Chapter 4
Heredity and Evolution
Chapter Outline
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The Genetic Principles Discovered by
Mendel
Mendelian Inheritance in Humans
Non-Mendelian Inheritance
Genetic and Environmental Factors
Modern Evolutionary Theory
Chapter Outline
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A Current Definition of Evolution Factors that
Produce and Redistribute Variation
Natural Selection Acts on Variation
Review of Genetics and Evolutionary Factors
New Frontiers in Research: Molecular
Applications in Forensic Anthropology
Selective Breeding
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A practice whereby animal and plant
breeders choose which animals will be
allowed to mate based on traits (such as
coat color, body size, shape of face) they
hope to produce in offspring.
Animals that don’t have the desirable
traits aren’t allowed to breed.
Genetic Principles
Discovered by Mendel
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Gregor Mendel (1822-1884) laid down the
basic principles of heredity.
He explored the ways physical traits could
be expressed in plant hybrids.
A hybrid is the offspring of individuals
that differ with regard to certain traits or
certain aspects of genetic makeup.
Gregor Mendel
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A portrait of Gregor
Mendel.
Traits Mendel Studied: Peas
Results: One Trait at a Time
Principle of Segregation
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Genes occur in pairs because
chromosomes occur in pairs.
During gamete production, members of
each gene pair separate so each gamete
contains one member of a pair.
During fertilization, the full number of
chromosomes is restored and members
of a gene or allele pairs are reunited.
Dominance and Recessiveness
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Recessive traits are not expressed in
heterozygotes.
For a recessive allele to be expressed, there
must be two copies of the allele.
Dominant traits are governed by an allele that
can be expressed in the presence of another,
allele.
Dominant alleles prevent the expression of
recessive alleles.
Homozygous
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Having the same allele at the same locus
on both members of a pair of
chromosomes.
Heterozygous
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Having different alleles at the same locus
on members of a pair of chromosomes.
Punnett square
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1⁄4 of the plants can
be expected to be
homozygous tall (TT).
1/2 can be expected
to be tall but will be
heterozygous (Tt).
The remaining 1⁄4 are
homozygous for the
recessive “short”
allele (tt).
Principle of
Independent Assortment
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The distribution of one pair of alleles
into gametes does not influence the
distribution of another pair.
The genes controlling different traits
are inherited independently of one
another.
Phenotype
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The observable or detectable physical
characteristics of an organism.
The detectable expressions of genotypes,
frequently influenced by environmental
factors.
Phenotypic Ratio
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The proportion of one phenotype to other
phenotypes in a group of organisms.
For example,Mendel observed that
there were approximately three tall
plants for every short plant in the F2
generation.
This is expressed as a phenotypic ratio
of 3:1.
Question
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Traits that have a range of phenotypic
expressions and show a continuum of
variation are termed
a) co-dominant.
b) polygenic.
c) polymorphic.
d) sex-linked.
Answer: b
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Traits that have a range of phenotypic
expressions and show a continuum of
variation are termed polygenic.
Question
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Genes exist in pairs in individuals; during
the production of gametes, the pairs are
separated so that a gamete has only one
of each kind. This is known as the
a)principle of segregation.
b)principle of independent assortment.
c)mitosis.
d)unification theory.
Answer: a
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Genes exist in pairs in individuals; during
the production of gametes, the pairs are
separated so that a gamete has only one
of each kind. This is known as the
principle of segregation.
Mendelian Traits
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Characteristics that are influenced by
alleles at only one genetic locus.
Examples include many blood types, such
as ABO.
Many genetic disorders such as sickle-cell
anemia and Tay-Sachs disease are also
Mendelian traits.
Mendelian Inheritance in
Humans
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Over 4,500 human trains are known to be
inherited according to Mendelian principles.
The human ABO blood system is an example of
a simple Mendelian inheritance.
The A and B alleles are dominant to the O
allele.
Neither the A or B allele are dominant to one
another; They are codominant and both traits
are expressed.
Results of Crosses When Traits Are
Considered Simultaneously
Some Mendelian Traits in
Humans: Dominant
Condition
Achondroplasia
Brachydactyly
Familial hypercholesterolemia
Manifestations
Dwarfism due to growth defects involving
the long bones of the arms and legs; trunk
and head
size usually normal.
Shortened fingers and toes.
Elevated cholesterol levels and
cholesterol plaque deposition;
a leading cause of heart disease, with
death frequently occurring
by middle age.
Some Mendelian Traits in
Humans: Recessive
Condition
Cystic fibrosis
Tay-Sachs
disease
Manifestations
Among the most common genetic disorders
among European Americans; abnormal
secretions of the exocrine glands, with
pronounced involvement of the pancreas;
most patients develop obstructive lung
disease.
Most common among Ashkenazi Jews;
degeneration of the nervous system
beginning at about 6 months of age; lethal by
age 2 or 3 years.
Question
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Mendel used the term dominant for
a) plants that were larger than others
of the same variety.
b) a trait that prevented another trait
from appearing.
c) a variety of pea plants that
eliminated a weaker variety.
d) a trait that "skipped" a generation.
Answer: b
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Mendel used the term dominant for a
trait that prevented another trait from
appearing.
Antigens
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Large molecules found on the surface of
cells.
Several different loci govern various
antigens on red and white blood cells.
Foreign antigens provoke an immune
response.
Codominance
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The expression of two alleles in
heterozygotes.
In this situation, neither allele is dominant
or recessive so they both influence the
phenotype.
ABO Genotypes and Associated
Phenotypes
Genotype
Antigens on
Red Blood Cells
ABO Blood Type
(Phenotype)
AA, AO
A
A
BB, BO
B
B
AB
A and B
AB
OO
None
O
Pedigree Chart
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A diagram showing
family relationships;
it’s used to trace the
hereditary pattern of
particular genetic
traits.
Autosomal Dominant Trait
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Inheritance of an autosomal dominant trait: a
human pedigree for brachydactyly. How can
individuals 5, 11, 14, 15, and 17 be unaffected?
Pattern of Inheritance of Autosomal
Dominant Traits
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(a) Diagram of possible gametes produced by one
parent with brachydactyly and another with normal
hands and fingers.
(b) Punnett square depicting possible genotypes in the
offspring of one parent with brachydactyly (Bb) and one
with normal hands and fingers (bb).
Partial Pedigree for Albinism
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Individuals 6 and 7, children of unaffected parents, are
affected. Four individuals are definitely unaffected
carriers. Which ones are they?
An African Albino
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An African albino.
This young man has
a greatly increased
likelihood of
developing skin
cancer.
Phenotypically Normal Parents,
Both Carriers of the Albinism Allele
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Offspring:
Homozygous dominants
(AA) with normal
phenotype, 25%
Heterozygotes, (carriers)
(Aa) with normal
phenotype, 50%
Homozygous recessives
(aa) with albinism, 25%.
Mendelian Disorders Inherited as XLinked Recessive Traits
Condition
Manifestations
G-6-PD
deficiency
Lack of an enzyme in red blood cells;
produces severe anemia in the presence of
certain foods and/or drugs.
One form; other forms can be inherited as
autosomal recessives; progressive weakness
and atrophy of muscles beginning in early
childhood; continues to progress
Muscular
dystrophy
Mendelian Disorders Inherited as XLinked Recessive Traits
Condition
Manifestations
Red-green color
blindness
Two separate forms, one involving the
perception of red and the other affecting the
perception of green.
Lesch-Nyhan
Impaired motor development noticeable by 5
months; progressive motor impairment,
disease diminished kidney function, selfmutilation, and early death.
Mendelian Disorders Inherited as XLinked Recessive Traits
Condition
Manifestations
Hemophilia
In hemophilia A, a clotting factor is missing;
hemophilia B is caused by a defective clotting
factor. Both produce abnormal internal and
external bleeding from minor injuries; severe
pain is a frequent accompaniment; without
treatment, death usually occurs before
adulthood.
Mendelian Disorders Inherited as XLinked Recessive Traits
Condition
Manifestations
Ichthyosis
A skin condition due to lack of an enzyme;
characterized by scaly, brown lesions on the
extremities and trunk. In the past, people with
this condition were sometimes exhibited in
circuses and sideshows as “the alligator man.”
Polygenic Inheritance
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Polygenic traits are continuous traits governed
by alleles at more than one genetic locus.
Continuous traits show gradations, there is a
series of measurable intermediate forms
between two extremes.
Skin color is a common example of a polygenic
trait it is governed by 6 loci and at least 12
alleles.
Discontinuous Distribution of
Mendelian Traits
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Shows the
discontinuous
distribution of ABO
blood type in a
hypothetical
population.
The expression of the
trait is described in
terms of frequencies.
Continuous Expression
of a Polygenic Trait
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Represents the continuous expression of
height in a large group of people.
Height
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Male students arranged according to height. The most
common height is 70 inches, which is the mean, or
average, for this group.
Pleiotropy
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Pleiotropy is a situation where a single gene
influences more than one phenotypic
expression.
Example: The autosomal recessive disorder
phenylketonuria (PKU).
Individuals who are homozygous for the PKU
allele don’t produce phenylketonurase, the
enzyme involved in the conversion of the amino
acid phenylalanine to the amino acid, tyrosine.
Mitochondrial Inheritance
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All cells contain mitochondria that convert
energy into a form that can be used by the cell.
Each mitochondrion contains several copies of
a ring-shaped DNA molecule, or chromosome.
Animals of both sexes inherit their mtDNA, and
all mitochondrial traits, from their mothers.
All the variation in mtDNA is caused by
mutation.
Heredity and Evolution
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Evolution works at four levels:
Molecular
Cellular
Individual
Population
The levels reflect different aspects of
evolution.
The Modern Synthesis
Evolution is a two-stage process:
1. The production and redistribution of variation
(inherited differences between individuals).
2. Natural selection acting on this variation
(whereby inherited differences, or variation,
among individuals differentially affect their
ability to reproduce successfully).
A Current Definition Of
Evolution
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From a modern genetic perspective, we
define evolution as a change in allele
frequency from one generation to the
next.
Allele frequencies are indicators of the
genetic makeup of an interbreeding group
of individuals known as a population.
Mutation
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Mutation is a molecular alteration in genetic
material:
For a mutation to have evolutionary
significance it must occur in a gamete (sex
cell).
Such mutations will be carried on one of the
individual's chromosomes.
During meiosis the chromosome carrying the
mutation will assort giving a 50% chance of
passing the allele to an offspring.
Gene Flow
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Gene flow is the exchange of genes between
populations.
If individuals move temporarily and mate in the
new population (leaving a genetic contribution),
they don’t necessarily remain in the population.
Example: The offspring of U.S. soldiers and
Vietnamese women represent gene flow, even
though the fathers returned to their native
population.
Genetic Drift
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Genetic drift is directly related to population
size.
Genetic drift occurs when some individuals
contribute a disproportionate share of genes to
succeeding generations.
Drift may also occur solely because the
population is small:
Alleles with low frequencies may simply not
be passed on to offspring, so they eventually
disappear from the population.
Genetic Drift
Founder Effect
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Genetic drift in which allele frequencies are
altered in small populations that are taken from,
or are remnants of, larger populations.
A new population will be established, and as
long as mates are chosen only within this
population, all the members will be descended
from the founders.
An allele that was rare in the founders’ parent
population but is carried by even one of
founders can become common.
Question
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When alleles are introduced into a
population from another population, this
is known as
a) genetic drift.
b) gene flow/migration.
c) founder effect.
d) bottleneck effect.
Answer: b
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When alleles are introduced into a
population from another population, this
is known as gene flow/migration.
Recombination
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In sexually reproducing species both
parents contribute genes to offspring.
The genetic information is reshuffled
every generation.
Recombination doesn’t change allele
frequencies, however, it does produce
different combinations of genes that
natural selection may be able to act on.
Natural Selection
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Natural selection provides directional change in
allele frequency relative to specific
environmental factors.
If the environment changes, selection pressures
also change.
If there are long-term environmental changes in
a consistent direction, then allele frequencies
should also shift gradually each generation.
Genetic Variation
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Cheetahs, like many
other species, have
passed through a
genetic bottleneck.
As a species, they
have little genetic
variation.
Sickle-cell Distribution in
the Old World
Life Cycle of the Parasite That
Causes Malaria
Levels of Organization in the
Evolutionary Process
Evolutionary
Factor
Mutation
Level
Evolutionary Process
DNA
Mutation
Chromosomes
Storage of genetic
information; ability to
replicate; influences
phenotype by production of
proteins
A vehicle for packaging and
transmitting DNA
Levels of Organization in the
Evolutionary Process
Evolutionary
Factor
Recombination
(sex cells only)
Level
Evolutionary Process
Cell
Natural selection
Organism
Basic unit of life, contains
chromosomes, divides for
growth and production of sex
cells
The unit that reproduces and
which we observe for
phenotypic traits
Drift, gene flow
Population
Changes in allele frequencies
between generations