Chapter 4
Heredity and Evolution
Selective Breeding
Genetic Principles
Discovered by Mendel
• Gregor Mendel (1822-1884) laid down
the basic principles of heredity.
• Plant hybrids
Traits Mendel Studied: Peas
Results: One Trait at a Time
Principle of Segregation
Dominance and Recessiveness
• Homozygous vs. heterozygous
• Recessive traits are not expressed in
heterozygotes.
• Dominant traits are governed by an allele that
can be expressed in the presence of another,
allele.
Punnett square
• 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
• 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
• The observable or detectable physical
characteristics of an organism.
• The detectable expressions of genotypes,
frequently influenced by environmental
factors.
• Phenotypic ratio - The proportion of one
phenotype to other phenotypes in a
group of organisms.
Mendelian Traits
• 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
• Over 4,500
human trains are
known to be
inherited
according to
Mendelian
principles.
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.
Antigens
• Large molecules found on the surface of
cells.
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
Codominance
• The expression of two alleles in
heterozygotes.
• In this situation, neither allele is
dominant or recessive so they both
influence the phenotype.
Pedigree Chart
Autosomal Dominant Trait
• 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
• (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
• Individuals 6 and 7, children of unaffected parents, are
affected. Four individuals are definitely unaffected carriers.
Which ones are they?
An African Albino
• An African albino. This
young man has a
greatly increased
likelihood of developing
skin cancer.
Phenotypically Normal Parents,
Both Carriers of the Albinism Allele
• Offspring: Homozygous dominants, Heterozygotes,
(carriers), or Homozygous recessives
X-linked traits
Mendelian Disorders Inherited as
X-Linked 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
X-Linked 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
X-Linked 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
X-Linked 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
• Polygenic traits
are continuous
traits governed
by alleles at
more than one
genetic locus.
Discontinuous Distribution of
Mendelian Traits
• 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
• Represents the continuous expression of height in a
large group of people.
Pleiotropy
• 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
• Animals of both sexes inherit their mtDNA, and
all mitochondrial traits, from their mothers.
• All the variation in mtDNA is caused by
mutation.
The Modern Synthesis
Evolution is a two-stage process:
1. The production and redistribution of
variation.
2. Natural selection acting on this variation.
–
A current definition of evolution - a change in
allele frequency from one generation to the next.
Mutation
• Mutation is a molecular alteration in genetic
material.
Gene Flow
• Gene flow is the exchange of genes between
populations.
Genetic Drift
• Genetic drift occurs when some individuals
contribute a disproportionate share of genes to
succeeding generations.
Genetic Drift
Founder Effect
• Genetic drift in
which allele
frequencies are
altered in small
populations that are
taken from, or are
remnants of, larger
populations.
Bottleneck Effect
Recombination
• In sexually
reproducing
species both
parents
contribute genes
to offspring and
the genetic
information is
reshuffled every
generation.
Natural Selection
• 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
• 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