Chapter 14 Mendel and the
Gene Idea
Genetic Theories
1. Blending Theory - traits were like paints
and mixed evenly from both parents.
2. Incubation Theory - only one parent
controlled the traits of the children.
Ex: Spermists and Ovists
3. Particulate Model - parents pass on traits
as discrete units that retain their identities
in the offspring.
Gregor Mendel
• Father of Modern Genetics.
• Mendel’s paper was published in 1866, but
was not recognized by Science until the early
1900’s.
Reasons for Mendel's Success
• Used an experimental approach.
• Applied mathematics to the study of natural
phenomena.
• Kept good records.
• Mendel was a pea
picker!
• He used peas as
his study
organism.
Why Use Peas?
•
•
•
•
•
Short life span.
Bisexual.
Many traits known.
Cross- and self-pollinating.
(You can eat the failures).
Cross-pollination
• Two parents.
• Results in hybrid offspring where the offspring
may be different than the parents.
Self-pollination
• One flower as both parents.
• Natural event in peas.
• Results in pure-bred offspring where the
offspring are identical to the parents.
Mendel's Work
• Used seven characters, each with two
expressions or traits.
• Example:
– Character - height
– Traits - tall or short.
Monohybrid or Mendelian Crosses
• Crosses that work with a single character at a
time.
Example - Tall X short
P Generation
• The Parental generation or the first two
individuals used in a cross.
Example - Tall X short
• Mendel used reciprocal crosses, where the
parents alternated for the trait.
Offspring
• F1 - first filial generation.
• F2 - second filial generation, bred by crossing
two F1 plants together or allowing a F1 to selfpollinate.
Results - Summary
• In all crosses, the F1 generation showed
only one of the traits regardless of which
was male or female.
• The other trait reappeared in the F2 at
~25% (3:1 ratio).
Mendel's Hypothesis
1. Genes can have alternate versions called
alleles.
2. Each offspring inherits two alleles, one from
each parent.
Mendel's Hypothesis
3. If the two alleles differ, the dominant allele
is expressed. The recessive allele remains
hidden unless the dominant allele is
absent.
Comment - do not use the term “strongest”
to describe the dominant allele.
Mendel's Hypothesis
4. The two alleles for each trait separate during
gamete formation. This now called: Mendel's
Law of Segregation
When does this occur?
Law of Segregation
Mendel’s Experiments
• Showed that the Particulate Model best fit the
results.
Vocabulary
• Phenotype - the physical appearance of the
organism.
• Genotype - the genetic makeup of the
organism, usually shown in a code.
– T = tall
– t = short
Helpful Vocabulary
• Homozygous - When the two alleles are the
same (TT/tt).
• Heterozygous- When the two alleles are
different (Tt).
6 Mendelian Crosses are Possible
Cross
Genotype
Phenotype
TT X tt
Tt X Tt
TT X TT
tt X tt
TT X Tt
Tt X tt
all Tt
1TT:2Tt:1tt
all TT
all tt
1TT:1Tt
1Tt:1tt
all Dom
3 Dom: 1 Res
all Dom
all Res
all Dom
1 Dom: 1 Res
Test Cross
• Cross of a suspected heterozygote with a
homozygous recessive.
– Ex: T_ X tt
If TT - all dominant
If Tt - 1 Dominant: 1 Recessive
Dihybrid Cross
• Cross with two genetic traits.
• Need 4 letters to code for the cross.
– Ex: TtRr
• Each Gamete - Must get 1 letter for each trait.
– Ex. TR, Tr, etc.
Number of Kinds of Gametes
• Critical to calculating the results of higher
level crosses.
• Look for the number of heterozygous traits.
Equation
The formula 2n can be used, where “n” = the
number of heterozygous traits.
Ex: TtRr, n=2
22 or 4 different kinds of gametes are possible.
TR, tR, Tr, tr
Dihybrid Cross
TtRr X TtRr
Each parent can produce 4 types of gametes.
TR, Tr, tR, tr
Cross is a 4 X 4 with 16 possible offspring.
Results
• 9 Tall, Red flowered
• 3 Tall, white flowered
• 3 short, Red flowered
• 1 short, white flowered
Or: 9:3:3:1
Law of Independent Assortment
• The inheritance of 1st genetic trait is NOT
dependent on the inheritance of the 2nd trait.
• Inheritance of height is independent of the
inheritance of flower color.
Comment
• Ratio of Tall to short is 3:1
• Ratio of Red to white is 3:1
• The cross is really a product of the ratio of
each trait multiplied together. (3:1) X (3:1)
Probability
• Genetics is a specific application of the rules
of probability.
• Probability - the chance that an event will
occur out of the total number of possible
events.
Genetic Ratios
• The monohybrid “ratios” are actually the
“probabilities” of the results of random
fertilization.
Ex: 3:1
75% chance of the dominant
25% chance of the recessive
Rule of Multiplication or Product Rule
• The probability that two alleles will come
together at fertilization, is equal to the
product of their separate probabilities.
Example: TtRr X TtRr
• The probability of getting a tall offspring is
¾.
• The probability of getting a red offspring is
¾.
• The probability of getting a tall red
offspring is ¾ x ¾ = 9/16
Comment
• Use the Product Rule to calculate the results
of complex crosses rather than work out the
Punnett Squares.
Rule of Addition
• Rule of Addition—the probability of an event that
can occur in two or more different ways
– Yy x Yy
• What is the chance of a heterozygous
offspring?
–¼ + ¼ = 1/2
PpYyRr x Ppyyrr (trihybrid cross)
• What is the chance that an offspring will have at least 2 recessive
traits?
– ppyyRr =
• ¼ x ½ x ½ = 1/16 (Rule of Multiplication)
– ppYyrr =
• ¼ x ½ x ½ = 1/16
– Ppyyrr =
• ½ x ½ x ½ = 2/16
– PPyyrr =
• ¼ x ½ x ½ = 1/16
– ppyyrr =
• ¼ x ½ x ½ = 1/16
Total = 6/16 = 3/8 (Rule of Addition)
Variations on Mendel
1.
2.
3.
4.
5.
Incomplete Dominance
Codominance
Multiple Alleles
Epistasis
Polygenic Inheritance
Incomplete Dominance
• When the F1 hybrids show a phenotype
somewhere between the phenotypes of
the two parents.
• Often a “dose” effect
Ex. Red X White snapdragons
F1 = all pink
F2 = 1 red: 2 pink: 1 white
Result
• No hidden Recessive.
• 3 phenotypes and 3 genotypes
(Hint! – often a “dose” effect)
– Red = CR CR
– Pink = CRCW
– White = CWCW
Another example
Codominance
• Both alleles are expressed equally in the
phenotype.
• Ex. MN blood group
– MM
– MN
– NN
Result
• No hidden Recessive.
• 3 phenotypes and 3 genotypes
(but not a “dose” effect)
Multiple Alleles
• When there are more than 2 alleles for a trait.
• Ex. ABO blood group
– IA - A type antigen
– IB - B type antigen
– i - no antigen
Result
• Multiple genotypes and phenotypes.
• Very common event in many traits.
Alleles and Blood Types
Type
A
B
AB
O
Genotypes
IA IA or IAi
IB IB or IBi
IAIB
ii
Comment
• Rh blood factor is a separate factor from the
ABO blood group.
• Rh+ = dominant
• Rh- = recessive
• A+ blood = dihybrid trait
Epistasis
• When 1 gene locus alters the expression
of a second locus.
• Ex:
– 1st gene: C = color, c = albino
– 2nd gene: B = Brown, b = black
Gerbils
In Gerbils
CcBb X CcBb
Brown X Brown
F1 = 9 brown (C_B_)
3 black (C_bb)
4 albino (cc__)
Result
• Ratios often altered from the expected.
• One trait may act as a recessive because it is
“hidden” by the second trait.
Epistasis in Mice
Problem
• Wife is type A
• Husband is type AB
• Child is type O
Question - Is this possible?
Comment - Wife’s boss is type O
Bombay Effect
•
•
•
•
Epistatic Gene on ABO group.
Alters the expected ABO outcome.
H = dominant, normal ABO
h = recessive, no A,B, reads as type O blood.
Genotypes
• Wife: type A (IA IA , Hh)
• Husband: type AB (IAIB, Hh)
• Child: type O (IA IA , hh)
Therefore, the child is the offspring of the wife
and her husband (and not the boss).
Bombay - Detection
• When ABO blood type inheritance patterns
are altered from expected.
Polygenic Inheritance
• Factors that are expressed as continuous
variation.
• Lack clear boundaries between the phenotype
classes.
• Ex: skin color, height
Genetic Basis
• Several genes govern the inheritance of the
trait.
• Ex: Skin color is likely controlled by at least 4
genes. Each dominant gives a darker skin.
Result
• Mendelian ratios fail.
• Traits tend to "run" in families.
• Offspring often intermediate between the
parental types.
• Trait shows a “bell-curve” or continuous
variation.
Genetic Studies in Humans
• Often done by Pedigree charts.
• Why?
– Can’t do controlled breeding studies in humans.
– Small number of offspring.
– Long life span.
Pedigree Chart Symbols
Male
Female
Person with trait
Sample Pedigree
Dominant Trait
Recessive Trait
Human Recessive Disorders
• Several thousand known:
– Albinism
– Sickle Cell Anemia
– Tay-Sachs Disease
– Cystic Fibrosis
– PKU
Sickle-cell Disease
• Most common inherited disease among
African-Americans.
• Single amino acid substitution results in
malformed hemoglobin.
• Reduced O2 carrying capacity.
• Codominant inheritance.
Tay-Sachs
• Eastern European Jews.
• Brain cells unable to metabolize type of lipid,
accumulation of causes brain damage.
• Death in infancy or early childhood.
Cystic Fibrosis
• Most common lethal genetic disease in the
U.S.
• Most frequent in Caucasian populations (1/20
a carrier).
• Produces defective chloride channels in
membranes.
Recessive Pattern Diseases
• Usually rare.
• Skips generations.
• Occurrence increases with consaguineous
matings.
• Often an enzyme defect.
Human Dominant Disorders
• Less common then recessives.
• Ex:
– Huntington’s disease
– Achondroplasia
– Familial Hypercholsterolemia
Inheritance Pattern
• Each affected individual had one affected
parent.
• Doesn’t skip generations.
• Homozygous cases show worse phenotype
symptoms.
• May have post-maturity onset of symptoms.
Genetic Screening
• Risk assessment for an individual inheriting a
trait.
• Uses probability to calculate the risk.
General Formula
R=FXMXD
R = risk
F = probability that the female carries the
gene.
M = probability that the male carries the
gene.
D = Disease risk under best conditions.
Example
• Wife has an albino parent.
• Husband has no albinism in his pedigree.
• Risk for an albino child?
Risk Calculation
• Wife = probability is 1.0 that she has the
allele.
• Husband = with no family record,
probability is near 0.
• Disease = this is a recessive trait, so risk is
Aa X Aa = .25
• R = 1 X 0 X .25
• R=0
Risk Calculation
• Assume husband is a carrier, then the risk is:
R = 1 X 1 X .25
R = .25
There is a .25 chance that any child will be
albino.
Common Mistake
• If risk is .25, then as long as we don’t have 4
kids, we won’t get any with the trait.
• Risk is .25 for each child. It is not dependent
on what happens to other children.
Carrier Recognition
• Fetal Testing
– Amniocentesis
– Chorionic villi sampling
• Newborn Screening
Fetal Testing
• Biochemical Tests
• Chromosome Analysis
Amniocentesis
•
•
•
•
Administered between 11 - 14 weeks.
Extract amnionic fluid = cells and fluid.
Biochemical tests and karyotype.
Requires culture time for cells.
Chorionic Villi Sampling
• Administered between 8 - 10 weeks.
• Extract tissue from chorion (placenta).
• Slightly greater risk but no culture time
required.
Newborn Screening
• Blood tests for recessive conditions that can
have the phenotypes treated to avoid
damage. Genotypes are NOT changed.
• Ex. Phenylketonuria (PKU)
Newborn Screening
• Required by law in all states.
• Tests 1 - 6 conditions.
• Required of “home” births too.
Multifactorial Diseases
• Where Genetic and Environment Factors
interact to cause the Disease.
• Becoming more widely recognized in
medicine.
Ex. Heart Disease
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•
•
•
Genetic
Diet
Exercise
Bacterial Infection
Genes & Environment
Summary
• Know the Mendelian crosses and their
patterns.
• Be able to work genetic problems (practice!).
• Watch genetic vocabulary.
• Be able to read pedigree charts.
• Be able to recognize and work with some of
the “common” human trait examples.