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CHAPTER 14
MENDEL AND THE GENE IDEA
How are traits transmitted?
One theory-The ____________ hypothesis- Genetic material
mixes much like yellow and blue paint makes
green
Another theory- The _______________ hypothesisParents pass on discrete heritable units - genes - that
retain their separate identities in offspring.
1. Mendel brought an experimental and
quantitative approach to genetics
• Gregory Mendel is the father of genetics
• entered Augustinian monastery in Czech republic in 1843
• University of Vienna from 1851 to 1853
• Influenced by a mathematician and a botanist to study
variation in plants
• Around 1857, Mendel began breeding garden peas in a
monastery to study inheritance
• Advantages of pea plants –
• many varieties with distinct heritable features (characters)
• Mendel had strict control over which plants mated with which.
Mendel cross-pollinated peas
_________________
_________________
1. Law of segregation
P1 x P2
F1
The F2 generation revealed two
principles of heredity:
1. _____________________
F1 x F1
2._______________________
_________________
F2
1. __________________two alleles for a character are packaged
into separate gametes
Evidence
F1
F2
purple x white
all purple
white + purple
224
705
= 1 to 3 ratio
1. Law of segregation
Mendel reasoned that the purple flower is a
_____________trait and the white flower is a
_________________trait.
• The plant colors were not “blended”
Mendel also found the same 3:1 ratio in other
crosses (e.g wrinkled and smooth seeds)
1. Law of segregation
Mendel developed a hypothesis to explain these
results that consisted of ________related ideas.
1. Alternative version of genes (______________)
account for variations in inherited characters.
• Example: The purple-flower
allele and white-flower
allele are two DNA
variations at the
flower-color locus.
Fig. 14.4
Locus for
flower color
2. For each ___________, an organism inherits two
alleles, one from each parent.
• E.g. a purple-flower allele from one parent and a whiteflower allele from the other.
1. Law of segregation
3. If two alleles differ, the_______________, is
fully expressed in the organism’s appearance.
•The ________________, has no noticeable effect on
the organism’s appearance.
•E.g. white-flower allele is recessive.
4. The two alleles for each character ___________
(separate) during gamete production.
• If different alleles are present, then 50% of the gametes
will receive one allele and 50% will receive the other.
• The separation of alleles into separate gametes is
summarized as Mendel’s ____________________
1. Law of segregation
• A ________________ predicts the results of a genetic
cross between individuals of known genotype.
Purple
White
PP x pp
Call the dominant allele “P”
Call the recessive allele “p”
F1
All Pp
Pp x Pp
Predicts 75% purple: 25% white
Or 3:1 ratio
F2
Purple
Vocabulary
1. Law of segregation
• ______________- An organism with two ________
alleles for a character
• _______________- An organism with two
________ alleles for a character
• __________- A description of an organism’s _____
• ___________- A description of its genetic makeup
•Example- For flower color in peas, both PP and
Pp plants have the same phenotype (purple) but
different genotypes (homozygous and
heterozygous).
2. Law of independent assortment
What about the law of independent assortment??
Law of _________ ___________each pair of alleles
segregates into gametes independently
• This law comes from Mendel’s observing inheritance
of ______ different characters
Color and 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).
So what happens when we cross YYRR x yyrr??
Known as a ______________
2. Law of independent assortment
• Predict- two characters are transmitted from
parents to offspring as a__________.
• The Y and R alleles and y and r alleles stay together.
•Thus, ________ the F1 to be
yellow, round seeds (YyRr)
and F2 to be 3:1
(yellow/round:green/wrinkled
•But ________ four classes
of gametes
(________________)
produced in equal amounts.
•These combinations
produce four distinct
phenotypes in a
___________ ratio.
9
3
3
1
Fig. 14.8
Mendelian inheritance reflects rule of
__________.
•Each toss of a coin is an independent event, just like
the distribution of alleles into gametes.
•Like a coin toss, each ovum from a heterozygous parent
has a _____ chance of carrying the dominant allele and a
_____ chance of carrying the recessive allele.
•The probability that two coins tossed at the same
time will land heads up is 1/2 x 1/2 = 1/4.
•Similarly, the probability that a heterozygous pea
plant (Pp) will produce a white-flowered offspring
(pp) is ______________.
• Predicting phenotypes in dihybrid crosses- The
multiplication rule.
• What is the probability that an F2 plant will have a
YYRR genotype from a heterozygous parent (YyRr)
• = 1/4 (YR ovum) x 1/4 (YR sperm)= 1/16
Y
y
Y
YY
Yy
y
Yy
¼ YY
yy
X
R
r
R
RR
Rr
r
Rr
¼ RR
rr
= 1/16
What are the chances of having a grandchild with the muscular
dystrophy (a recessive disease)?
Given: Grandparents are RR and rr, where “r” is the disease allele.
Their grandson has married a carrier.
Grandparents
Parents
R
R
r
Rr
Rr
r
Rr
Rr
F1 (Parents)- no disease
R
r
R
RR
Rr
r
Rr
rr
F2 - ¼ with disease
2. The relationship between genotype
and phenotype is ___________________
• Mendel’s peas were relatively simple genetically.
• Each character is controlled by a____________________.
• Each gene has only_________________, one of which is
completely dominant to the other.
Many alleles show_____________________–
heterozygotes show a distinct intermediate
phenotype, not seen in homozygotes.
Others show _______________ in which
both alleles affect the phenotype
e.g. Two genes encode distinct proteins
on blood cells responsible for blood-typing
red
F1
Fig. 14.10
white
pink
red
F2
white
pink
• Dominance/recessiveness relationships have
three important points.
1. They range from_____________________, to
incomplete dominance, to codominance.
2. They reflect the mechanisms by which specific
alleles are expressed in the phenotype and
_________ involve the ability of one allele to
subdue another at the level of DNA.
3. They do not determine or correlate with the
relative abundance of alleles in a population.
• The genes covered so far affect _________
phenotypic character.
• Most genes are_____________, affecting more
than one phenotypic character.
• Example: A single mutation can contribute to diabetes,
eye disease, gangrene infections, and heart disease.
• Phenotype depends on ______________________.
• 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______________________.
• 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.
• A ______________emphasis on single genes and
single phenotypic characters presents an
_____________ perspective on heredity and
variation.
• A more comprehensive theory of Mendelian
genetics must view organisms______________.
3. Pedigree analysis reveals Mendelian
patterns in human inheritance
• In a_____________________, information about the
presence/absence of a particular phenotypic trait is
collected from as many individuals in a family as
possible and______________________.
• The distribution of these characters is then mapped
on the ____________________.
A pedigree can help us understand the past and to predict the
future.
• Example- the occurrence of widows peak (W) is
______________ to a straight hairline (w).
Grandparents
Parents
Kids
• If the kids lacks a
widow’s peak, but both
parents have widow’s
peaks, then her parents
must be ____________ for
that gene
Fig. 14.14
The chance of having a widow’s peak is
(1/2 [WW] + 1/4 [Ww]) = _______
4. Many human disorders follow Mendelian
patterns of inheritance
• Thousands of genetic disorders, including disabling
or deadly hereditary diseases, are inherited as simple
__________________________.
• From mild (albinism) to life-threatening (cystic fibrosis).
•__________________ may have no clear phenotypic
effects, they are carriers who may transmit a recessive allele
to their offspring.
•Most people with ____________________are born to
______________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.
Examples of recessive disorders
1. _________________-one of every 2,500 whites of
European descent.
• One in 25 whites is a _____________.
• 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.
Examples of recessive disorders
2. ___________________________
• Caused by a dysfunctional enzyme that fails to break down
specific brain lipids.
• Symptoms- seizures, blindness, and degeneration of
motor and mental performance a few months after birth.
• Child dies after a few years.
• Among ____________________(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.
Examples of recessive disorders
3. ___________________
Figure not in text
• The most common
inherited disease among
blacks (1:400 African
Americans)
• Cause- substitution of a
single amino acid in
hemoglobin.
• Effects- sickle-cell
hemoglobin crystallizes
into long rods.
• This deforms red blood
cells into a sickle shape.
• Multiple problems
A pleiotropic disease
Examples of recessive disorders
• Why does sickle cell disease remain in the
population?
• Individuals with one sickle-cell allele have increased
resistance to malaria (whereas those with normal alleles
die)
• Its relatively high frequency in African Americans
is a vestige of their African roots.
Examples of recessive disorders
Why is incest genetically dangerous?
• Relatively unlikely that two carriers of the same
rare harmful allele will meet and mate.
• However, ___________________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.
• Example- _______________in royal British lineages
• Most societies and cultures have laws or taboos
forbidding marriages between close relatives.
Examples of ____________disorders
1. __________________, 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
• Why? because if a lethal dominant kills an offspring
• One example -__________________________, a
degenerative disease of the nervous system that is fatal.]
•The ________________________________has no obvious
phenotypic effect until an individuals is about 35 to 45 years
old.
Many disorders normally involve multiple genesthese are called _________________
Multifactorial disorders include heart disease, diabetes,
cancer, alcoholism, and certain mental illnesses, such a
schizophrenia and manic-depressive disorder.
• The genetic component is typically polygenic.
5. Technology is providing new tools for
genetic testing and counseling
• Most children with recessive disorders are born to
parents with a normal phenotype.
• A ________ to assessing risk is identifying if
prospective parents are carriers of the recessive trait.
• However, issues of confidentiality, discrimination,
and adequate information and counseling arise.
A hypothetical case
• Consider a hypothetical couple, John and Carol, who
are planning to have their first child.
• In both of their families’ histories a recessive lethal
disorder is present and both John and Carol had
brothers who died of the disease.
• While neither John and Carol nor their parents have the
disease, their parents must have been carriers (Aa x Aa).
• John and Carol each have a 2/3 chance of being carriers
and a 1/3 chance of being homozygous dominant.
• The probability that their first child will have the disease
= 2/3 (chance that John is a carrier) x 2/3 (chance that
Carol is a carrier) x 1/4 (chance that the offspring of two
carriers is homozygous recessive) = 1/9.
• If their first child is born with the disease, we know that
John and Carol’s genotype must be Aa and they both are
carriers.
• The chance that their next child will also have the
disease is 1/4.
• In utero testing for a disorder.
1. ____________- 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
____________.
• 2. _____________________performed as early as the
eighth to tenth week of
pregnancy.
• Extracts a sample of fetal tissue
from the chrionic villi of the
placenta are karyotyped.
Fig. 14.17a
Fig. 14.17b
• Other techniques-_________________________
allow fetal health to be assessed visually in utero.
• usually reserved for cases in which the risk of a genetic
disorder or other type of birth defect is relatively great.
• If fetal tests reveal a serious disorder, the parents face the
difficult choice of terminating the pregnancy or preparing to
care for a child with a genetic disorder.
• 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, _______________________
• This disorder occurs in one in 10,000 to 15,000 births.
• Accumulate the amino acid phenylalanine in the blood
to toxic levels.
• Leads to mental retardation.
• If the disorder is detected, a special diet low in
phenyalalanine usually promotes __________
development.