Transcript Ch 14 Lecture

Ch. 14:
Mendel and the Gene Idea
I. Introduction
A. Heritable traits (brown eyes, green eyes,
blue eyes) are passed down from parents
to offspring.
B. Blending hypothesis: Offspring should
have a blend of parental traits.
(Yellow + Blue = Green)
 This blending hypothesis is incorrect.
C. Particulate inheritance: parental genes
retain their separate identities, and are
sorted and then passed on to future
generations.
D. Gregor Mendel documented particulate
inheritance by studying sweet pea plants.
II. Gregor Mendel
A. An Augustinian monk who studied
the sciences and math. His studies
influenced his experimentation in genetics.
B. In 1857, Mendel began breeding garden
peas to study inheritance.
1. Peas were an ideal subject to study:
a. Many heritable traits (color, height,
pod shape, etc.)
b. Easy to control mating
- Self-pollination
- Cross-pollination
C. Mendel’s experiment:
1. Mendel cross-pollinated to hybridize
two contrasting, true-breeding pea
varieties.
- True-breeding: P generation
- Offspring: F1 generation
2. Mendel then allowed for the F1
generation to self-pollinate to produce
the F2 generation.
3. Mendel quantitative analysis of the
F2 generation enabled him to come up
with two important principles of heredity:
a. The law of segregation
b. The law of independent assortment
D. Law of segregation: two alleles for a
trait are packaged into separate gametes.
1. When the P generation cross-pollinated,
the F1 generation were all purple.
2. When the F1
generation
self-pollinated,
the F2
generation had
both purple
and white.
The ratio between purple and white
flowers in the F2 generation were 3:1.
Mendel reasoned
that though the
F1 generation had
no white flowers,
they must have
carried the
heritable trait for
white flower color.
Mendel said that the purple color gene
was “dominant” and the white color
gene was “recessive.”
3. Mendel found similar 3:1 ratios of traits
in the F2 generation when he conducted
crosses for these traits:
Example: Round v. Wrinkled pea seeds
F1 = 100% Round
F2 = 75% Round, 25% Wrinkled
4. Mendel stated that different genes
(different alleles) account for variations
in inherited characteristics.
Ex. Purple and
white color genes
have different
DNA sequences.
5. Mendel also stated that an organism
inherits two alleles for each trait, one
from each parent.
-Diploid
-Homologous chromosomes
6. If two alleles inherited are different, then
the dominant allele will be expressed.
The recessive allele will have no effect.
7. The two alleles segregate during gamete
production; homologues will separate
when gametes are made.
 This is the Law of Segregation
8. Mendel’s Law of Segregation accounts
for his observation of 3:1 in the F2
generation:
a.The F1 generation will create two kinds
of gametes = half will have the white
allele, half will have the purple allele.
b. During self-pollination, the gametes
will combine randomly to form four
combinations.
c. A Punnett Square can be made to
predict the results of a genetic cross:
9. An organism with two identical alleles
for a character is homozygous for that
trait.
10.An organism with two different alleles
for a character is heterozygous for that
trait.
11.An organisms genetic makeup is called
its genotype.
12.An organisms physical traits is called
its phenotype.
 Two organisms can have the same
phenotype but different genotypes.
The only way
to produce a
white offspring
is to have two
recessive
traits
(homozygous
recessive).
13.You cannot predict correctly the
genotype of an organism with a
dominant phenotype.
 A purple flower could be PP, or Pp.
a. The only way to know the genotype of
an organism with
a dominant traits
is by doing a
“test-cross.”
E. Law of Independent Assortment: Each
pair of alleles segregates into gametes
independently.
1. Mendel did other experiments that
followed the inheritance of two different
characters in a dihybrid cross
(v. monohybrid).
2. Mendel studied seed shape and color:
Y = Yellow, y = Green
R = Round, r = wrinkled
3. Mendel crossed true-breeding plants
that had yellow, round seeds (YYRR)
with true-breeding plants that has
green, wrinkled seeds (yyrr).
4. One possibility is that the two
characteristics could be transmitted as
a package.
 F1 = Yellow, Round
 F2 = 3 Yellow, Round:
1 Green, Wrinkled
** However, this was not the results of
Mendel’s experiment.
5. Instead, the two characteristics
segregate independently of one another.
F1 = Yellow, Round
Their gametes can have four
combinations: YR, Yr, yR, yr
Therefore, the ratios for the F2
generation is:
F2 = 9 Yellow, Round (Yy, Rr):
3 Yellow, Wrinkled (Yy, rr):
3 Green, Round (yy, Rr):
1 Green, Wrinkled (yy, rr)
** Whenever Mendel did a dihybrid cross,
he always got the 9:3:3:1 ratio. This
can be explained as the result of the
“Law of Independent Assortment.”
III.Extending Mendelian Genetics
A. Mendel chose to study pea characteristics
that had simple genetic; each phenotypic
characteristic was determined by one gene.
B. However, in most cases, the relationship
between phenotype and genotype is
rarely simple.
C. Incomplete
Dominance: F1 hybrids (heterozygotes)
show a intermediate phenotype:
Example:
Snapdragons
P = Red, White
F1 = 100% Pink
F2 = 25% Red
50% Pink
25% White
Mendel’s peas
showed that
heterozygous
and homozygous
dominant plants
had the same
phenotype. This is
complete dominance.
D. Codominance: two alleles affect the
phenotype in distinguishable ways.
Example: Blood types A, B, AB, and O
Tay-Sach’s Disease
E. Because an allele is dominant does not
necessarily mean that it is more common
in a population than the recessive allele.
Example: Polydactyly is a dominant gene.
However, the recessive allele is far more
prevalent.
F. Multiple Alleles: Most genes have more
than one allele in a population.
1. Example: Blood alleles A, B, and O
with four possible phenotypes for blood
type.
a. A and B alleles are codominant.
b. Both A and B alleles are dominant to
O.
c. A type blood = genotype AA or AO.
A type blood has oligosaccharides on
the surface of the RBCs. A type blood
also produces antibodies against B
type blood.
d. B type blood = genotype BB or BO.
B type blood has oligosaccharides on
the surface of RBCs. B type blood
also produces antibodies against A
type blood.
e. Individuals with O type blood have
neither A or B type oligosaccharides
on their RBCs. They do however
produce antibodies against both A
and B blood types.
f. Matching compatibility for blood type
is crucial for transfusions.
- O type blood = universal donor
- AB type blood = universal recipient
G. Pleiotropy: Genes have multiple effects.
Example: Sickle Cell gene has multiple of
effects.
H. Epistasis: a gene on one locus can alter
the effects of another gene on a different
locus.
Example: Coat color in mice is determined
by two genes.
1. The epistatic gene, determines whether
or not pigment will be deposited into
hair.
Presence of pigment ( C ) is dominant
to no presence ( c ).
2. Pigment color black (B) is dominant to
brown pigment (b).
3. A mouse with cc will be an albino
regardless of genotypes for hair color
black or brown.
4. A cross between
two mice that
are heterozygous
for both genes
(Cc, Bb) follows
the law of
independent
assortment:
However, the
ratio will be
9 black:3 brown:
4 white
I. Polygenic Inheritance: an additive effect
of two or more genes on a single
phenotypic character.
1. Example: Human skin color is due to
more than 3 separate genes. For
simplicity sake, however, we will
consider just 3 genes for determining
skin color.
a. A, B, and C are the 3 different genes,
all incompletely dominant to the
alleles (a, b, and c).
b. An individual with AABBCC genotype
will be very dark. An individual with
AaBbCc genotype will have an
intermediate shade.
c. Because the alleles have a cumulative
effect, an individual with genotype
AaBbCc will have the same skin color
as an individual with genotype AABbcc.
d. A cross between
two individuals
with AaBbCc
genotypes
would result
in a bellshaped
curve,
called a
normal
distribution.
J. The environmental impact on phenotype:
1. Example: Nutrition can influence height.
Exercise influences build. Sun exposure
influences skin color. Practice improves
intelligence tests.
 Identical twins?
2. The products of a genotype can be a
wide range of variation. This phenotypic
range due to the environment is called
the norm of reaction.
Norm of reaction:
colors of hydrangea
flowers, range from
blue to pink, depending
on the acidity of soil.
IV. Mendelian Inheritance in Humans
A. While peas are an easy subject to study
genetics, humans are not.
1. Human generation span is too long.
2. Parents produce few offspring.
3. Breeding experiments is socially
unacceptable.
B. Pedigree analysis reveals Mendelian
patterns in human inheritance.
1. Phenotypic information is gathered from
as many members of a family across
generations.
2. The information can then be mapped
onto a family tree.
3. Example: Widow’s peak (W) is dominant
over a straight hairline (w). We can try
and predict the genotypes of individuals
in a family tree.
a. If an individual has no widow’s peak
but both his/her parents have a
widow’s peak, we can predict that both
parents are heterozygous.
4. A pedigree can help us understand the
past and predict the future.
C. Many human disorders follow Mendelian
patterns of inheritance.
1. Thousands of genetic disorders can be
inherited as recessive traits.
2. Genetic disorders can range from mild
to life-threatening.
3. Heterozygous individuals are
phenotypically normal but are “carriers”
of the disorder.
4. Most individuals that are born with a
disorder are born to carriers with normal
phenotypes.
a. Two carriers have ¼ probability of
having a child with the disorder, ½
probability of having carriers, and ¼
free.
5. Some genetic disorders are found more
commonly among people of certain
ethnic backgrounds.
a. Cystic fibrosis: 1 in 2,500 caucasians
of european descent. 1 in 25
caucasians are carriers. CF is caused
by a defective Cl- membrane protein
that causes build up of mucus in the
lungs, pancreas, & digestive system.)
b. Tay-Sachs: 1 in 3,600 births in
Ashkenazic Jews. T-S is caused by
a defective enzyme that cannot break
down certain lipids in the brain. This
causes brain degeneration.
c. Sickle-Cell Anemia:
1 in 400 African
Americans. It is
caused by a
substitution in one
amino acid of the
hemoglobin protein.
When oxygen levels
are low in blood, red
blood cells deform
into a sickle shape.
This sickling can cause a number of results.
This sickling has pleiotropic effects.
The non-sickling allele is incompletely
dominant to the sickle-cell allele.
Heterozygous individuals are carriers
and can suffer some symptoms of
the disease under blood oxygen
stress. Both normal and abnormal
hemoglobins are synthesized.
Individuals that are heterozygous
are also resistant to malaria, a
parasite that spends part of its life
cycle inside RBCs.
5. Dominant Inherited Disorders:
a. Achondroplasia, a form of dwarfism,
has an incidence of one case in
10,000 people.
b. Lethal dominant alleles are much less
common than lethal recessives
because an offspring with a lethal
dominant will die before passing the
allele on to future generations.
c. A lethal dominant allele can be passed
down to generations if the onset of
the disease is later in life.
Example: Huntington’s Disease, a
degenerative brain disorder; onset
bt. 35-45 years of age.
-Offspring born to a parent who has
the allele for Huntington’s disease has
a 50% chance of inheriting the
disease and the disorder.
-Molecule geneticists have recently
discovered that the gene for HD is
found on the tip of chromosome #4.
d. Polydactyly: a rare dominant
phenotype
6. Multifactorial Diseases: Diseases caused
by genetics and the environment.
Example: Heart disease, diabetes,
cancer, alcoholism, schizophrenia, and
manic-depressive disorder.
7. Albinism is a rare condition that is
inherited as a recessive phenotype in
many animals, including humans.
D. Technology is providing new tools for
genetic testing and counseling.
1. Many hospitals have genetic counselors
that can provide information to
prospective parents who are concerned
about a family history of a specific
disease.
2. Using Mendelian probability (Punnett
Squares), one can determine the risks
of passing on lethal genes.
3. Tests determine in utero if a child has a
disorder. One technique is called an
amniocentesis.
-can be done after 14-16 weeks of
pregnancy.
-fetal cells are extracted and karyotyped.
-other disorders can be detected from
chemicals in the amniotic fluid.
4. Chorionic villus sampling (CVS) can
allow faster karyotyping and can be
performed as early as the eighth to
tenth week of pregnancy.
-Fetal tissue is extracted from the
chrionic villi of the placenta.
5. Ultrasound and fetoscopy, allow fetal
health to be assessed visually in utero.
-Both fetoscopy and amniocentesis
cause complications in about 1% of
cases. They can cause maternal
bleeding or fetal death.
-These techniques are usually reserved
for cases in which the risk of a genetic
disorder or other type of birth defect is
relatively great.
After the results of a test are revealed,
the parents must face the difficult decision
of terminating the pregnancy or preparing
to care for a child with a genetic disorder.
6. Some genetics tests can be done after
the child is born.
-Example: PKU - phenyketonuria
-1 in 10,000-15,000 births
-causes a build-up of the amino acid
phenylalanine, and its derivative
phenypyruvate in the blood to toxic
levels.
-this build-up can cause mental
retardation.
-if the genetic test is given at birth, a
child can be given a special diet low in
phenylalanine, which usually promotes
normal growth.