th
4
Exam is Thursday, December 9
Review session will be at 5:00 PM
Wednesday, December 8
Final Exam
Final exam will be Dec. 16, 8:00-10:00 AM
Yellow Sheets:
You will be allowed to put whatever you want onto
one side of one yellow sheet of paper (to be
handed out in class).
You will attach your yellow sheet to your exam and
hand it in when you are finished.
Final Exam
The format and style of the final will be similar to
the regular exams.
Bring a calculator
Applications of the H-W Law:
Calculate Heterozygote Frequency
If you know the frequency of any one genotype, you
can calculate the frequency of the other genotypes.
This is especially useful if you are interested in
identifying the frequency of carriers for a specific
condition like cystic fibrosis.
Cystic fibrosis is a recessive condition occurring in
about 1 of every 2,500 people (1/2500 = 0.0004)
Calculating Carrier Frequency
Remember, p + q = 1.0
The incidence of cystic fibrosis is 0.0004, so
q2 = 0.0004
q = 0.0004 = 0.02
p + 0.02 = 1.0
p = 1.0 – 0.02 = 0.98
Calculating Heterozygote Frequency
In the H-W equation, heterozygote frequency = 2pq,
so
2pq = 2(0.98)(0.02)
= 0.04 or 4%, about 1 in 25 people
Factors that influence allele and
genotype frequencies
These are the influences that keep a population from
reaching H-W equilibrium.
1. Natural Selection
2. Mutation
3. Migration
4. Genetic Drift
5. Nonrandom Mating
Natural Selection
The H-W assumption is that all individuals have an
equal chance of survival to reproductive age and
equal chance of reproductive success.
Any difference in survival or ability to reproduce is
called natural selection.
Natural selection is the strongest force that alters
allele frequencies and is one of the most important
factors inducing genetic changes.
Natural Selection
Selection occurs whenever some individuals within a
population have an advantage in survival or
reproduction over other individuals.
These advantages ultimately translate into increased
contribution to future generations, or fitness.
Genotypes with high rates of reproductive success
are said to have high fitness.
Natural Selection and Quantitative
Traits
Most phenotypic traits are controlled by multiple loci
and the environment.
Polygenic traits also respond to selection, which can
be classed into three types:
1. Directional
2. Stabilizing
3. Disruptive
Directional Selection
Directional selection: Selection of desirable traits.
Selected traits usually represent phenotypic extremes.
Used by plant and animal breeders, but also occurs in
nature. In nature, the selective agent is usually an
environmental change.
Stabilizing Selection
Stabilizing selection favors intermediate types,
meaning both phenotypic extremes are selected
against.
Tends to keep a population adapted to its
environment.
Disruptive Selection
Disruptive selection is selection against
intermediates and for phenotypic extremes.
Occurs in populations with a heterogeneous
environment.
Mutation
Because the number of possible genotypes is so
large, at any given time, a population will only
represent a small fraction of the possible
genotypes.
Mendelian assortment and recombination produce
new allele combinations, but do not produce new
alleles.
Mutation
Mutational events occur at random, i.e., without
regard to possible benefits or detriments to the
individual.
However, by creating new alleles, mutation can be a
significant force causing allele frequencies to
change.
Migration
Populations may be geographically separated and
respond to different selective pressures such that
their allele frequencies differ.
When individuals from one population move into the
other, allele migration occurs.
Genetic Drift
Genetic drift is the random fluctuation in allele
frequencies caused by chance deviation.
Accurately predicting the genetic ratios (1:1, 3:1,
1:2:1 etc.) requires large numbers of individuals
and mating pairs.
If only small numbers are used, the probability of
deviation from expected ratios increases.
Genetic Drift
Small populations are created when they are
separated from the larger population.
1.
2.
Disruptive event, like a war or epidemic
Migration
Non-Random Mating
Non-random mating does not alter allele frequencies,
but does alter genotype frequencies.
The most important form of non-random mating is
inbreeding, or mating between relatives.
Inbreeding
Inbreeding increases the percentage of individuals
homozygous for recessive alleles previously
concealed in heterozygous individuals.
Developmental Genetics
Developmental Genetics
Genes control development of a fertilized egg into a
cohesive individual composed of millions of cells
organized into tissues and organs with distinct
functional and structural properties.
Basic Events
1.
2.
3.
4.
Cytoplasmic localization
Cell-cell interaction
Determination
Differentiation
Cytoplasmic Localization
Following fertilization, initial cell divisions produce
cells with different distributions of maternal
cytoplasm contributed by the oocyte.
In different cells, the cytoplasm exerts different
influences on the genetic material of the different
cells, ultimately resulting in differences in
transcription.
Cell-Cell Interaction
Physical contact and signal molecules produced by
one cell influence another cell.
Cells in immediate proximity create a
microenvironment that influence development
within that environment and the ultimate structure
and function.
Determination
Determination is the point at which the
developmental fate of a cell becomes fixed.
Determination
Differentiation is the process by which a cell
achieves final form and function.
Differentiation follows determination.
Master Regulators
Master regulators are genes that act as switches.
When the switch is flipped, the number of
developmental pathways is reduced. The switch
commits the cell to move along a specific pathway.
Most master regulators are binary, meaning there are
only two possible alternatives. When the switch is
activated, there is only one.
These genes are called binary switch genes
Binary Switch
Most binary switch genes are identified by isolation
of non-functional mutations.
The binary switch gene triggering differentiation of
the eye in Drosophila is called eyeless.
The eyeless gene is expressed in all embryonic cells
that will give rise to the eye.
When eyeless is lost, cells normally destined to
become part of the eye degenerate later in
development and die.
Binary Switch Genes
In the absence of eyeless, eyes do not form.
If eyeless is expressed in other tissues such as the
leg, wing or antenna, eyes will form in those
locations.
Therefore, eyeless is a master regulator directing eye
development.
Maternal Effect Genes
Maternal effect genes deposit mRNA and/or proteins
into the oocyte cytoplasm.
Maternal effect genes may be distributed evenly
throughout the cytoplasm or may be concentrated
in particular areas.
Distribution can affect the concentration of maternal
effect products in embryonic cells.
Gradients
Concentration of maternal effect gene products in
cells produces a gradient that provides positional
information and directs formation of the anteriorposterior orientation and body segments.
Genes that Control Development
Gap genes: transcription factors that activate pair
rule genes.
Pair-rule genes: products divide the embryo into
smaller regions about two segments wide and in
turn, activate segment polarity genes.
Segment polarity genes: divide segments into
anterior and posterior compartments.
Collectively, these genes define the fields of action
for selector genes that specify the identity of each
segment.