Mendel and
Heredity
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).
Gregor Mendel
Father
of Modern Genetics.
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.
You can control mating.
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
Purple X white
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.
Another Sample Cross
P1
F1
F2
Tall X short (TT x tt)
all Tall (Tt)
3 tall to 1 short
(1 TT: 2 Tt: 1 tt)
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.
Mendel's Hypothesis
4. The two alleles for each trait
separate during gamete
formation. This now called:
Mendel's Law of Segregation
Law of Segregation
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.
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.
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.
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.
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.
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)
= CR CR
Pink = CRCW
White = CWCW
Red
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.
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.
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.
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.
Sex-Linked Traits
 Trait
that is on a sex
chromosome…more X linked traits
than Y because X is much larger
 Sex chromosomes: genes that
determine the gender of an individual
 Autosomes: remaining genes
Common sex linked
diseases
Hemophilia
Duchenne muscular
dystrophy
Red-green color
blindness
Genetic Screening
Risk
assessment for an
individual inheriting a trait.
Uses probability to calculate
the risk.
Carrier Recognition
Fetal
Testing
Amniocentesis
Chorionic
Newborn
villi sampling
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