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Mendelian Genetics
A lesson in…why I can roll
my tongue but my father
cannot..
A. Introduction
Every day we observe heritable variations
(eyes of brown, green, blue, or gray) among
individuals in a population.
These traits are transmitted from parents to
offspring.
One mechanism for this transmission is the
“blending” hypothesis.
 This hypothesis proposes that the genetic
material contributed by each parent mixes
in a manner analogous to the way blue and
yellow paints blend to make green.
 Over many generations, a freely mating
population should give rise to a uniform
population of individuals.
However, the “blending” hypothesis
appears incorrect as everyday
observations and the results of
breeding experiments contradict its
predictions.
An alternative model, proposes that
parents pass on discrete heritable
units - genes - that retain their
separate identities in offspring.
Modern genetics began in an abbey
garden, where a monk names Gregor
Mendel documented this type of
inheritance.
B. Mendel brought an experimental and
quantitative approach to genetics
Mendel grew up on a small farm in what is
today the Czech Republic.
In 1843, Mendel entered an Augustinian
monastery.
He studied at the University of Vienna from
1851 to 1853 where he was influenced by a
physicist who encouraged experimentation
and the application of mathematics to
science and a botanist who sparked Mendel’s
interest in the causes of variation in plants.
These influences gelled in Mendel’s
experiments.
After the university, Mendel taught at
the Brunn Modern School and lived in
the local monastery.
The monks at this monastery had a
long tradition of interest in the
breeding of plants, including peas.
Around 1857, Mendel began breeding
garden peas to study inheritance.
Pea plants have several advantages
for genetics.

Pea plants are available in many varieties
with distinct heritable features
(characters) with different variants
(traits).
Another advantage of peas is that
Mendel had strict control over which
plants mated with which.
 Each pea plant has male (stamens)
and female (carpal) sexual organs.
 In nature, pea plants typically selffertilize, fertilizing ova with their
own sperm.
 However, Mendel could also move
pollen from one plant to another to
cross-pollinate plants.

Click on
animation
In a typical breeding experiment,
Mendel would cross-pollinate
(hybridize) two contrasting, truebreeding pea varieties.
 The true-breeding parents are the P
generation and their hybrid
offspring are the F1 generation.
Mendel would then allow the F1
hybrids to self-pollinate to produce an
F2 generation.
It was mainly Mendel’s quantitative
analysis of F2 plants that revealed the
two fundamental principles of
heredity: the law of segregation and
the law of independent assortment.
C. Mendel’s Experiments and
Conclusions
If the blending model were correct, the
F1 hybrids from a cross between
purple-flowered and white-flowered
pea plants would have pale purple
flowers.
Instead, the F1 hybrids all had purple
flowers, just a purple as the
purple-flowered parents.
When Mendel allowed the F1 plants to
self-fertilize, the F2 generation
included both purple-flowered and
white-flowered plants.
 The white trait, absent in the F1,
reappeared in the F2.
Based on a large sample size, Mendel
recorded 705 purple-flowered F2
plants and 224 white-flowered F2
plants from the original cross.
The results appeared in a 3:1 ratio of
purple to white flowers
Mendel reasoned that the inherited
factor for white flowers was present
in the F1 plants, but it did not affect
flower color.
 Purple flower is a dominant trait
and white flower is a recessive trait
since the purple overpowered or
masked the appearance of the
white.
The reappearance of white-flowered
plants in the F2 generation indicated
that the white trait was not diluted or
“blended” by coexisting with the
purple-flower factor in F1 hybrids.
Mendel found similar 3 to 1 ratios of
two traits among F2 offspring when
he conducted crosses for six other
characters, each represented by two
different varieties.
For example, when Mendel crossed
two true-breeding varieties, one of
which produced round seeds, the
other of which produced wrinkled
seeds, all the F1 offspring had round
seeds, but among the F2 plants, 75%
of the seeds were round and 25%
were wrinkled.
animation
Mendel developed a hypothesis to
explain these results that
consisted of four related ideas.
1. Variations in inherited traits
are due to different “factors” or
different versions of a gene called
alleles.
 Different alleles vary somewhat
in the sequence of nucleotides
at the specific locus of a gene.
The purple-flower
allele and white-flower
allele are two DNA
variations at the
flower-color locus.
P
p
2. For each trait, an organism inherits two
alleles, one from each parent.
 A diploid organism inherits one set of
chromosomes from each parent.
 Each diploid organism has a pair of
homologous chromosomes and therefore
two copies of each locus.
These alleles may be identical, as in the
true-breeding plants of the P generation.
(HOMOZYGOUS)
Alternatively, the two alleles may differ.
(HETEROZYGOUS)
 In the flower-color example, the F1
plants inherited a purple-flower allele
from one parent and a white-flower allele
from the other.
3. If two alleles differ, then one,
the dominant allele, is fully
expressed in the organism’s
appearance.
The other, the recessive allele,
has no noticeable effect on the
organism’s appearance.
 Mendel’s F1 plants had purple
flowers because the purpleflower allele is dominant and
the white-flower allele is
recessive.
animation
4. The two alleles for each
character segregate
(separate) during gamete
production (The Law of
Segregation)
This segregation of alleles
corresponds to the
distribution of homologous
chromosomes to different
gametes in meiosis.
If an organism has identical alleles
(homozygous) for a particular trait,
then that allele exists as a single copy
in all gametes.

If different alleles are present
(heterozygous), then 50% of the
gametes will receive one allele and
50% will receive the other.

The separation of alleles into
separate gametes (during meiosis)
is summarized as Mendel’s law of
segregation.
Mendel’s law of segregation accounts
for the 3:1 ratio that he observed in
the F2 generation.
The F1 hybrids will produce two
classes of gametes, half with the
purple-flower allele and half with the
white-flower allele.
During self-pollination, the gametes
of these two classes unite randomly.
This can produce four equally likely
combinations of sperm and ovum.
A Punnett
square
predicts the
results
of a genetic
cross
between
individuals
of known
genotype.
Video about
Punnett
squares
A Punnett square analysis of the
flower-color example demonstrates
Mendel’s model.



One in four F2 offspring will inherit two
white-flower alleles and produce white
flowers.
Half of the F2 offspring will inherit one
white-flower allele and one purple-flower
allele and produce purple flowers.
One in four F2 offspring will inherit two
purple-flower alleles and produce purple
flowers too.
Mendel’s model accounts for the 3:1
ratio in the F2 generation
Genetics has some unique, useful
vocabulary.
An organism with two identical alleles for a
character is homozygous for that character.
Organisms with two different alleles for a
character is heterozygous for that character.
A description of an organism’s traits or
outward appearance is its phenotype.
A description of its genetic makeup is its
genotype.
 Ex. PP = homozygous (pure) dominant
 pp = homozygous recessive
 Pp = heterozygous (hybrid)
For flower color in peas, both PP and
Pp plants have the same phenotype
(purple) but different genotypes
(homozygous and heterozygous).
The only way to
produce a white
phenotype is to
be homozygous
recessive (pp)
for the flowercolor gene.
It is not possible to predict the
genotype of an organism with a
dominant phenotype.

The organism must have one dominant
allele, but it could be homozygous
dominant or heterozygous.
A test cross is used to determine the
genotype when the organism shows
the dominant phenotype by breeding
the organism with another that shows
the recessive condition.
The outcome of the offspring help
solve the mystery genotype of the
parent.
D. The law of independent assortment
and dihybrid crosses.
Mendel’s experiments that followed the
inheritance of flower color or other
characters focused on only a single
character via monohybrid crosses.
He conduced other experiments in
which he followed the inheritance of
two different characters, a dihybrid
cross.
In one dihybrid cross experiment,
Mendel studied the inheritance of
seed color and seed shape.


The allele for yellow seeds (Y) is
dominant to the allele for green seeds
(y).
The allele for round seeds (R) is
dominant to the allele for wrinkled seeds
(r).
Mendel crossed true-breeding plants
that had yellow, round seeds (YYRR)
with true-breeding plants that has
green, wrinkled seeds (yyrr).
Mendel hypothesized that the two pairs of
alleles segregate independently of each
other.
 The presence of one specific allele for
one trait has no impact on the presence
of a specific allele for the second trait.
In our example, the F1 offspring would still
produce yellow, round seeds.
However, the F1’s produced gametes, genes
would be packaged into gametes with all
possible allelic combinations.
 Four classes of gametes (YR, Yr, yR, and
yr) would be produced in equal amounts.
F1 __ __ __ ___ x __ __ __ __
What are the phenotypic
results of the F2 generation?
Round and yellow _______
Round and green _______
Wrinkled and yellow _______
Wrinkled and green _______
When sperm with four classes of
alleles and ova with four classes of
alleles combined, there would be
16 equally probable ways in which
the alleles can combine in the F2
generation.
These combinations produce four
distinct phenotypes in a 9:3:3:1
ratio.
This was consistent with Mendel’s
results.
Mendel repeated the dihybrid cross
experiment for other pairs of
characters and always observed a
9:3:3:1 phenotypic ratio in the F2
generation.
Each character appeared to be
inherited independently. One trait
does not determine the outcome of
another.
The independent assortment of each
pair of alleles during gamete
formation is now called Mendel’s law
of independent assortment.
E. Mendelian inheritance reflects rule of
probability (Using the product law)
Mendel’s laws of segregation and
independent assortment reflect the
same laws of probability that apply to
tossing coins or rolling dice.
The probability scale ranged from zero
(an event with no chance of occurring)
to one (an event that is certain to
occur).
 For exampe, the probability of
tossing heads with a normal coin is
1/2.
When tossing a coin, the outcome of
one toss has no impact on the
outcome of the next toss.
Each toss is an independent event,
just like the distribution of alleles into
gametes.
 Like a coin toss, each ovum from a
heterozygous parent has a 1/2
chance of carrying the dominant
allele and a
1/2 chance of carrying the recessive
allele.
 The same odds apply to the sperm.
We can use the rule of multiplication(THE
PRODUCT LAW)->the chance that two or
more independent events will occur
together in some combination is equal to
the product of each event occurring
independently.
 Compute the probability of each
independent event.
 Then, multiply the individual probabilities
to obtain the overall probability of these
events occurring together.
 The probability that two coins tossed at
the same time will land heads up is 1/2 x
1/2 = 1/4.
Using the product law
Similarly, we can apply
the same concept in a
dihybrid cross.
 If two heterogyzous YyRr
x YyRr (yellow and round)
are crossed, what is the
chance that a yellow
wrinkled offspring will be
produced?

In a monohybrid cross involving
just the Yy x Yy alleles, there is a
¾ chance that the offspring will be
yellow.

In another monohybrid cross
between Rr x Rr, there is a ¼
chance the offspring will be
wrinkled.

Multiplying ¾ x ¼ = 3/16 gives
you the chance that these events
will occur together!

We can combine the product law to
solve complex problems in Mendelian
genetics.
Let’s determine the probability of
finding all recessive phenotypes for
three traits resulting from a trihybrid
cross between pea plants : PpYyRr x
Ppyyrr.
1st: Pp x Pp
2nd: Yy x yy
3rd: Rr x rr
Chance for recessive is ¼ x ½ x ½ =
1/16
F. Mendel discovered the behavior of
genes: a review
Mendel’s experiments
succeeded because he counted
so many offspring and was able
to discern this statistical
feature of inheritance and had
a keen sense of the rules of
chance.
Mendel’s laws of independent
assortment and segregation explain
heritable variation in terms of
alternative forms of genes that are
passed along according to simple rule
of probability.
These laws apply not just to garden
peas, but to all other diploid
organisms that reproduce by sexual
reproduction.
G. Things Mendel Never Knew…
In the 20th century, geneticists have
extended Mendelian principles not only
to diverse organisms, but also to
patterns of inheritance more complex
than Mendel described.
In fact, Mendel had the good fortune to
choose a system that was relatively
simple genetically.


Each character (but one) is controlled by a
single gene.
Each gene has only two alleles, one of
which is completely dominant to the other.
However, in some cases one allele is
not completely dominant over another.
Some traits exhibit incomplete
dominance where one allele does not
overpower another. Instead, the two
alleles combine to produce a third
phenotype that is a blended effect of
the parents.


Offspring of a cross between individuals
of parents with the blended or
heterozygous condition will show three
phenotypes
The phenotypic and genotypic ratios are
identical, 1:2:1.
Examples of incomplete
dominance?
Red and white snapdragons produce pink
snapdragons in the F1 generation.
Japanese 4 o’clock flowers…red and white
make pink.
In Andalusian fowls, black feathered chickens
crossed with white make a blue colored
chicken.
Cats with long tails bred with those without
tails make a short tailed cat.
Yellow rats crossed with white rats produced a
cream colored offspring.
A clear example of incomplete
dominance is seen in flower color
of snapdragons.
 How can we use a punnett
square to show the F1 ratios of
a red snapdragon crosses with a
white one? Crossing 2 F1s?
What letters can we use to
illustrate incomplete dominance?
Since both alleles are dominant,
you can’t use a capital and lower
case letter.
The following is acceptable:
Red = RR, White = R’R’, Pink = RR’
Red = CRCR, White = CWCW, Pink =
CRCW
Not as acceptable but will work…
Red = RR, White = WW, Pink = RW
At the other extreme from complete
dominance is codominance in which
two alleles both show up
simultaneously to produce a third
phenotype.
For example, the ABO blood
groups of humans are due to
the presence of two specific
antigens on the surface of red
blood cells.
 People of type AB blood have
both molecules and both alleles
present.

In another example of
codominance, red cows
crossed with white bulls
make roan colored offspring.
Roan coloration is due to the
presence of both red and white
hairs expressed simultaneously.
Because an allele is dominant does
not necessarily mean that it is more
common in a population than the
recessive allele.


For example, polydactyly, in which
individuals are born with extra fingers or
toes, is due to an allele dominant to the
recessive allele for five digits per
appendage.
However, the recessive allele is far more
prevalent than the dominant allele in the
population.
 399 individuals out of 400 have five digits per
appendage.
Most genes have more than two alleles in a
population (multiple alleles)
The ABO blood groups in humans are
determined by three alleles, IA, IB, and i
(even though each individual can only have
two of these alleles).


Both the IA and IB alleles are dominant to the i
allele
The IA and IB alleles are codominant to each
other.
Because each individual carries two alleles,
there are six possible genotypes and four
possible blood types.




Individuals that are IA IA or IA i are type A
and place type A oligosaccharides on the
surface of their red blood cells.
Individuals that are IB IB or IB i are type B
and place type B oligosaccharides on the
surface of their red blood cells.
Individuals that are IA IB are type AB and
place both type A and type B
oligosaccharides on the surface of their
red blood cells.
Individuals that are ii are type O and
place neither oligosaccharide on the
surface of their red blood cells.
Matching compatible blood groups is
critical for blood transfusions because
a person produces antibodies against
foreign blood factors.

If the donor’s blood has an A or B
oligosaccharide that is foreign to the
recipient, antibodies in the recipient’s
blood will bind to the foreign molecules,
cause the donated blood cells to clump
together, and can kill the recipient.
http://www.bing.com/images/search?q=blood+types+and+red+blood+cells+and+antibodies&view=detailv2&&id=7065B4FA5DB2C858663177F
2EC5A8F3BFFCDE456&selectedIndex=0&ccid=TZzfE1UZ&simid=608031348362645359&thid=OIP.M4d9cdf135519e7b37acb5cdf1bd76540H0&aja
xhist=0
Who can you donate and
receive blood from?
Blood type
Can receive
from?
Can donate to?
A
A and O
A and AB
B
B and O
B and AB
AB (universal
recipient)
O (universal
donor)
AB, A, B and O AB
O
O, AB, A and B
What is the Rh factor?
The Rh factor is another antigen
found on the surface of the red
blood cells.
It was first identified in the rhesus
monkey.
If you have the rh antigen, you are
positive (RR or Rr)
If you don’t have it, you are
negative (rr)
Click on website
Some characters do not fit the either-or
basis that Mendel studied.
Quantitative characters vary in a population
along a continuum
These are usually due to polygenic
inheritance, the additive effects of two or
more genes on a single phenotypic
character.
 For example, skin color in humans is
controlled by at least three different
genes.
 Imagine that each gene has two alleles,
one light and one dark, that demonstrate
incomplete dominance.
 An AABBCC individual is dark and aabbcc
is light.
A cross between two AaBbCc
individuals (intermediate skin shade)
would produce offspring covering a
wide range of shades.
 Individuals with intermediate skin
shades would be the most likely
offspring, but very light and very
dark individuals are possible as
well.
 The range of phenotypes forms a
normal
distribution.
Phenotype depends on environment and
genes.
 A single tree has leaves that vary in size,
shape, and greenness, depending on
exposure to wind and sun.
 For humans, nutrition influences height,
exercise alters build, sun-tanning
darkens the skin, and experience
improves performance on intelligence
tests.
 Even identical twins, genetic equals,
accumulate phenotypic differences as a
result of their unique experiences.
The relative importance of genes and the
environment in influencing human
characteristics is a very old and hotly
contested debate.
What is epigenetics and how can environmental factors
affect gene expression making even identical twins
different?
the study of changes in gene activity that do not involve
alterations to the genetic code but still get passed down
to at least one successive generation.
These patterns of gene expression are governed by the
cellular material — the epigenome — that sits on top of
the genome, just outside it (hence the prefix epi-, which
means above).
It is these epigenetic "marks" that tell your genes to
switch on or off, to speak loudly or whisper.
It is through epigenetic marks that environmental factors
like diet, stress and prenatal nutrition can make an
imprint on genes that is passed from one generation to
the next.
http://learn.genetics.utah.edu/con
tent/epigenetics/control/
http://www.pbs.org/wgbh/nova/search/
results/?q=epigenetics
Chemical tags alter the
genome
Methyl groups cause
the genome to be
tightly wound up
The DNA cannot be
read and the gene is
turned off.
Acetyl groups cause
the genome to be
loosely wound
The DNA is easily
read and the protein
involved is
produced. The gene
is turned on.
G. Pedigree analysis reveals Mendelian
patterns in human inheritance
Rather than manipulate mating
patterns of people, geneticists analyze
the results of matings that have already
occurred.
In a pedigree analysis, information
about the presence/absence of a
particular phenotypic trait is collected
from as many individuals in a family as
possible and across generations.
The distribution of these characters is
then mapped on the family tree.
For example, the occurrence
of widows peak (W) is
dominant to a straight
hairline (w).
 The relationship among
alleles can be integrated
with the phenotypic
appearance of these traits to
predict the genotypes of
members of this family.

• For example, if an individual in the third
generation lacks a widow’s peak, but
both her parents have widow’s peaks,
then her parents must be heterozygous
for that gene
• If some siblings in the second generation
lack a widow’ peak and one of the
grandparents (first generation) also lacks
one, then we know the other grandparent
must be heterozygous and we can
determine the genotype of almost all
other individuals.
We can use the same family tree to
trace the distribution of attached
earlobes (f), a recessive
characteristic.
 Individuals with a dominant allele (F)
have free earlobes.
 Some individuals may be ambiguous,
especially if they have the dominant
phenotype and could be
heterozygous or homozygous
dominant.

A pedigree can help us understand the
past and to predict the future.
We can use the normal Mendelian rules,
including multiplication and addition, to
predict the probability of specific
phenotypes.

For example, these rules could be used to
predict the probability that a child with
WwFf parents will have a widow’s peak and
attached earlobes.
 The chance of having a widow’s peak is 3/4 in a
Ww x Ww cross.
 The chance of having attached earlobes is 1/4
[ff] in a Ff x Ff cross.
 This combination has a probability of 3/4 x 1/4
= 3/16.
Pedigree analysis helps us track a
particular trait in a family
Example: A family in Kentucky has an
unusual trait of blue skin.
It is an autosomal recessive condition.
Look at the Fugate family tree:
H. Many human disorders follow Click on
Mendelian patterns of inheritance website
Thousands of genetic disorders, including
disabling or deadly hereditary diseases, are
inherited as simple recessive traits.
 These range from the relatively mild
(albinism) to life-threatening (cystic
fibrosis).
The recessive behavior of the alleles occurs
because the allele codes for either a
malfunctioning protein or no protein at all.
 Heterozygotes have a normal phenotype
because one “normal” allele produces
enough of the required protein.
A recessively inherited disorder shows up
only in homozygous individuals who inherit
one recessive allele from each parent.
Individuals who lack the disorder are either
homozygous dominant or heterozygotes.
While heterozygotes may have no clear
phenotypic effects, they are carriers who
may transmit a recessive allele to their
offspring.
Most people with recessive disorders are
born to carriers with normal phenotypes.
 Two carriers have a 1/4 chance of having
a child with the disorder, 1/2 chance of a
carrier, and 1/4 free.
One such disease is cystic fibrosis which strikes
one of every 2,500 whites of European descent.
 One in 25 whites is a carrier.
 The normal allele codes for a membrane protein
that transports Cl- between cells and the
environment.
 If these channels are defective or absent, there
are abnormally high extracellular levels of
chloride that causes the mucus coats of certain
cells to become thicker and stickier than
normal.
 This mucus build-up in the pancreas, lungs,
digestive tract, and elsewhere favors bacterial
infections.
 Without treatment, affected children die before
five, but with treatment can live past their late
20’s.
Tay-Sachs disease is another lethal
recessive disorder.




It is caused by a dysfunctional enzyme
that fails to break down specific brain
lipids.
The symptoms begin with seizures,
blindness, and degeneration of motor
and mental performance a few months
after birth.
Inevitably, the child dies after a few
years.
Among Ashkenazic Jews (those from
central Europe) this disease occurs in one
of 3,600 births, about 100 times greater
than the incidence among non-Jews or
Mediterranean (Sephardic) Jews.
The most common inherited disease
among blacks is sickle-cell disease.
 It affects one of 400 African
Americans.
 It is caused by the substitution of a
single amino acid in hemoglobin.
 When oxygen levels in the blood of
an affected individual are low,
sickle-cell hemoglobin crystallizes
into long rods.
 This deforms red blood cells into a
sickle shape.
This sickling creates a cascade of
symptoms, demonstrating the
pleiotropic effects of this allele.
Doctors can use
regular blood
transfusions to
prevent brain
damage and new
drugs to prevent
or treat other
problems.
At the organismal level, the non-sickle
allele is incompletely dominant to the
sickle-cell allele.


Carriers are said to have the sickle-cell
trait.
These individuals are usually healthy,
although some suffer some symptoms of
sickle-cell disease under blood oxygen
stress.
At the molecule level, the two alleles
are codominant as both normal and
abnormal hemoglobins are
synthesized.
The high frequency of heterozygotes with
the sickle-cell trait is unusual for an allele
with severe detrimental effects in
homozygotes.


Interestingly, individuals with one sickle-cell
allele have increased resistance to malaria, a
parasite that spends part of its life cycle in red
blood cells.
In tropical Africa, where malaria is common, the
sickle-cell allele is both a boon and a bane.
 Homozygous normal individuals die of
malaria, homozygous recessive individuals die
of sickle-cell disease, and carriers are
relatively free of both.
Its relatively high frequency in African
Americans is a vestige of their African
roots.
Normally it is relatively unlikely that
two carriers of the same rare harmful
allele will meet and mate.
However, consanguineous matings,
those between close relatives,
increase the risk.

These individuals who share a recent
common ancestor are more likely to carry
the same recessive alleles.
Most societies and cultures have laws
or taboos forbidding marriages
between close relatives.
Although most harmful alleles are recessive,
many human disorders are due to dominant
alleles.
For example, achondroplasia, a form of
dwarfism, has an incidence of one case in
10,000 people.


Heterozygous individuals have the dwarf
phenotype.
Those who are not achodroplastic dwarfs,
99.99% of the population are homozygous
recessive for this trait.
Lethal dominant alleles are much less
common than lethal recessives because if a
lethal dominant kills an offspring before it
can mature and reproduce, the allele will not
be passed on to future generations.
A lethal dominant allele can escape
elimination if it causes death at a
relatively advanced age, after the
individual has already passed on the
lethal allele to his or her children.
One example is Huntington’s disease,
a degenerative disease of the nervous
system.


The dominant lethal allele has no obvious
phenotypic effect until an individuals is
about 35 to 45 years old.
The deterioration of the nervous system
is irreversible and inevitably fatal.
Any child born to a parent who has
the allele for Huntington’s disease has
a 50% chance of inheriting the
disease and the disorder.
Recently, molecular geneticists have
used pedigree analysis of affected
families to track down the
Huntington’s allele to a locus near the
tip of chromosomes 4.
While some diseases are inherited in a
simple Mendelian fashion due to alleles at a
single locus, many other disorders have a
multifactorial basis.


These have a genetic component plus a
significant environmental influence.
Multifactorial disorders include heart disease,
diabetes, cancer, alcoholism, and certain mental
illnesses, such a schizophrenia and manicdepressive disorder.
At present, little is understood about the
genetic contribution to most multifactorial
diseases

The best public health strategy is education
about the environmental factors and healthy
behavior.
I. Technology is providing new tools for
genetic testing and counseling
The risk that a particular genetic
disorder will occur can sometimes
be assessed before a child is
conceived or early in pregnancy.
Many hospitals have genetic
counselors to provide information
to prospective parents who are
concerned about a family history of
a specific disease.
Most children with recessive disorders are
born to parents with a normal phenotype.
A key to assessing risk is identifying if
prospective parents are carriers of the
recessive trait.
Recently developed tests for several
disorders can distinguish between normal
phenotypes in heterozygotes from
homozygous dominants.
The results allow individuals with a family
history of a genetic disorder to make
informed decisions about having children.
However, issues of confidentiality,
discrimination, and adequate information
and counseling arise.
Tests are also available to determine
in utero if a child has a particular
disorder.
One technique, amniocentesis, can be
used beginning at the 14th to 16th
week of pregnancy to assess the
presence of a specific disease.
 Fetal cells extracted from amniotic
fluid are cultured and karyotyped to
identify some disorders.
 Other disorders can be identified
from chemicals in the amniotic
fluids.
A second technique, chorionic villus
sampling (CVS) can allow faster
karyotyping and can be performed as
early as the eighth to tenth week of
pregnancy.


This technique extracts a sample of fetal
tissue from the chorionic villi of the
placenta.
This technique is not suitable for tests
requiring amniotic fluid.
Some genetic tests can be detected at birth
by simple tests that are now routinely
performed in hospitals.
One test can detect the presence of a
recessively inherited disorder,
phenylketonuria (PKU).




This disorder occurs in one in 10,000 to 15,000
births.
Individuals with this disorder accumulate the
amino acid phenylalanine and its derivative
phenypyruvate in the blood to toxic levels.
This leads to mental retardation.
If the disorder is detected, a special diet low in
phenyalalanine usually promotes normal
development.
J. Mendel’s laws were confirmed by the
chromosomal theory of inheritance
Around 1900, cytologists and
geneticists began to see parallels
between the behavior of chromosomes
and the behavior of Mendel’s factors.



Chromosomes and genes are both present
in pairs in diploid cells.
Homologous chromosomes separate and
alleles segregate during meiosis.
Fertilization restores the paired condition
for both chromosomes and genes.
K. Morgan discovered how genes can be
linked to the sex chromosomes
Thomas Hunt Morgan was the first to
associate a specific gene with a specific
chromosome in the early 20th century.
Like Mendel, Morgan made an
insightful choice as an experimental
animal, Drosophila melanogaster, a
fruit fly species that eats fungi on fruit.


Fruit flies are prolific breeders and have a
generation time of two weeks.
Fruit flies have three pairs of autosomes
and a pair of sex chromosomes (XX in
females, XY in males).
Morgan spent a year looking for
variant individuals among the flies he
was breeding.

He discovered a single male fly with
white eyes instead of the usual red.
The normal character phenotype is
the wild type.
Alternative
traits are
mutant
phenotypes.
When Morgan crossed his white-eyed
male with a red-eyed female, all the
F1 offspring had red eyes,

The red allele appeared dominant to the
white allele.
Crosses between the F1 offspring
produced the classic 3:1 phenotypic
ratio in the F2 offspring.
Surprisingly, the white-eyed trait
appeared only in males.

All the females and half the males had
red eyes.
Morgan concluded that a fly’s eye
color was linked to its sex.
Sex Linkage
When a gene is sex linked, it
means that it is carried on either
the X or Y chromosome.
Only a few genes are carried on
the Y chromosome and are said
to be “holandric”.
An example of a holandric trait is
hair on the earlobe (a hairy
pinna)
Who are the only individuals
able to get holandric traits?
Sex Linkage on the X
chromosome
Most sex linked trait are carried
on the X chromosome.
Examples include hemophilia,
colorblindness, Duchennes
muscular dystrophy and malepatterned baldness.
Why are males more likely to get
these sex linked traits than
females?
If a sex-linked trait is due to a recessive
allele, a female has this phenotype only if
homozygous.

Heterozygous females will be carriers.
Because males have only one X
chromosome (hemizygous), any male
receiving the recessive allele from his
mother will express the trait.
The chance of a female inheriting a double
dose of the mutant allele is much less than
the chance of a male inheriting a single
dose.
Therefore, males are far more likely to
inherit sex-linked recessive disorders than
are females.
Several serious human disorders are
sex-linked.
Duchenne muscular dystrophy affects
one in 3,500 males born in the United
States.

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Affected individuals rarely live past their
early 20s.
This disorder is due to the absence of an
X-linked gene for a key muscle protein,
called dystrophin.
The disease is characterized by a
progressive weakening of the muscles
and loss of coordination.
Hemophilia is a sex-linked recessive
trait defined by the absence of one or
more clotting factors.

These proteins normally slow and then
stop bleeding.
Individuals with hemophilia have
prolonged bleeding because a firm
clot forms slowly.

Bleeding in muscles and joints can be
painful and lead to serious damage.
Individuals can be treated with
intravenous injections of the missing
protein.
In fruit flies:
Females (XX) may
have two red-eyed
alleles and have
red eyes or may be
heterozygous and
have red eyes.
 Males (XY) can
only have one
allele for either red
or white since they
have only one X
chromosome.

Sex Linked Punnett Squares
Most of the sex linked traits are recessive.
Therefore, write a superscript over only the X
chromosome to indicate the presence of the
trait involved.
For example, in colorblindness or hemophilia,
let N= normal, n = colorblind or hemophiliac
A female can be XNXN or XNXn (normal) or
XnXn (have the disease)
A male can only be XNY or XnY. He only needs
one dose of the gene to be either normal or
have the disease.
Do the punnett squares as you would a
monohybrid cross but use both X with the “N”
alleles attached and Y to show the results.
Cross a hemophiliac male with a female that is
not a hemophiliac but whose father was.
In fruit flies, eye color is a sex linked trait. Red
(wildtype) is dominant to white eyes.
Show a cross between a homozygous pure wildtype
eyed female and a white -eyed male.
Note: SRY gene
controls proteins
necessary for male
sexual development
L. Linked genes tend to be inherited
together because they are located on the
same chromosome
Each chromosome has hundreds or
thousands of genes.
Genes located on the same
chromosome, linked genes, tend to be
inherited together because the
chromosome is passed along as a unit.
Results of crosses with linked genes
deviate from those expected according
to independent assortment.
Morgan observed this linkage and its
deviations when he followed the
inheritance of characters for body
color and wing size.


The wild-type body color is gray (B)and
the mutant black (b).
The wild-type wing size is normal (V) and
the mutant has vestigial wings (v).
Morgan test crossed F1 heterozygous
females (BbVv) with homozygous
recessive males (bbvv).
According to independent
assortment, this should produce 4
phenotypes in a 1:1:1:1 ratio.
Surprisingly, Morgan observed a
large number of wild-type (graynormal) and double-mutant
(black-vestigial) flies among the
offspring.
 These phenotypes correspond
to those of the parents.
animation
Morgan reasoned that body color and
wing shape are usually inherited
together because their genes are on
the same chromosome.
B V
B V
• The other two phenotypes (grayvestigial and black-normal) were
fewer than expected from
independent assortment (and totally
unexpected from dependent
assortment).
• These new phenotypic variations
must be the result of crossing over
(aka recombination)
In contrast, linked genes, genes
located on the same chromosome,
tend to move together through
meiosis and fertilization.
Under normal Mendelian genetic
rules, we would not expect linked
genes to recombine into assortments
of alleles not found in the parents.
The results of Morgan’s testcross for body
color and wing shape did not conform to
either independent assortment or complete
linkage.
 Under independent assortment the
testcross should produce a 1:1:1:1
phenotypic ratio.
 If completely linked, we should expect to
see a 1:1:0:0 ratio with only parental
phenotypes among offspring.
Most of the offspring had parental
phenotypes, suggesting linkage between
the genes.
However, 17% of the flies were
recombinants, suggesting recombination
(crossing over)
Morgan proposed that some
mechanism occasionally exchanged
segments between homologous
chromosomes.

This switched alleles between homologous
chromosomes.
The actual mechanism, crossing over
during prophase I, results in the
production of more types of gametes
than one would predict by Mendelian
rules alone.
The occasional production of
recombinant gametes during prophase
I accounts for the occurrence of
recombinant phenotypes in Morgan’s
testcross.
M. Geneticists can use recombination
data to map a chromosome’s genetic loci
One of Morgan’s students,
Alfred Sturtevant, used
crossing over of linked genes to
develop a method for
constructing a chromosome
map.
This map is an ordered list of
the genetic loci along a
particular chromosome.
Sturtevant hypothesized that the
frequency of recombinant offspring
reflected the distances between
genes on a chromosome.
The farther apart two genes are, the
higher the probability that a crossover
will occur between them and
therefore a higher recombination
frequency.

The greater the distance between two
genes, the more points between them
where crossing over can occur.
Sturtevant used the test cross design to
map the relative position of three fruit fly
genes, body color (b), wing size (vg), and
eye color (cn).

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The recombination frequency between cn and b
is 9%.
The recombination frequency between cn and
vg is 9.5%.
The recombination
frequency between
b and vg is 17%.
The only possible
arrangement of these
three genes places
the eye color gene
between the other two.
Sturtevant expressed the distance
between genes, the recombination
frequency, as map units.

One map unit is equivalent to a 1%
recombination frequency.
You may notice that the three
recombination frequencies in our
mapping example are not quite
additive: 9% (b-cn) + 9.5% (cn-vg) >
17% (b-vg).
This results from multiple crossing
over events.
Some genes on a chromosome are so far
apart that a crossover between them is
virtually certain.
In this case, the frequency of
recombination reaches is its maximum
value of 50% and the genes act as if found
on separate chromosomes and are
inherited independently.
 In fact, several genes studies by Mendel
are located on the same chromosome.
 For example, seed color and flower
color are far enough apart that linkage
is not observed.
 Plant height and pod shape should
show linkage, but Mendel never
reported results of this cross.
Sturtevant and his
colleagues were able
to map the linear
positions of genes in
Drosophila into four
groups, one for each
chromosome.
Map units indicate relative
distance and order, not precise
locations of genes.
More recent techniques show
the absolute distances
between gene loci in DNA
nucleotides.
N. Alterations of chromosome number or
structure cause some genetic disorders
Nondisjunction occurs when
problems with the meiotic spindle
cause errors in daughter cells.
This may occur if tetrad
chromosomes do not separate
properly during meiosis I.
 Alternatively, sister chromatids may
fail to separate during meiosis II.


Note: This is a chromosomal aberration not a specific gene
mutation!
As a consequence of nondisjunction,
some gametes receive two of the same
type of chromosome and another
gamete receives no copy.
Offspring results from fertilization of a
normal gamete with one after
nondisjunction will have an abnormal
chromosome number or aneuploidy.
 Trisomic cells have three copies of a
particular chromosome type and
have 2n + 1 total chromosomes.
One trisomy condition, Down
syndrome, is due to three copies
of chromosome 21.
 It affects one in 700 children
born in the United States.
Although chromosome 21 is the
smallest human chromosome, it
severely alters an individual’s
phenotype in specific ways.
Most cases of Down syndrome result
from nondisjunction during gamete
production in one parent.
The frequency of Down syndrome
correlates with the age of the mother.

This may be linked to some agedependent abnormality in the spindle
checkpoint during meiosis I, leading to
nondisjunction.
Trisomies of other chromosomes also
increase in incidence with maternal
age, but it is rare for infants with
these autosomal trisomies to survive
for long.
Klinefelter’s syndrome, an XXY male, occurs
once in every 2000 live births.
 These individuals have male sex organs,
but are sterile.
 There may be feminine characteristics,
but their intelligence is normal.
Males with an extra Y chromosome (XYY)
tend to somewhat taller than average.
Trisomy X (XXX), which occurs once in every
2000 live births, produces healthy females.
Monosomy X or Turner’s syndrome (X0),
which occurs once in every 5000 births,
produces phenotypic, but immature females.
Organisms with more than two
complete sets of chromosomes, have
undergone polyploidy.
This may occur when a normal
gamete fertilizes another gamete in
which there has been nondisjunction
of all its chromosomes.

The resulting zygote would be triploid
(3n).
Alternatively, if a 2n zygote failed to
divide after replicating its
chromosomes, a tetraploid (4n)
embryo would result from subsequent
successful cycles of mitosis.
Polyploidy is relatively common
among plants and much less common
among animals.

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The spontaneous origin of polyploid
individuals plays an important role in the
evolution of plants.
Both fishes and amphibians have
polyploid species.
Recently, researchers
in Chile have
identified a new
rodent species
which may be
the product of
polyploidy.
Breakage of a chromosome can lead
to four types of changes in
chromosome structure.
A deletion occurs when a
chromosome fragment lacking a
centromere is lost during cell division.

This chromosome will be missing certain
genes.
A duplication occurs when a fragment
becomes attached as an extra
segment to a sister chromatid.
An inversion occurs when a
chromosomal fragment
reattaches to the original
chromosome but in the reverse
orientation.
In translocation, a chromosomal
fragment joins a nonhomologous
chromosome.
 Some translocations are
reciprocal, others are not.
Deletions and duplications are common
in meiosis.

Homologous chromatids may break and
rejoin at incorrect places, such that one
chromatid will lose more genes than it
receives.
A diploid embryo that is homozygous
for a large deletion or male with a
large deletion to its single X
chromosome is usually missing many
essential genes and this leads to a
lethal outcome.

Duplications and translocations are
typically harmful.
Structural alterations of chromosomes
can also cause human disorders.
Deletions, even in a heterozygous
state, cause severe physical and
mental problems.
One syndrome, cri du chat, results
from a specific deletion in
chromosome 5.


These individuals are mentally retarded,
have a small head with unusual facial
features, and a cry like the mewing of a
distressed cat.
This syndrome is fatal in infancy or early
childhood.
O. Mutations are caused
by environmental factors
Radiation from the bombing of Hiroshima and
the nuclear accident at Chernobyl are causes
for mutagenic agents.
A person exposed to such high doses of
radiation may have:
•
•
•
•
•
•
•
•
a drop in blood cell counts
hair loss
nausea
vomiting
diarrhea
skin burns
an eventual increased risk for blood and thyroid cancers
eventual death as a result of organ failure
http://www.dailyinfographic.com/wp-content/uploads/2011/03/radiation-infographic.jpg
P. Mitochondrial DNA has genes too!
Not all of a eukaryote cell’s genes are
located in the nucleus.
Other genes are found on small circles
of DNA in mitochondria and
chloroplasts.
These organelles reproduce
themselves.
Their cytoplasmic genes do not display
Mendelian inheritance.

They are not distributed to offspring
during meiosis.
Because a zygote inherits all its
mitochondria only from the ovum, all
mitochondrial genes in mammals
demonstrate maternal inheritance.
Several rare human disorders are produced
by mutations to mitochondrial DNA.
 These primarily impact ATP supply by
producing defects in the electron
transport chain or ATP synthase.
 Tissues that require high energy supplies
(for example, the nervous system and
muscles) may suffer energy deprivation
from these defects.
 Other mitochondrial mutations may
contribute to diabetes, heart disease, and
other diseases of aging.