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’s law of
segregation
• The two members
of an allele pair
segregate
(separate) from
each other during
the production of
gametes.
• Shown using a
Punnett square.
– 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.
– 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.
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 and People
– In incomplete
dominance, F1
hybrids have an
appearance in
between the
phenotypes of the
two parents.
Incomplete Dominance in
Plants and 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.
Figure 9.20
Polygenic Inheritance
– Polygenic inheritance is the additive effects
of two or more genes on a single
phenotype.
Figure 9.21
The Role of Environment
– Many human characteristics result from a
combination of heredity and environment.
Figure 9.22
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.
Figure 9.24
Genetic Recombination:
Crossing Over
– Two linked genes
• Can give rise to four different gamete
genotypes.
• Can sometimes cross over.
Crossing Over
Figure 9.25
Figure 9.26
Linkage Maps
– Early studies of crossing over were
performed using the fruit fly Drosophila
melanogaster.
– Studies using Drosophila
• Developed a method for mapping gene loci.
• Resulted in linkage maps.
Figure 9.27
Sex Chromosomes and SexLinked Genes
– Sex chromosomes
• Influence the inheritance of certain traits.
Sex Determination in Humans
and Fruit Flies
– Sex chromosomes
• Are designated X and Y.
• Determine an individual’s sex.
Figure 9.28
Sex-Linked Genes
– Sex-linked genes
• Are any genes located on a sex chromosome.
• Were discovered during studies on 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.
Figure 9.31
– Hemophilia
• Is a blood-clotting disease.
– Hemophilia
• Is a blood-clotting disease.
Figure 9.32
– Duchenne muscular dystrophy
• Is characterized by a progressive weakening
and loss of muscle tissue.
The Rules of Probability
– The rule of multiplication states that
• The probability of a compound event is the
product of the separate probabilities of the
independent events.
Figure 9.11