Chapter 14 Mendel and the Gene Idea
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Chapter 14
Mendel and the Gene Idea
Inheritance
• The passing of traits from
parents to offspring.
• Humans have known about
inheritance for thousands of
years.
Genetics
• The scientific study of the
inheritance.
• Genetics is a relatively “new”
science (about 150 years).
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 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
self-pollinate.
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
terms “strongest” to describe
the dominant allele.
Allele for purple flowers
Homologous
pair of chromosomes
Locus for flower-color gene
Allele for white flowers
Allele
Purple
White
Mendel's Hypothesis
4. The two alleles for each trait
separate during gamete
formation and end up in
different games.
This now called:
Mendel's Law of Segregation
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 all Tt all Dom
Tt X Tt 1TT:2Tt:1tt 3 Dom: 1 Res
TT X TT all TT all Dom
tt X tt all tt all Res
TT X Tt 1TT:1Tt all Dom
Tt X tt 1Tt:1tt 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.
• Ex: TtrrGG X TtRrgg
Solution
“T’s” = Tt X Tt = 3:1
“R’s” = rr X Rr = 1:1
“G’s” = GG x gg = 1:0
Product is:
(3:1) X (1:1) X (1:0 ) = 3:3:1:1
Dominance vs Phenotype
• A dominant allele does not
subdue a recessive allele;
alleles don’t interact.
• Alleles are simply variations in
a gene’s nucleotide sequence.
Variations on Mendel
1. Incomplete Dominance
2. Codominance
3. Multiple Alleles
4. Epistasis
5. 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 Genotypes
A IA IA or IAi B IB IB or IBi
AB IAIB
O 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.
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
• Galactosemia
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.
Dominance vs Phenotype
• For any character,
dominance/recessiveness
relationships of alleles depend
on the level at which we
examine the phenotype.
Example -Tay-Sachs
• Disease is fatal; a dysfunctional
enzyme causes an
accumulation of lipids in the
brain.
• At the organismal level, the
allele is recessive.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Tay-Sachs
• At the biochemical level, the
phenotype (i.e., the enzyme
activity level) is incompletely
dominant.
• At the molecular level, the
alleles are codominant.
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
• 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.
Homework
• Read Chapter 14 (Hillis – 8)
• Chapter 14 – Mon. 1/28
Genetic Screening
• Risk assessment for an
individual inheriting a trait.
• Uses probability to calculate
the risk.
General Formula
R = F X M X D
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. 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
• Genetic
• Diet
• Exercise
• Bacterial Infection
Genes & Environment
Summary
• Know the Mendelian crosses
and their patterns.
• Be able to work simple genetic
problems (practice).
• Watch genetic vocabulary.
• Be able to read pedigree
charts.
Summary
• Be able to recognize and work
with some of the “common”
human trait examples.
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