Download Chapter 4 Heredity and Evolution

Chapter 4
Heredity and Evolution
Genetic Principles
Discovered by Mendel
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Gregor Mendel (1822-1884)
 basic principles of heredity.
Crossed different strains of purebred
plants and studied their progeny.
Studied common garden peas (only one
trait at a time).
His work illustrates the basic rules of
inheritance.
Principle of Segregation
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Genes occur in pairs (like chromosomes).
During gamete production, members of each
gene pair separate.
During fertilization, the full number of
chromosomes is restored (allele pairs are
reunited).
Homozygous- same allele at same locus on
both members of a chromosome pair. (i.e.TT, tt)
Heterozygous- two different alleles at the same
locus on a chromosome pair.
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.
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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Mendelian principles apply to over 4,500 human
traits.
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 (codominant and both traits are
expressed).
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.
Skin color is a common example of a
polygenic trait it is governed by 6 loci and
at least 12 alleles.
Mitochondrial Inheritance
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Convert energy into a form that can be
used by the cell.
Animals inherit their mtDNA (and mt 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.
2. Natural selection acting on this variation
(affect ability to reproduce & survive)
Evolution = Mutation or Natural Selection
A Current Definition Of
Evolution
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Modern Genetics: evolution= 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.
Small changes + Time + Natural Selection= Evolution
Mutation
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.
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 from larger
populations.
A new population will be established.
(all members will be descended from the founders)
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An allele that was rare in the founders’ parent
population but is carried by even one of the
founders can eventually become common.
Example: French Canadians in Quebec
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.
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
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.