Download Chapter 4 Heredity and Evolution

Chapter 4
Heredity and Evolution
Chapter Outline
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Genetic Principles Discovered by Mendel
Mendelian Inheritance in Humans
Polygenic Inheritance
Genetic and Environmental Factors
Mitochondrial Inheritance
Chapter Outline
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Modern Evolutionary Theory
Factors That Produce and Redistribute
Variation
Natural Selection Acts on Variation
Review of Genetics and Evolutionary
Factors
Genetic Principles
Discovered by Mendel
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Gregor Mendel (1822-1884) laid down the
basic principles of heredity.
Crossed different strains of purebred
plants and studied their progeny.
Worked with common garden peas and
considered only one trait at a time.
His work illustrates the basic rules of
inheritance.
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,
different allele.
 Dominant alleles prevent the expression of
recessive alleles in heterozygotes.
Alleles
Punnett square
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.
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.
Some Mendelian Traits in
Humans: Dominant
Condition
Manifestations
Achondroplasia
Dwarfism due to growth defects
involving the long bones of the arms
and legs; trunk and head size usually
normal.
Shortened fingers and toes.
Brachydactyly
Familial hypercholesterolemia
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
Manifestations
Cystic fibrosis
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.
Tay-Sachs
disease
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
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.
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, which makes it very useful for
studying genetic change over time.
Heredity and Evolution
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Evolution works at four levels:
 Molecular
 Cellular
 Individual
 Population
The levels reflect different aspects of
evolution and are integrated in a way that
produces evolutionary change.
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.
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 the
founders can eventually become common.
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.
Sickle-cell Distribution in
the Old World
Malaria Distribution in the
Old World
Levels of Organization in the
Evolutionary Process
Evolutionary
Factor
Mutation
Mutation
Level
Evolutionary Process
DNA
Storage of genetic
information; ability to
replicate; influences
phenotype by production of
proteins
A vehicle for packaging and
Chromosomes
transmitting DNA
Levels of Organization in the
Evolutionary Process
Evolutionary
Factor
Level
Evolutionary Process
Basic unit of life, contains
Recombination
chromosomes, divides for
Cell
growth and production of sex
(sex cells only)
cells
The unit that reproduces and
Natural
Organism
which we observe for
selection
phenotypic traits
Changes in allele
Drift, gene flow Population
frequencies between
generations
New Technologies
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Polymerase chain reaction (PCR) makes it
possible to analyze and identify DNA as small
as one molecule and produce multiple copies of
the original DNA.
Recombinant DNA techniques allow scientists
to transfer genes from the cells of one species
into the cells of another.
Genetic manipulation is controversial due to
safety and environmental concerns.
Quick Quiz
1. 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.
2. 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.
correct: 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.
3. 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.
4. 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.
5. The most complete definition of biological
evolution is
a) change.
b) mutation.
c) survival of the fittest.
d) a change in allele frequency from one
generation to the next.
Answer: d
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The most complete definition of
biological evolution is a change in allele
frequency from one generation to the
next.