Population Dynamics
Humans, Sickle-cell Disease, and
Malaria
How does a population of humans
become resistant to malaria?
Natural Selection
• Overproduction
• Environmental
pressure/competition
• Pre-existing individual
variation
• Heritable traits
• Happens over generations
(time)
• Happens in populations (not
single individuals)
• Offspring must be viable and
fertile
The Origins of Genetic
Variation
– Offspring of sexual reproduction are
genetically different from their parents and
from one another.
– Meiosis
• Random mutations
• Crossing over
• Independent assortment of chromosomes
– Random fertilization
Meiosis and comparing it to Mitosis
The Origins of Genetic
Variation
– Offspring of sexual reproduction are
genetically different from their parents and
from one another.
– Meiosis
• Random mutations
• Crossing over
• Independent assortment of chromosomes
– Random fertilization
Intergenerational Mutation Rate
• By how many
mutations does your
genome differ from
your parents
genome?
• Roach, et al., Science
(2010) found about 60
mutations, 30 from
each parent, that
occurred during
meiosis.
Hemophilia in the Royal Family:
Hypothesis - hemophilia allele
arose through mutation in
gamete of Queen Victoria’s
mother or father.
Crossing Over
– In crossing over,
• Homologous
chromosomes
exchange genetic
information.
• Genetic
recombination
occurs.
Independent Assortment of
Chromosomes
– In independent assortment, every
chromosome pair orients independently of
the others during meiosis.
Random Fertilization
– The human egg cell is fertilized randomly
by one sperm, leading to genetic variety in
the zygote.
Natural Selection
• Overproduction
• Environmental
pressure/competition
• Pre-existing individual
variation
• Heritable traits
• Happens over generations
(time)
• Happens in populations (not
single individuals)
• Offspring must be viable and
fertile
Heritable Variation and Patterns
of Inheritance - Ch 9
– Gregor Mendel
• Was the first
person to analyze
patterns of
inheritance.
• Deduced the
fundamental
principles of
genetics.
Figure 9.6a
Monohybrid Crosses
– A monohybrid cross is a cross between
parent plants that differ in only one
characteristic.
– Mendel developed
four hypotheses
from the
monohybrid cross:
• There are
alternative forms of
genes, called
alleles.
• For each
characteristic, an
organism inherits
two alleles, one from
each parent.
• Alleles can be
dominant or
recessive.
• Gametes carry only
one allele for each
inherited
characteristic.
– Mendel developed four
hypotheses from the
monohybrid cross:
• There are alternative
forms of genes, called
alleles.
• For each
characteristic, an
organism inherits two
alleles, one from each
parent.
• Alleles can be
dominant or recessive.
• Gametes carry only
one allele for each
inherited
characteristic.
– Mendel developed four
hypotheses from the
monohybrid cross:
• There are alternative
forms of genes, called
alleles.
• For each
characteristic, an
organism inherits two
alleles, one from each
parent.
• Alleles can be
dominant or recessive.
• Gametes carry only
one allele for each
inherited
characteristic.
– Mendel developed four
hypotheses from the
monohybrid cross:
• There are alternative
forms of genes, called
alleles.
• For each
characteristic, an
organism inherits two
alleles, one from each
parent.
• Alleles can be
dominant or recessive.
• Gametes carry only
one allele for each
inherited
characteristic.
– Mendel developed four
hypotheses from the
monohybrid cross:
• There are alternative
forms of genes, called
alleles.
• For each
characteristic, an
organism inherits two
alleles, one from each
parent.
• Alleles can be
dominant or recessive.
• Gametes carry only
one allele for each
inherited
characteristic.
– Phenotype
• An organism’s physical traits; what it
looks like.
– Genotype
• An organism’s genetic makeup; what
genes it has.
Genetic Alleles
and Homologous Chromosomes
Figure 9.7
Independent Assortment of
Chromosomes
– In independent assortment, every
chromosome pair orients independently of
the others during meiosis.
Figure 9.5
Dihybrid cross
Is the mating of parental varieties differing in two
characteristics.
– Mendel’s law of
independent
assortment states
that
• Each pair of
alleles segregates
independently of
the other pairs
during gamete
formation.
Figure 9.23
Using a Testcross to Determine
an Unknown Genotype
– A testcross is a mating between
• An individual of unknown genotype and a
homozygous recessive individual.
Family Pedigrees
•Shows the history of a trait in a family.
•Allows geneticists to analyze human traits.
Human Disorders Controlled
by a Single Gene
Variations On Mendel’s Laws
– Some patterns of genetic inheritance are
not explained by Mendel’s laws.
•
•
•
•
Incomplete dominance
Codominance
Pleiotropy
Polygenic Inheritance
Incomplete Dominance in
Plants
– In incomplete
dominance, F1
hybrids have an
appearance in
between the
phenotypes of the
two parents.
Incomplete Dominance in
People
– In incomplete dominance, F1 hybrids have
an appearance in between the phenotypes
of the two parents.
ABO Blood Type: An Example of
Multiple Alleles and
Codominance
– The ABO blood
groups in
humans are an
example of
multiple alleles.
– The immune system produces blood
proteins
• That may cause clotting when blood cells of a
different type enter the body.
Pleiotropy and Sickle-Cell
Disease
– Pleiotropy is the
impact of a single
gene on more than
one characteristic.
• Sickle-cell disease is
an example.
Polygenic Inheritance
– Polygenic inheritance is
the additive effects of two
or more genes on a single
phenotype.
The Role of Environment
– Many human characteristics result from a
combination of heredity and environment.
The Chromosomal Basis of
Inheritance
– The chromosome theory of inheritance
states that
• Genes are located at specific positions on
chromosomes.
• The behavior of chromosomes during meiosis
and fertilization accounts for inheritance
patterns.
Figure 9.23
Linked Genes
– Linked genes
• Are located close together on a chromosome.
• May be inherited together.
The Process of Science: Are
Some Genes Linked?
– Using the fruit fly Drosophila melanogaster,
Thomas Hunt Morgan determined
• That some genes were linked based on the
inheritance patterns of their traits.
What to
expect if the
genes follow
the rules of
independent
assortment
GL
gl
gl
gL
GgLl ggll ggLl
50% parental
phenotypes
Gl
Ggll
50% recombinant
phenotypes
Unexpected
Results!!
Figure 9.24
Linked Genes
The “P”, “a”, and “b” genes are linked
because they are on the same chromosome.
What to
expect if the
genes are
linked.
gl
GL
gl
GgLl
ggll
100% parental
phenotypes
Figure 9.24
Unexpected
Results!!
Figure 9.24
Genetic Recombination:
Crossing Over
– Two linked genes
• Can give rise to four different gamete
genotypes because of crossing over!
Figure 9.25
Figure 9.26
Linkage Maps
– Studies using Drosophila
• Developed a method for mapping gene loci.
• Resulted in linkage maps.
If these genes are
linked on one
chromosome:
How far apart
are they?
Figure 9.24
Figure 9.27
Sex-Linked Genes
– Sex-linked genes
• Are any genes located on a sex chromosome.
• Were discovered during studies on fruit flies.
Sex Determination in Humans
and Fruit Flies
– Sex chromosomes
• Are designated X and Y.
• Determine an individual’s sex.
Figure 9.28
Eye color is a sex-linked gene in fruit flies
Figure 9.29
Inheritance patterns of a sex-linked gene
Figure 9.30
Sex-Linked Disorders in
Humans
– A number of human conditions result from
sex-linked (X-linked) genes.
– Red-green color blindness
• Is characterized by a malfunction of lightsensitive cells in the eyes.
– Hemophilia
• Is a blood-clotting disease.
Figure 9.32