Download Chapter 14 Notes - Parkway C-2

Chapter 14 - Gregor Mendel and the
Gene Idea
A. Bergeron
+AP Biology
PCHS
Do Now - Now Where Did I Put that Bug Spray?
In fruit flies a recessive mutation in either of two independently
assorting genes, brn and prp prevents the synthesis of
wild-type (i.e. normal) red eye pigment. Homozygotes for either
of these mutations have brownish purple eyes. But
heterozygotes for both of these mutations have wild type eyes.
1) If double heterozygotes are crossed with each other what
are the phenotypes and expected phenotypic ratios of the
offspring?
2) Which mode of inheritance seems to be at play in this
problem?
3) Where would you predict the brn and prp genes to be located
in relation to one another?
Hint: Let the wild type alleles of each gene be brn+ and prp+,
respectively.
Mendel’s Conclusions (deduced from a series of
hypotheses that he tested experimentally)
1. Alternative versions of genes (i.e. alleles) account for
variations in inherited characters/traits
2. An organism inherits two alleles (two versions of the
same gene) from each parent (see next slide)
3. Dominant and recessive alleles for each character/trait
may exist (But what does it mean to be dominant?
Recessive?)
4. The alleles for each character/trait segregate during
gamete production (i.e. meiosis)
Two Alleles for Every Trait (most of the time…)
Recall that offspring inherit one chromosome (of each pair)
from Mom and Dad
Together, these chromosomes make-up a homologous pair
The location (i.e. locus)
of a particular gene is
the same on both
chromosomes but
the allele may be different
Allele: alternative
forms of the same
gene
Mendel’s Genetic “Laws”
1. Principle of Segregation
Alleles for different traits
segregate independently (as
long as the alleles are on
SEPARATE chromosomes)
Equal probability of passing
on either allele to the F2
generation (see example)
Segregation of allelic pairs
occurs during meiosis
(anaphase I)
Restoration of paired alleles
occurs at fertilization
Mendel’s Principle of Segregation
Mendel hypothesized that
there were two factors (i.e.
alleles) for a particular trait
Each parent passes on one
allele to their offspring
But why?
Recall that homologous
chromosomes separate
into different cells during
meiosis I
As a result, each gamete
will receive only only allele
for a particular trait
Useful Genetics Vocabulary
1. Homozygous - Having two identical alleles for a particular
trait
-Homozygotes are “true-breeding” or pure bred organisms
-Can only pass on one type of allele to offspring
2. Heterozygous - Having two different alleles for a particular
trait
!
-Heterozygotes are often referred to as hybrids or
!
carriers
3. Phenotype - An organism’s traits that are based on its
genotype (Note: Phenotype is not necessarily the outward,
physical appearance)
4. Genotype - The genetic make-up of an organism
!
-Genotype determines phenotype
Mendel’s Genetic “Principles”
2. Principle of Independent Assortment
Each allele pair segregates
independently of the other
allele pairs during gamete
formation (i.e. meiosis)
Mendel’s Law of Independent Assortment
(Assume genes are on separate chromosomes)
Mom
Dad
A
a
A
a
B
b
b
B
A
a
A
a
B
b
b
B
A
A
a
a
B
B
b
b
Sex
cells
A
A
a
a
b
b
B
B
Independent Assortment
Important Genotypic and Phenotypic Ratios
Type of
Parent
Cross
Genotypes
Monohybrid Heterozygous
Genotype
Ratio
1:2:1
Phenotype
Ratio
3:1
Dihybrid
Complicated
9:3:3:1
Heterozygous
(for both traits)
Ratios can be predicted with a Punnett square (but Punnett
squares are evil!) and there is an easier way…
Do Now - Using Probability to Solve Genetics Problems
What is the probability that in a trihybrid cross between two
organisms with the genotypes AaBbCc and AaBbCc that the
parents will produce an offspring with the genotype aabbcc?
(Hint: Consider each pair of alleles as a single “coin flip”)
What is the probability that the two parents will produce an
offspring that is dominant for the traits controlled by genes
A and C but recessive for the trait controlled by gene B?
Using Probability to Solve Genetics Problems
The probability of an event ranges from 0 (not going to happen)
to 1 (certain to occur)
The probabilities of all outcomes of an event must add up to 1
Random events are independent of one another
!
-The outcome of one event does not influence the
!
outcome of a previous or future event
Using Probability to Solve Genetics Problems
Rule of Multiplication
-The probability that independent events will occur
simultaneously is equal to the product of their individual
probabilities
Question - Determine the probability that you will role two
“6’s” in a row on a single die. Three “6’s?” Seven “6’s?”
Question - In a Mendelian cross between pea plants that are
heterozygous for flower color (Pp), what is the probability that
the offspring will be homozygous recessive?
Question - What is the probability that in a trihybrid cross
between two organisms with the genotypes AaBbCc and
AaBbCc that the parents will produce an offspring with the
genotype aabbcc? (Hint: Consider each pair of alleles as a
single “coin flip”)
Using Probability to Solve Genetics Problems
2. Rule of Addition (Either-or Rule)
-Probability that an event will occur is the sum of the
probabilities of each way that it can occur.
-Useful when you want to know the probability of an event
happening when there are several ways for the event
to occur
Question - If two parents are heterozygous for a trait, what
is the probability, that their child will be heterozygous as well?
Question - What is the probability of tossing three coins
simultaneously and obtaining two heads and one tail?
Question - If pea plants with the genotypes PpYyRr and
Ppyyrr are crossed, what is the probability that offspring will
be recessive for at least 2 of the 3 traits?
Using Math to Solve Genetics Problems
(Wait…Mr. B Knows Math?)
If only two alleles exist for a particular character/trait, the distribution of
phenotypes among the offspring of a cross can be modeled with a
binomial distribution.
Question - Imagine that a couple has chosen to have 3
children. How likely is it that two of the children will be boys
and one will be a girl?
Let p = probability of having a male child
Let q = probability of having a female child
p+q=1
p = q = 1/2
Family Composition
3 boys
Order of birth
bbb
Calculation
pxpxp
Prob.
p3
2 boys and 1 girl
bbg
bgb
gbb
pxpxq
pxqxp
qxpxp
3p2q
bgg
gbg
ggb
pxqxq
qxpxq
qxqxp
ggg
qxqxq
1 boy and 2 girls
3 girls
3q2p
q3
3p2q = 3 (1/2)2(1/2) = 3/8
Genetics Practice Problems
Question - Calculate the probability that two parents heterozygous
for the recessive allele producing albinism (a) will have one albino
child in a family of five.
Question - Imagine that a couple has chosen to have five children.
How likely is it that three of the children will be boys and two will be
girls?
Assume that the ability to hear is controlled by 3 pairs (6 alleles) of
independently assorting genes and that normal hearing occurs when
an individual has the genotype A_ or B_. The presence of DD,
however, causes deafness, no matter what other allelic combinations
are present.
Question: Write out all of the possible genotypes of the offspring
produced from a mother with the genotype AABBDd and a father with
the genotype AaBbDd using the underscore format.
Question: Predict the phenotype ratios of the offspring that would be
produced if the above parents were to have children.
Genetics Practice Problems
Flower color in pea plants is determined by two gene pairs (A/a
and B/b). For the first gene pair, the ‘A’ allele is dominant to the
‘a’ allele, while for the second gene pair, the ‘B’ allele is
dominant to the ‘b’ allele. If at least one dominant allele from
each gene is present, the flowers are purple. All other
genotypes have white flowers.
1. What is the phenotype of a plant that has the Aabb
genotype?
2. Using the symbols provided above, write all possible
genotypes for a plant that has purple flowers.
3. If two plants that have the AaBb genotype are crossed, what
is the phenotypic ratio of the offspring?
Genetics Practice Problems
Fur color in mice is governed by genes concerned with
producing and distributing melanin. At one gene location, a
dominant allele (B) specifies dark brown and a recessive gene
(b) specifies a tan coat. At another gene location, a dominant
allele (C) shuts down melanin production while a recessive allele
(c) allows for the deposition of melanin.
4. Show the expected phenotype ratios in the offspring resulting
from crossing a true breeding (i.e. homozygous) brown
mouse with an albino mouse that is heterozygous at both
loci.
5. What are the possible genotypes of the offspring?
Genetics Practice Problems
Phenylketonuria (PKU) is an inherited disease determined by a
recessive allele. If a woman and her husband are both carriers,
what is the probability of each of the following?
6. All three of their children will be normal.
7. One or more of the three children will have the disease.
Given two cats with the genotypes llSsdd and LlSsDd, what is
the probability of the following? (Short hair is dominant to long
hair, white spotted coat is dominant to not spotted, and diluted
color is dominant to non-diluted color).
8. A cat having short hair
9. A cat having the genotype llssdd
10. A cat having the short hair, white-spotted, and non-diluted
phenotype
Do Now – Non-Mendelian Inheritance Patterns
1. What is the principle difference between codominance and
incomplete dominance at the phenotypic level? At the
cellular level? At the molecular level?
2. In Labrador retrievers, brown fur color is dominant over yellow
fur color. A dog breeder wishes to produce brown puppies
rather than yellow puppies. She has two brown males and
one yellow female. What could she do to verify that her
male “studs” will be more likely to produce brown puppies?
What Does It Mean to Be Dominant? Recessive?
Recall that genes encode the instructions for making a
particular protein
Dominant alleles can be thought of as the instructions for
synthesizing a “normal, functional” protein
!
Although there are exceptions to this rule...
!
Example: Dominant negatives
Typically (but not always) recessive alleles encode
instructions for making a non-functional protein
If a protein is non-functional, it will affect the organism’s
phenotype
Mendelian Genetics Extensions
Incomplete Dominance
Neither allele is “completely
dominant”
Organisms that possess the
heterozygous phenotype
have a phenotype which is
intermediate between the two
“extremes”
Does not support blending
hypothesis because original
phenotypes will reappear if
F1 organisms are crossed
Genetic Disease (Tay Sachs)
Tay Sachs disease is a recessively inherited disease in humans
Brain cells of Tay Sachs babies lack a lipid-metabolizing
enyzme.
!
-Lipids accumulate in brain causing disease symptoms
Heterozygotes are symptom-free
!
-Normal allele appears completely dominant and Tay
!
Sachs allele is recessive
However, heterozygotes actually make “equal” amounts of
the normal and dysfunctional enzymes
!
-The presence of the normal enzyme prevents
!
heterozygotes from developing the disease (i.e. gene
!
dosage effect)
Extensions of Mendelian Inheritance Patterns Codominance and Multiple Alleles
More than one allele contributes to the phenotype of an
organism however phenotype is not intermediate between two
“extremes”
Example: Human blood types
The Dominance Spectrum
Complete
Dominance
(A is Dominant)
AA and Aa have
the same
phenotype
Incomplete
Dominance
(A is Incompletely
Dominant)
Aa = intermediate phenotype.
Phenotype is intermediate
between the phenotypes
of the two homozygotes
Codominance
(No “pure” dominance)
Aa = Both alleles are
equally expressed
in phenotype
Do Now - Cholesterol Metabolism
1. From which type of biological molecule is cholesterol derived?
2. How do you think cholesterol is transported through the body?
3. What are some of the advantages and disadvantages of
cholesterol in the body? Try to be as specific as possible.
3-
Endocytosis
• Internalization of
substances by
formation of a
vesicle
• Types
– Phagocytosis
– Pinocytosis
– Receptor-mediated
endocytosis
3-
Pinocytosis and
Receptor-Mediated Endocytosis
3-
HMG-CoA Reductase
3-
Cholesterol Metabolism
Ingested cholesterol and fatty acids
cannot be transported through the
bloodstream (Why?)
Cholesterol molecules and fatty acids are
transported in chylomicrons (“fat droplets”)
Chylomicron - a small, membrane-bound
vesicle composed of phospholipids and protein
Chylomicrons are transported through the bloodstream to
adipocytes (i.e. fat cells) where fatty acids are stored
Remaining cholesterol molecules are transported to the liver
Cells in the liver break down the chylomicron releasing the
cholesterol
Cholesterol Metabolism
3-
Most dense
(fatty acid composition)
Chylomicron
High cholesterol
(food intake)
Chylomicron remnant
VLDL
LDL
Least dense
HDL (scavenger)
Low cholesterol 3-
Cholesterol Metabolism
3-
AP Biology - Genetics Quiz
1. In humans, the allele for short fingers (F) is dominant over
the allele for long fingers (f). If a person with short fingers
who had one parent with long fingers “mates” with a person
who has long fingers and this couple have three children,
what are the chances that two of their three children will
have short fingers?
2. Genes A, B, and B assort independently and are dominant
to their respective alleles a, b, and c. Two triple
heterozygotes (AaBbCc) are crossed.
What is the probability that a given offspring will be
phenotypically dominant for one of the dominant traits?
3-
AP Biology - Genetics Quiz
3. The recessive allele s causes Drosophila to have small wings
and s+ allele results in a normal wing phenotype. This gene is
known to be located on the X chromosome.
a. If a small winged male is crossed with a homozygous wildtype female, what ratio of normal to small winged flies can be
expected in each sex in the F1?
b. If F1 flies are intercrossed (mated with each other), what F2
progeny ratios are expected?
c. What progeny ratios are expected if F1 females are
backcrossed to their father?
3-
Testcross
Used to determine the genotype of a “parent” with the
dominant phenotype but an unknown genotype
Dominant parent “crossed/mated” with a homozygous
recessive parent
Genotype of the dominant parent can be determined based on
the offspring produced from the cross
If offspring with ONLY the dominant phenotype are produced,
the parent must have the homozygous dominant genotype
If offspring with the dominant and the recessive phenotype are
produced, the parent must have the heterozygous genotype
Sample Testcross
Epistasis
Expression of one gene is dependent on the expression of a
second gene
Example
Coat color is controlled by one set of
genes (For simplicity, let’s use
R for black allele and r for brown allele).
In cats, the dilute gene (d) is
responsible for lightening the coat
color
Dilute gene is recessive to dense gene (D)
The presence of dense allele (D) will produce a
cat with a dark coat color; absence of dense will produce a cat
with lightened coat color
Mendelian Genetics Extensions - Epistasis
One would predict 9:3:3:1
phenotype ratio in a
dihybrid cross
If ratios of offspring do
not adhere to predicted
ratios something else
must be happening
Epistasis - Ability for one
gene to influence the
phenotype brought about
by a second gene
Epistasis
Expression of one gene can “mask” or inhibit the expression of a
second gene
One human trait in which epistasis may occur is in the occurrence
of deafness in a child
Question: Assume that this trait is controlled by 3 pairs of genes
and that normal hearing occurs when an individual has the genotype
A_ or B_. The presence of DD, however, causes deafness, no matter
what other gene combinations are present
1. Write out all of the possible genotypes of the offspring
produced from a mother with the genotype AABBDd and
a father with the genotype AaBbDd
2. Predict the phenotype ratios of the offspring that would
be produced if the above parents were to have children
Genetic Imprinting
Does it matter which allele is
inherited from each parent?
In some cases, the answer is
yes.
Genomic imprinting can
inactivate one of the two copies
(i.e. alleles) of a particular gene
in an organism
New imprint
corresponds to
sex of parent
Addition of methyl groups to DNA
nucleotides is responsible for
imprinting
Genetic Imprinting
Special enzymes responsible for
adding methyl (e.g. -CH3) group
to nitrogen base