Transcript Mendel_and_the_genetic_engine

The Genetic Engine
How Genetics works
The Code of Life
Gregor Mendel 1822-1884;
Austrian monk who performed the first
comprehensive and systematic
genetic experiments.
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Certain strains of peas bred
true – tall plants give tall plants
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Short peas bred gave short
plants
A Mendelian Genetic Primer
Genes come in pairs
The pairs separate in
the formation of
gametes.
Each form of a
particular gene
is an allele.
The members of the pair may be identical
(homozygous) or non-identical (heterozygous).
Generally speaking
• Adults are diploid
• Gametes are haploid
• However – many
plants may have
polyploid cycles or
tissues
• http://www.cellsalive.c
om/meiosis.htm
• Some organisms may
have more than two
coppies:
• Polyploid.
Genes, Alleles, and Chromosomes
Genes, Alleles, and Chromosomes
Mendel’s hypothesis
1. Each adult possessed two sets
of genes, one contributed from
each parent.
a. A gene may be dominant or
recessive
b. The combination of genes
(homozygous or
heterozygous determine the
expression of traits)
Mendel's genetic model served provide an
understanding of years of data collected on pea
plant breeding
Mendel’s Monohybrid
Cross – P to F1
Simple cross
• Each parent is
homozygous
• All the offspring are
heterozygous – and
all look alike
• The genotype “ratio”
A Punnett square,
is 100% heterozygous
something we’ll
cover in a moment.
Second
generation cross
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Parents are heterozygous
The offspring includes:
One homozygous dominant
One homozygous recessive
Two heterozygous
The phenotype ratio is 3
dominant to one recessive
expression (3:1)
Phenotype vs Genotype
Years of
experimentation
on pea plants
lead to an
understanding of
Genetic
consistency
Characters
investigated by
Mendel
Dihybrid Cross
Principles of
Segregations and
Independent
Assortment.
The offspring will have the ratio
9:3:3:1 ratio 
NOT the 3:1 ratio we would get if
the genes did not undergo
independent assortment
Not just peas
• In the cross Aa x Aa,
where A is a
dominant allele for
(standard)
pigmentation
• a is a recessive allele
for no pigmentation
(albinism), ¾ of
offspring will be wild
type and ¼ will be
albino.
Multiple Alleles
• Many genes are
present in 3 or more
versions (alleles) –
this is known as
multiple alleles.
• The human ABO
blood group is
determined by three
alleles (IA, IB, and i)
of a single gene.
A Molecular Perspective of Genes
The answer to the question “What’s a
gene?” depends on we’re interested in.
At the molecular level: a gene is a sequence of
DNA capable of producing some element of
biological function.
• Biological function:
• It may be an observable trait,
(like skin color),
• A cellular property, ( cell cycle),
• A molecular property, like the
three dimensional shape of a
protein.
Alleles at the Molecular Level
Each form of a gene is an allele.
The standard (wild type) and altered
(mutant) forms of the gene associated
with hemoglobin and sickle cell anemia
provide an example.
The DNA sequences of both alleles of the
“hemoglobin gene” are 99.9% identical – a
single nucleotide difference makes for a
single amino acid difference, which makes
for a difference in protein shape, function
and, ultimately, phenotype.
normal
red blood
cell
sickled
red blood
cell
Mutation asas
Villain
Mutation
Villain
Cancerous growths that
Cancerous growths that
result
from loss of a protein
result from loss of a
that
polices
forDNA
errors.
protein
thatDNA
polices
for errors.
Cancer Incidence Increases Sharply with Age
Cancer Incidence Increases Sharply with Age
The increase is due at least in part to the age-related accumulation of
The increase
is due
at least
in part to the age-related accumulation of multiple
multiple
mutations
in single
cells.
mutations in single cells.
Genetics and Evolution
• While mutations can be seen as Villains – they
are also the “hero’s” of evolution
• It is only by mutation that genetic variations are
added to a population- leading to long term
change over time
• The effects of evolution are felt by individuals,
but it is the population as a whole that actually
evolves. Evolution is simply a change in
frequencies of alleles in the gene pool of a
population.
Hardy Weinberg Equilibrium:
Evolution will not take place if these
conditions are met:
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1. mutation is not occurring
2. natural selection is not occurring
3. the population is infinitely large
4. all members of the population breed
5. all mating is totally random
6. everyone produces the same number of
offspring
• 7. there is no migration in or out of the
population
The Genetic Engine part II;
Darwin and Natural Selection
Hardy Weinberg (link to Video
introduction)
Modern
Biology &
Genetics
• Gregor
Mendel’s
genetic theory
of inheritance
• The
connection
between traits
and heredity
explored
The connection between Mendel
and Hardy Weinberg
• Use phenotype to
identify frequency of
recessive trait
• Take the square root to
identify frequency of
the recessive allele
• Use this to get
frequency of the
dominant allele
Sample: 4 recessive
individuals out of 100
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Frequency of the recessive phenotype will be equal to q2
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Note, q = frequency of the recessive allele
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if 4 out of 100 are recessive, q2 = .04
So, q = the square root of .04 = .2
Heterozygous dominant frequency can now be calculated
• P=1-q
• P =. 1-.2 = .8
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Heterozygous dominant frequency can now be calculated
• 2pq = 2(.2)(.8) = .32
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Check your work!
• p2 + 2pq= q2 = 1
• .64 + .32 +. 04 = 1.0
Coloration in the Scarlet tiger moth
• Coloration in this species had been previously shown
to behave as a single-locus, two-allele system with
incomplete dominance. Data for 1612 individuals are
given below:
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White-spotted (AA) =1469
Intermediate (Aa) = 138
Little spotting (aa) =5
Calculate the following frequencies:
A=
a=
AA =
Aa =
aa =
– Link for answer
More Practice
• One in 1700 US Caucasian newborns have
cystic fibrous. (use C for normal which is
dominant over c for cystic fibrous)
– What percent of the above population have cystic
fibrous (note, this is cc = q2)
– What is the frequency of the recessive allele? (q)
– What is the frequency of the dominant allele? (p)
– Calculate or state, the frequency of the 3 phenotypes
– Link to answers
Yet more practice
If 9% of an African population is born with a severe
form of sickle-cell anemia (ss), what percentage
of the population will be more resistant to
malaria because they are heterozygous (Ss) for
the sickle-cell gene?
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Calculate q2 (Phenotype frequency)
Calculate q (Recessive Allele frequency)
Calculate p
(dominant allele frequency)
Calculate 2pq and q2 & CHECK YOUR WORK
Link to answer
Conditions for Hardy Weinberg
Five conditions needed to insure gene pool
stability (null condition for “no evolution”)
1. A large breeding population
2. Random mating
3. No change in allelic frequency due to
mutation
4. No immigration or emigration
5. No natural selection
What about natural selection?
• video
• Multiple generations of
selection – natural or
artificial will change a
character substantially
• Cumulative selective
response (in one
generation)
• Cumulative selection
differential (sum of the
effect over all generations)
How might stabilizing selection work?
• Why shouldn’t evolution
favor really, really, really,
really really tall people?
• Mortality rates for very
small and very large infants
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• Stabilizing selection favors
intermediate characteristics
Disruptive selection favors
extremes: very large or
small bills
• Red portion of bar graph represents young that did
not survive (Black- Bellied Seed cracker)
Disruptive selection can lead to
speciation
“Truisms” about natural selection
• Natural selection can “happen” if the trait undergoing
selective pressure is genetically determined
• Natural selection can only work toward traits' that
increase fitness for survival and reproduction
• Natural selection acts by changing the frequency of
alleles in the gene pool over time – thus phenotypes do
not “disappear” all at once.
• Evolutionary rates are linked to generation time
• For vertebrates like man with long generation times, ,
evolutionary change is slow, massive mutations tend to
be eliminated quickly
Evolution?
• Natural selection favors organisms that
have traits that are “good enough”; and
usually acts on just a few characteristics
– Extremes are rarely selected for
Many important life forms have
short generation times
• HIV: the ultimate evolver
• Resistance in bacteria