Chapter 14: Mendel and Sexual Life Cycles
Blending Hypothesis: The idea that genetic material contributed by the two parents mixes the
way blue and yellow paints blend to make green
Particulate model: Parents pass on discrete heritable units (genes) that retain their separate
identities in offspring
I. Mendel’s Model: A Case Study In the Scientific Process
A. Mendel’s Experimental Approach
1. Character: Heritable feature that varies among individuals
2. Trait: Each variant for a character (purple, white)
B. Controlled which plants mated with which
1. Petals enclose female (carpels) and male (stamens)
2. Self fertilization: Pollen from stamens falls on carpels of the same flower
a. Sperm from the pollen fertilizes the ova in the carpel
3. Cross-pollination: Fertilization between different plants
a. Removed immature stamens before they produced pollen
b. Dusted pollen from another plant onto the emasculated flowers
4. Whether ensuring self-pollination or executing cross-pollination
a. Always sure of the parentage of new seeds
C. True-breeding: When plants self-pollinate all their offspring are of the same variety
D. Hybridization: Crossing two true breeding varieties
E. Monohybrid cross: Cross that tracks the inheritance of a single character (flower
color)
F. P generation: (Parental generation) True breeding parents
G. F1 generation: (First filial) Hybrid offspring
I. F2 generation: (Second filial) Result of allowing F1s to self-pollinate
II. Mendel’s Law of Segregation
A. P1: True breeding purple parent with true breeding white parent
1. F1: All purple
2. F2: White reappeared
3. 705 purple and 244 white
B. Heritable factor for white flowers did not disappear in F1
1. Dominant: Purple
2. Recessive: White
C. Developed a hypothesis
1. Alternative versions of genes (different alleles) account for variations in
inherited traits
2. Alleles: Alternative versions of a gene
a. For each character, an organism inherits two genes, one from each
parent
1. Matching alleles (True breeding peas)
3. If the two alleles differ:
a. Dominant: Allele that is fully expressed in the organism’s appearance
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b. Recessive: Allele that has no noticeable effect on the organism’s
appearance
4. The two alleles for each character segregate (separate) during gamete
production
1. If contrasting alleles are present (F1 hybrids)
a. 50% of gametes receive dominant allele
b. 50% of gametes receive recessive allele
2. Law of segregation: The separation of alleles into separate gametes
B. One test of Mendel’s segregation hypothesis is whether or not it can account for the
3:1 ratio in the F2
1. F1 produces 2 classes of gametes
2. When alleles separate
a. Half get a purple allele
b. Half get a white allele
3. During self-pollination, gametes unite randomly
a. Ovum with purple flower allele has equal chance of being fertilized by a
sperm with a purple flower allele
b. Same is true for an ovum with a white flower allele
c. 4 equally likely combinations of sperm and ovum
4. Punnet square: Device used to predicted the probable results of a genetic cross
between individuals of a known genotype
III. Useful Genetic Vocabulary
A. Homozygous: An organism having a pair of identical alleles for a character
B. Heterozygous: Organisms having two different alleles for a character
1. Cross dominant and recessive homozygotes
a. All offspring will have a combination of non-matching alleles (Pp)
b. Not true breeding
C. Genotype: An organism’s genetic makeup
1. An organism’s appearance does not always reflect its genotype
2. Pp and PP: Same phenotype different genotypes
D. Phenotype: Organisms’ physiological traits
1. Ex: Purple flowers
E. Test Cross: Breeding of a recessive homozygote with an organism of dominant
phenotype, but unknown genotype
1. Determine whether a plant with purple flowers is homozygous or
heterozygous?
2. Cross it with a plant having white (recessive) flowers
3. Appearance of offspring will reveal genotype of the purple flowered parent
a. All purple: Parent was homozygous purple
b. Purple and white: Parent was heterozygous
1. 1:1 phenotypic ratio of Pp and pp
IV. Law of Independent Assortment: The independent segregation of each pair of alleles
during gamete formation
A. Monohybrids: F1 hybrids produced by a cross that tracks the inheritance of a single
character
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B. Dihybrids: F1 plants differing in two characters
1. Cross YYRR and yyrr
2. 2 pairs of alleles segregate independently of each other
a. YR, Yr, yR, yr
b. 4 classes of sperm are mixed with 4 classes of ova
1. 16 probable ways in which the alleles can combine in the F2
2. 4 phenotypic categories
V. Inheritance as a Game of Chance
A. Probability scale ranges from 0-1
1. 1: Certain to happen
2. 0: Certain not to occur
3. Probability of all possible outcomes for an event must add up to 1
B. Coin tossing
1. 1/2 chance for heads every time
2. The outcome of any particular toss is unaffected by what has happened on
previous trials
a. Independent events
3. Will a Pp heterozygote carry allele P or allele p
C. Rule of Multiplication
1. Multiplying individual probabilities to obtain the overall probability of events
occurring in combination
a. Two coins landing heads up at the same time
b. 1/2 x 1/2 = 1/4
2. F1 monohybrid cross (Pp x Pp)
a.
3. Dihybrid crosses (YyRr)
a. Probability that gamete will carry Y and R alleles is ¼
b.
D. Rule of Addition
1. The probability of an event that can occur in two or more different ways is the
sum of the separate probabilities of those ways
3. Probability of an F2 heterozygote:
E. Using Probability to solve genetics problems
1. Trihybrid Cross: Cross of organisms with 3 differing characters
a. Calculate fraction of offspring predicted to exhibit recessive phenotypes
for at least 2 of the 3 traits
b. List all genotypes that fulfill this condition
-ppyyRr, ppYyrr, Ppyyrr, Ppyyrr, ppyyrr
c. Multiply individual probabilities for the allele pairs
d. Use rule of addition to pool probabilities
F. Particulate behavior of genes
1. Larger the sample the closer the results will come to our predictions
VI. Extending Mendelian Genetics
A. Incomplete Dominance: F1 hybrids have an appearance somewhere in between the
phenotypes of the two parental varieties
B. Incomplete dominance: F1 hybrids have an appearance somewhere in between
phenotypes of 2 parental varieties
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1. Active allele does not compensate for inactive allele
a. R: Codes for enzyme that makes red pigment
1. Incompletely dominant over r
b. r: Defective enzyme that cannot make pigment
2. Ex: Red snapdragons are crossed with white ones
a. F1 hybrids: Pink
b. F2
C. Complete dominance: Phenotypes of heterozygote and dominant homozygote are
indistinguishable
D. Codominance: Both alleles are separately manifest in the phenotype
1. Ex: Human Blood groups M, N, and MN
a. 2 molecules on the surface of blood cells
c. A single gene locus
2.Tay-Sachs disease: Brain cells of baby are unable to metabolize gangliosides
a. Children who inherit 2 copies of the Tay-Sachs allele have the disease
1. On the organism level: Recessive
2. On the biochemical level: Intermediate phenotype characteristic
of incomplete dominance
a. Enzyme deficiency can be detected in heterozygotes
b. Lack symptoms of the disease
1. Half the normal amount of functional enzyme is
sufficient
3. Molecular Level: Codominant
E. Dominant doesn’t mean more common in the population
1.Ex: Polydactyly
F. Multiple Alleles
1. Most genes exist in more than two allelic forms
a. ABO Blood groups
b. 6 genotypes are possible
2. Antibodies: Specific proteins that work against foreign blood factors
a. Agglutination: Blood cells clump together
b. IAIB are codominant
G. Pleiotropy: The ability of a gene to affect an organism in many ways
H. Epistasis: Situation in which a gene at one locus alters the phenotypic expression of a
gene at a second locus
I. Polygenic Inheritance: Additive effect of two or more genes on a single phenotypic
character
1. Quantitative characters: Characters vary in population along a continuum (in
graduations)
2. Ex: Skin pigmentation
a. Controlled by at least 3 separately inherited genes
1. A, B, C
2. Incompletely dominant over a, b, c
J. Environmental Impact on Phenotype
1. Product of genotype is not a rigidly defined phenotype
2. Variation due to environmental influence
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3. Norm of reaction: The phenotypic range for a genotype
a. Broadest for polygenic characters
4. Multifactoral: Many factors, both genetic and environmental, collectively
influence phenotype
VI. Mendelian Inheritance in Humans
A. Humans are not a good genetic subject
1. Human generation span: 20 years
2. Few offspring
3. Breeding experiments are socially unacceptable
B. Human Pedigrees: Family tree describing interrelationships of parents and children
across generations
1. Widows peak
2. Earlobes
3. Follow Mendelian patterns of inheritance
a. Disabling or lethal disorders are analyzed by geneticists, physicians,
genetic counselors
4. Thousands of genetic disorders are known to be inherited as simple recessive
traits
a. Mild: Albinism
1. Lack of pigmentation
2. Susceptibility to skin cancers and vision problems
b. Life threatening: cystic fibrosis
5. Genes code for proteins of specific function
a. Allele that causes genetic disorder codes for malfunctional protein or
no protein
b. Carriers: Heterozygotes who are phenotypically normal with regard to
disorder but may transmit recessive allele to the offspring
VII. 3 Disorders
A. Cystic Fibrosis: Characterized by mucus build up in pancreas, lungs, digestive tracts
and other organs
Most common lethal genetic disease in US
1. 1/2500 whites of European descent
a. 1/25 whites are carriers
2. Normal allele codes for a membrane protein that functions in chloride ion
transport between certain cells and the extracellular fluid
3. Chloride ion channels are defective or absent in plasma membranes
a. 2 recessive alleles
b. High concentration of extracellular chloride
causes mucus coats of certain cells to become thicker and stickier than
normal
1. Bacterial infections
4. Untreated children die before age 5
a. Treatment: Pounding chest to clear mucus from clogged airways
b. Daily doses of antibiotics
c. US survive into late 20s and beyond
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B. Tay-Sachs disease: Dysfunctional enzyme fails to break down brain lipids of a
certain class
1. Recessive allele
2. Symptoms manifest a few months after birth
a. Seizures, blindness, degeneration of motor and mental performance
3. High incidence among Ashkenazic Jews
a. 1/3600 births
b. 100 times greater than in non-Jews or Sephardic Jews
(Mediterranean)
C. Sickle-cell disease: Characterized by sickle shaped blood cells
1. Most common inherited disease among blacks
2. Affects 1/400 African Americans
3. Substitution of single amino acid in hemoglobin
4. When O2 is low (High altitude or physical stress)
a. Hemoglobin molecules crystallize by aggregating into long rods
b. Deform cells into sickle shape
3. Pleiotropy: Multiple effects of double dose of sickle-cell allele
a. Brain damage (prevented in children by blood transfusions)
b. New drugs help prevent and treat other problems
4. Heterozygotes: Have the sickle cell trait
a. Fraction suffer some symptoms in extended reduction of blood O2
b. 1/10 African Americans have sickle-cell trait
c. Increases resistance to malaria
VIII. Rare that 2 carriers will meet and mate
A. Consanguineous (same blood) matings
1. Double lines in pedigrees
a. Inbred domestic and zoo animals
2. Most societies and cultures have laws or taboos forbidding marriages between
close relatives
b. Stillbirths, birth defects
IX. Dominantly Inherited Disorders
A. Achondroplasia: Form of dwarfism
1. 1/10 000
2. Heterozygous individual show dwarf phenotype
3. All who are not dwarfs (99.99% of pop)
a. Homozygous for recessive allele
B. Huntington’s disease: Degenerative disease of nervous system caused by lethal
dominant allele
1. No phenotypic effect until individual is 35 to 45
2. Irreversible, fatal
3. Child: 50 % chance of inheriting allele
-Aa x aa
4. Locus: Near tip of chromosome 4
5. Now possible to test for
a. Dr. Nancy Wexler of Columbia University and the Hereditary Disease
Foundation
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C. Lethal dominant alleles are much less common
1. Both arise by mutation in DNA of sperm or egg
2. Lethal dominant allele kills offspring before it can reproduce
X. Genetic Testing and Counseling
A. Carrier Recognition
1. Tests that can distinguish between normal individuals who are heterozygous
and normal individuals who are homozygous
a. Available for some disorders (increases each year)
1. Tay-Sachs
2. Sickle-cell anemia
3. cystic fibrosis
2. These new methods could be abused
a. If confidentiality is violated: Carriers could be stigmatized
1. Health /Life insurance issues
2. Employment
3. Enough genetic counselors must be available to help interpret
results
XI. Fetal Testing
A. Amniocentesis: A technique for determining genetic abnormalities in a fetus by the
presence of certain chemicals or defective fetal cells in the amniotic fluid, obtained by
aspiration from a needle inserted into the uterus
1. Performed on cells grown in lab from fetal cells that had sloughed off into
amniotic fluid
a. Takes several weeks
b. Cells can be used for karyotyping to identify certain chromosomal
defects
B. Chorionic Villi Sampling (CVS): Physician suctions off a small amount of fetal
tissue from projections of the embryonic membrane (chorion)
1. Cells proliferate rapidly
2. Enough to karyotype within 24 hours
3. Can be done between 8 and 10 weeks
4. Risks are being investigated
C. Ultrasound: Uses sound waves to produce image of
fetus
1. Noninvasive
2. Used to locate fetus during amniocentesis
3. No known risks
D. Fetoscopy: Needle thin tube containing viewing scope
1. Fiber optics to transmit light
2. Allows physician to examine fetus for anatomical deformities
E. Amniocentesis and Fetoscopy have a 1% risk of harming mother (bleeding) or fetus
(death)
1. Reserved for cases in which chances for a genetic disorder or other birth defect
is high
2. Women over 35
XII. Newborn Screening: Routinely performed in US hospitals
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A. Phenylketonuria (PKU): 1/10 000 - 15000 in US (recessive)
1. Cannot break down phenylalanine
2. This and its by-product: phenylpyruvate
accumulates to toxic levels in the blood causing mental retardation
3.Special diet, low in phenylalanine, allows normal development
XIII. Multifactoral Disorders: Genetic component plus significant environmental factors
A. Heart disease
B. Diabetes,
C. Cancer
D. Alcoholism
E. Schizophrenia
F. Manic-depressive disorder
G. Hereditary component; Sometimes polygenic
1. Many genes affect cardiovascular health
a. More prone to heart attacks and strokes
2. Life style: Exercise, diet, dealing with stress, lack of smoking
a. Reduces risk of heart disease and some kinds of cancer
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