelian principles apply in animals. He
coined the terms genetics, zygote, F1, F2, and
allelomorph (which was shortened to allele).
W. L. Johannsen named Mendelian factors genes in
1909, from the Greek genos, meaning “birth.”
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W. Farabee in 1905 was the first to demonstrate
Mendelian principles in humans, showing that
brachydactyly is inherited as a simple dominant
trait.
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The study of the phenotypic records
of a family over several generations is
pedigree analysis. The individual
upon whom the study focuses is the
propositus (male) or proposita
(female).
The symbols of pedigree analysis are
summarized in,
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Recessive traits are usually
the result of a mutation
causing loss or modification
of a gene product.
◦ Albinism is an example.
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Deleterious recessive alleles
persist in the population
because heterozygous
individuals carry the allele
without developing the
phenotype and so are not at
a selective disadvantage.
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Characteristics of recessive inheritance of a
relatively rare trait:
◦ a. Parents of most affected individuals have normal
phenotypes but are heterozygous.
◦ b.Mating of heterozygotes will produce 3⁄4 normal progeny
and 1⁄4 with the recessive phenotype.
◦ c. If both parents have the recessive trait, all their progeny
will usually also have the trait.
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Dominant trait is mutation causing a function to
be gained because of an altered gene product
capable of a new activity.
◦ Achondroplasia is an example.
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Dominant alleles produce a distinct phenotype
when in a heterozygote with wild type allele.
◦ Due to the rarity of dominant mutant alleles causing
recognizable traits, homozygous dominant individuals
are very unusual.
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Characteristics of dominant inheritance of a
relatively rare trait:
◦ a. Affected individuals have at least one affected parent.
◦ b.The trait is present in every generation.
◦ c. Offspring of an affected heterozygote will be 1⁄2 affected
and 1⁄2 wild type.
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Other examples include:
◦ a. Autosomal dominant polycystic kidney disease (ADPKD).
◦ b.Brachydactyly.
◦ c. Marfan syndrome.