Transcript Topic 14: Mendelian Genetics

+
Drawing from the Deck of Genes

What genetic principles account for the passing of traits from parents
to offspring? Two opposing theories!
Blending
Genetic material blends like
paint
Leads to homogenous
population
Doesn’t explain
“reappearance” of traits
Particulate
Genetic material is discrete
packets of info
Genes can be shuffled and
passed on, like cards
Suggests reappearance of traits
Gregor
Mendel:
The
father
of
genetics
+
+ Concept 14.1: Mendel used the
scientific approach to identify two
laws of inheritance

Advantages of pea plants for
genetic study




There are many varieties with
distinct heritable features, or
characters (such as flower
color); character variants (such
as purple or white flowers) are
called traits
Mating can be controlled
Each flower has spermproducing organs (stamens)
and an egg-producing organ
(carpel)
Cross-pollination (fertilization
between different plants)
involves dusting one plant with
pollen from another
Figure 14.2a
+
TECHNIQUE
1
Remove “purple” stamen
Transfer “white” pollen to “purple”
carpel
2
Parental
generation
(P)
Stamens
3
Carpel
4
Pollinated carpel grows
and bears seeds
Plant the peas!
Figure 14.2b
+
RESULTS
First filial
generation
offspring
(F1)
Examined offspring
5
+

Mendel chose to track only those characters that occurred in two
distinct alternative forms: Purple of white – nothing in between

He also used varieties that were true-breeding: purple plants
always give rise to purple; white to white, etc.

In a typical experiment, Mendel mated two contrasting, true-breeding
varieties, a process called hybridization
The true-breeding parents are
the P generation
The hybrid offspring of the P
generation are called the F1
generation
When F1 individuals self-pollinate
or cross- pollinate with other F1
hybrids, the F2 generation is
produced
EXPERIMENT
The Law of
Segregation
 When
Mendel crossed
contrasting, truebreeding white- and
purple-flowered pea
plants, all of the F1
hybrids were purple
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
The Law of
Segregation
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
Purple
flowers
White
flowers
All plants had purple flowers
Self- or cross-pollination
The Law of
Segregation
When Mendel crossed
the F1 hybrids, many of
the F2 plants had purple
flowers, but some had
white
Mendel discovered a
ratio of about three to
one, purple to white
flowers, in the F2
generation
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purpleflowered
plants
224 white
flowered
plants
+ Mendel’s Results Explained:
Heritable Factors

Mendel reasoned that only the
purple flower factor was
affecting flower color in the F1
hybrids

Mendel called the purple flower
color a dominant trait and the
white flower color a recessive
trait

The factor for white flowers was
not diluted or destroyed
because it reappeared in the F2
generation
+ Mendel’s Traits

Mendel observed the same pattern of
inheritance in six other pea plant
characters, each represented by two
traits

What Mendel called a “heritable
factor” is what we now call a gene
+ Mendel’s Model

Mendel developed a hypothesis to explain the 3:1 inheritance
pattern he observed in F2 offspring

These concepts can be related to what we now know about genes
and chromosomes
Mendel’s Hypotheses
Alternative versions
of genes create
variation
For each character,
two copies are
inherited
If the 2 versions differ,
then 1 will determine
the appearance, not
both
2 versions segregate
during game
formation
+ Concept 1: Alternative Versions of
Genes Account for Variation in
Inherited Characteristics: Alleles

For example, the gene for
flower color in pea plants
exists in two versions,.

These alternative versions of
a gene are now called alleles
+ Alleles and Loci
Allele for purple flowers
Locus for flower-color gene
Pair of
homologous
chromosomes
Allele for white flowers
Each gene resides at a specific locus on a specific chromosome!
+ Concept 2: For each character, an
organism inherits two alleles, one
from each parent

Mendel made this deduction without
knowing about the role of
chromosomes!

The two alleles at a particular locus
may be identical, as in the truebreeding plants of Mendel’s P
generation

Alternatively, the two alleles at a
locus may differ, as in the F1 hybrids
+
Concept 3: Dominant vs. Recessive

If the two alleles at a locus differ,
then one (the dominant allele)
determines the organism’s
appearance, and the other (the
recessive allele) has no
noticeable effect on appearance

In the flower-color example, the F1
plants had purple flowers because
the allele for that trait is dominant
+
Concept 4: Law of Segregation

The two alleles for a heritable
character separate (segregate)
during gamete formation and
end up in different gametes

Thus, an egg or a sperm gets
only one of the two alleles that
are present in the organism

This segregation of alleles
corresponds to the distribution
of homologous chromosomes to
different gametes in meiosis
Punnett Squares
and Mendel’s Ratio

Segregation model accounts for
the 3:1 ratio he observed in the
F2 generations

The possible combinations of
sperm and egg can be shown
using a Punnett square, a
diagram for predicting the results
of a genetic cross between
individuals of known genetic
makeup

A capital letter represents a
dominant allele, and a lowercase
letter represents a recessive
allele
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
Punnett Squares
and Mendel’s Ratio
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 p
2 P
Punnett Squares
and Mendel’s Ratio
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 p
2 P
Sperm from F1 (Pp) plant
F2 Generation
P
Eggs from
F1 (Pp) plant
p
3
P
p
PP
Pp
Pp
pp
:1
Punnett Square for “Tall” plants
Phenotype
vs.
Genotype
3
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Pp
(heterozygous)
1
2
1
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
The Testcross: Testing Dominant Phenotype

An individual could be either homozygous dominant or heterozygous
genotypically but have the same phenotype

The answer is to carry out a testcross: breeding the mystery individual with a
homozygous recessive individual

If any offspring display the recessive phenotype, the mystery parent must be
heterozygous
Figure 14.7
+
TECHNIQUE
Dominant phenotype,
unknown genotype:
PP or Pp?
Predictions
If purple-flowered
parent is PP
Sperm
p
p
Recessive phenotype,
known genotype:
pp
or
If purple-flowered
parent is Pp
Sperm
p
p
P
Pp
Eggs
P
Pp
Eggs
P
p
Pp
Pp
Pp
Pp
pp
pp
RESULTS
or
All offspring purple
1/
2
offspring purple and
1/ offspring white
2
+ The Law of Segregation: 1 Trait

Mendel derived the law of segregation by following a single
character

The F1 offspring produced in this cross were monohybrids,
individuals that are heterozygous for one character

A cross between such heterozygotes is called a monohybrid
cross
+ The Law of Independent
Assortment: 2 Traits

Mendel identified his second law of inheritance by following two
characters at the same time

Crossing two true-breeding parents differing in two characters produces
dihybrids in the F1 generation, heterozygous for both characters

A dihybrid cross, a cross between F1 dihybrids, can determine whether
two characters are transmitted to offspring as a package or
independently
Figure 14.8
EXPERIMENT
+
YYRR
P Generation
yyrr
yr
Gametes YR
F1 Generation
Predictions
YyRr
Hypothesis of
dependent assortment
Hypothesis of
independent assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
Sperm
1/
2
YR
1/
2
2
YR
YyRr
YYRR
Eggs
1/
2
1/
4
YR
4
Yr
4
yR
4
yr
Eggs
yr
YyRr
3/
yyrr
1/
4
YR
1/
4
1/
Yr
4
yR
1/
4
yr
yr
1/
1/
4
1/
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
4
Phenotypic ratio 3:1
1/
9/
16
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
The Law of Independent Assortment

Using a dihybrid cross, Mendel developed the law of independent
assortment

The law of independent assortment states that each pair of alleles
segregates independently of each other pair of alleles during gamete
formation

Strictly speaking, this law applies only to genes on different, nonhomologous
chromosomes or those far apart on the same chromosome. Genes located
near each other on the same chromosome tend to be inherited together
+
Concept 14.2: The laws of probability
govern Mendelian inheritance

Mendel’s laws of segregation and independent assortment reflect the
rules of probability

When tossing a coin, the outcome of one toss has no impact on the
outcome of the next toss

In the same way, the alleles of one gene segregate into gametes
independently of another gene’s alleles
+
The Multiplication and Addition Rules
Applied to Monohybrid Crosses

The multiplication rule states that the probability that two or more
independent events will occur together is the product of their individual
probabilities: Example – What is the likelihood of tossing 2 coins and
getting 2 heads?
1

2
1
2
Segregation in a heterozygous plant is like flipping a coin: Each gamete
has a chance of carrying the dominant allele and a chance of carrying
the recessive allele
Probability
in Crosses
Probability in an
F1 monohybrid
cross can be
determined using
the multiplication
rule to find
possible
genotype
combinations
Rr
Segregation of
alleles into eggs

Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
2
Eggs
4
r
2
r
R
R
1/
1/
r
2
R
R
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
Probability
in Crosses


The addition rule
states that the
probability that any
one of two or more
exclusive events
will occur is
calculated by
adding together
their individual
probabilities
What’s the
likelihood of
heterozygous
offspring?
Rr
Segregation of
alleles into eggs

Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
2
Eggs
4
r
2
r
R
R
1/
1/
r
2
R
R
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
+
Solving Complex Genetics Problems with
the Rules of Probability

We can apply the multiplication and addition rules to predict the outcome of
crosses involving multiple characters

In calculating the chances for various genotypes, each character is
considered separately, and then the individual probabilities are multiplied

Attempt to answer the following questions: see Dihybrid Cross worksheet
Math of Crosses:
Faster than Punnett Squares
Probability of YYRR  1/4 (probability of YY)  1/4 (RR)  1/16
Probability of YyRR  1/2 (Yy)
 1/4 (RR)  1/8
Three or More Traits!
+
PpYyRr x Ppyyrr
ppyyRr
ppYyrr
Ppyyrr
PPyyrr
ppyyrr
1/ (yy)  1/ (Rr)
(probability
of
pp)

4
2
2
1/  1/  1/
4
2
2
1/  1/  1/
2
2
2
1/  1/  1/
4
2
2
1/  1/  1/
4
2
2
1/
Chance of at least two recessive traits
 1/16
 1/16
 2/16
 1/16
 1/16
 6/16 or 3/8
+ Concept 14.3: Inheritance patterns
are often more complex - IT’S NOT
USUALLY 1 GENE!

The relationship between genotype and phenotype is rarely as
simple as in the pea plant characters Mendel studied

Many heritable characters are not determined by only one gene with
two alleles

However, the basic principles of segregation and independent
assortment apply even to more complex patterns of inheritance
+ Section 1: Extending Mendelian
Genetics for a Single Gene

Inheritance of characters by a single gene may deviate from simple
Mendelian patterns in the following situations:

When alleles are not completely dominant or recessive

When a gene has more than two alleles

When a gene produces multiple phenotypes
+ Degrees of Dominance

Complete dominance occurs when phenotypes of the heterozygote
and dominant homozygote are identical

In incomplete dominance, the phenotype of F1 hybrids is somewhere
between the phenotypes of the two parental varieties – looks like a
blend!

In codominance, two dominant alleles affect the phenotype in separate,
distinguishable ways
Incomplete
Dominance
2 alleles for color in
snapdragons: CR and CW
Neither allele is dominant!
CRCR – Red phenotype
CWCW – White phenotype
CRCW Pink phenotype
Heterzygous phenotype is new
third phenotype in between the
homozygous - blend
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
Incomplete
Dominance
If F1 generation is crossed,
homozygous genotypes will
reappear
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
F1 Generation
Gametes 1/2 CR
Pink
CRCW
1/
2
CW
Incomplete
Dominance
What are the probabilities
of Red, White, and Pink
phenotypes in the F2?
P Generation
White
CWCW
Red
CRCR
CR
Gametes
CW
F1 Generation
Pink
CRCW
What are the genotype and
phenotype rations?
1/
Gametes 1/2 CR
2
CW
Sperm
F2 Generation
1/
2
CR
1/
2
CW
Eggs
1/
2
CR
1/
2
CW
CRCR CRCW
CRCW CWCW
+ Codominance: 2 alleles affect the
phenotype in separately – both
present

Homozygous phenotypes display
only 1 antigen

Heterozygous phenotype has both
present

Heterozygous has both traits visible
+
Incomplete Dominance
vs. Codominance
+
The Relation Between
Dominance and Phenotype

A dominant allele does not subdue a recessive allele; alleles don’t
interact that way

Alleles are simply variations in a gene’s nucleotide sequence

For any character, dominance/recessiveness relationships of
alleles depend on the level at which we examine the phenotype
+

The Relation Between Dominance and
Phenotype
Tay-Sachs disease is fatal; a
dysfunctional enzyme causes an
accumulation of lipids in the brain

At the organismal level, the allele
is recessive

At the biochemical level, the
phenotype (i.e., the enzyme
activity level) is incompletely
dominant  half normal
enzymes activity

At the molecular level, the alleles
are codominant – equal amounts
of functional and dysfunctional
enzyme are made
+ Polydactyly: Frequency of Dominant
Alleles

Dominant alleles are not necessarily more common in populations
than recessive alleles

Polydactyly: The dominant allele has an extra digit!

For example, one baby out of 400 in the United States is born with
extra fingers or toes
Section 2: Multiple Alleles: Most Genes
Have More Than 2 Forms!

For example, the four phenotypes of the ABO blood group in
humans are determined by three alleles for the enzyme (I) that
attaches A or B carbohydrates to red blood cells: IA, IB, and i.
The enzyme
encoded by the
IA allele adds the
A carbohydrate,
whereas the
enzyme encoded
by the IB allele
adds the B
carbohydrate;
the enzyme
encoded by the i
allele adds
neither
Figure 14.11
+
(a) The three alleles for the ABO blood groups and their
carbohydrates
IA
Allele
Carbohydrate
IB
i
none
B
A
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
A
B
AB
O
Red blood cell
appearance
Phenotype
(blood group)
+
Pleiotropy

Most genes have multiple phenotypic effects, a property called
pleiotropy

For example, pleiotropic alleles are responsible for the multiple
symptoms of certain hereditary diseases, such as cystic fibrosis and
sickle-cell disease
+ Extending Mendelian Genetics
for Two or More Genes

Some traits may be determined by two or more genes
Epistasis
a gene at one locus
alters the phenotypic
expression of a gene at
a second locus
One gene for pigment
color: B – black; b brown
One gene to determine
if pigment gets
deposited (E – color; e –
no color
BbEe
Eggs
1/
4 BE
1/
4 bE
1/
4 Be
1/
4
be
Sperm
1/ BE
4
1/
BbEe
4 bE
1/
4 Be
1/
4 be
BBEE
BbEE
BBEe
BbEe
BbEE
bbEE
BbEe
bbEe
BBEe
BbEe
BBee
Bbee
BbEe
bbEe
Bbee
bbee
9
: 3
: 4
AaBbCc
AaBbCc
Sperm
1/
1/
8
8
1/
1/
Eggs
8
1/
1/
8
8
1/
8
1/
1/
8
8
8
8
1/
8
1/
8
1/
1/
Polygenic
Inheritance –
Continuum for
a trait

8
1/
8
1/
8
1/
8
Phenotypes:
Number of
dark-skin alleles:
1/
64
0
6/
64
1
15/
64
2
20/
64
3
15/
64
4
6/
64
5
1/
64
6
Quantitative variation
usually indicates
polygenic
inheritance, an
additive effect of two or
more genes on a single
phenotype
+ Nature and Nurture: The Environmental
Impact on Phenotype

Another departure from Mendelian genetics arises when the
phenotype for a character depends on environment as well as
genotype

The norm of reaction is the phenotypic range of a genotype
influenced by the environment
Hydrangea flowers
of the same
genotype range
from blue-violet to
pink, depending on
soil acidity
+ Concept 14.4: Many human traits follow
Mendelian patterns of inheritance


Humans are not good subjects for genetic research

Generation time is too long

Parents produce relatively few offspring

Breeding experiments are unacceptable
However, basic Mendelian genetics endures as the foundation of
human genetics
+ Pedigree Analysis

A pedigree is a family tree that describes the interrelationships of
parents and children across generations

Inheritance patterns of particular traits can be traced and
described using pedigrees
Widow’s
peak
No widow’s
peak
Attached
earlobe
Free
earlobe
Figure 14.15
+Key
Male
1st
generation
Affected
female
Affected
male
Female
Mating
1st
generation
Ww
ww
ww
Ww
2nd
generation
Ww
ww
3rd
generation
WW
or
Ww
Widow’s
peak
ff
ff
(a) Is a widow’s peak a dominant or
recessive trait?
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd
generation
ww
No widow’s
peak
ff
Ff
2nd
generation
FF or Ff
Ww ww ww Ww
Ff
Offspring
Attached
earlobe
Free
earlobe
b) Is an attached earlobe a dominant
or recessive trait?
Inherited Disorders
Recessive
Disorders
• These range from relatively mild to lifethreatening
• Only occur if person is Homozygous recessive
• Heterozygous persons are called carriers
Dominant
Disorders
• Dominant allele is less common
• Dominant, lethal diseases are not common!
Multifactorial
Disorders
• Have a genetic component and significant
environmental influence
+ Recessive Allele Behavior

If a recessive allele that causes a disease is rare, then the chance
of two carriers meeting and mating is low

Consanguineous matings (i.e., matings between close relatives)
increase the chance of mating between two carriers of the same
rare allele

Most societies and cultures have laws or taboos against marriages
between close relatives
Recessive Diseases
Albanism
+
Parents
Normal
Aa
Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
+
Recessive Diseases
Cystic Fibrosis

Cystic fibrosis is the most
common lethal genetic disease
in the US: 1 in 2,500 of
European descent

The cystic fibrosis allele
results in defective or absent
chloride transport channels in
plasma membranes leading to
a buildup of chloride ions
outside the cell

Symptoms include mucus
buildup in some internal
organs and abnormal
absorption of nutrients in the
small intestine
+
Recessive Diseases
Sickle-Cell Disease: A Genetic Disorder
with Evolutionary Implications

Sickle-cell disease affects 1 in 400 African-Americans

The disease is caused by the substitution of a single amino acid in the
hemoglobin protein in red blood cells

In homozygous individuals, all hemoglobin is abnormal (sickle-cell)

Symptoms include physical weakness, pain, organ damage, and even
paralysis
+
Recessive Diseases
Sickle-Cell Disease: A Genetic Disorder
with Evolutionary Implications

Heterozygotes (said to have sickle-cell
trait) are usually healthy but may suffer
some symptoms

About one out of ten African Americans
has sickle cell trait, an unusually high
frequency of an allele with detrimental
effects in homozygotes

Heterozygotes are less susceptible to
the malaria parasite, so there is an
advantage to being heterozygous
+Dominantly Inherited Disorders

Achondroplasia: form of dwarfism caused by a rare dominant allele
Parents
Dwarf
Dd
Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
Normal
d
Dd
Dwarf
dd
Normal
Eggs
Huntington’s Disease:
A Late-Onset Lethal Disease

The timing of onset of a disease significantly affects its inheritance

Huntington’s disease is a degenerative disease of the nervous system
with no obvious phenotypic effects until the individual is about 35 to 40
years of age
Once the
deterioration of
the nervous
system begins the
condition is
irreversible and
fatal
+
Multifactorial Disorders

Many diseases, such as heart disease, diabetes, alcoholism,
mental illnesses, and cancer have both genetic and environmental
components

Little is understood about the genetic contribution to most
multifactorial diseases
+
Counseling Based on Mendelian Genetics
and Probability Rules

Using family histories, genetic counselors help couples determine
the odds that their children will have genetic disorders

Probabilities are predicted on the most accurate information at the
time; predicted probabilities may change as new information is
available
+
Tests for Identifying Carriers

For a growing number of diseases, tests are available that identify
carriers and help define the odds more accurately
+
Fetal Testing

In amniocentesis, the liquid that bathes the fetus is removed and
tested

In chorionic villus sampling (CVS), a sample of the placenta is
removed and tested

Other techniques, such as ultrasound and fetoscopy, allow fetal
health to be assessed visually in utero
Video: Ultrasound of Human Fetus I
© 2011 Pearson Education, Inc.
Figure 14.19
+(a) Amniocentesis
1
(b) Chorionic villus sampling (CVS)
Ultrasound monitor
Amniotic
fluid
withdrawn
Ultrasound
monitor
Fetus
1
Placenta
Chorionic villi
Fetus
Placenta
Uterus
Cervix
Cervix
Uterus
Suction
tube
inserted
through
cervix
Centrifugation
Fluid
Fetal
cells
Several hours
2
Several
weeks
Biochemical
and genetic
tests
Several
hours
Fetal cells
2
Several hours
Several weeks
3
Karyotyping
+
Newborn Screening

Some genetic disorders can be detected at birth by simple tests that
are now routinely performed in most hospitals in the United States
Figure 14.UN03
+
Relationship among
alleles of a single gene
Complete dominance
of one allele
Description
Heterozygous phenotype
same as that of homozygous dominant
Incomplete dominance Heterozygous phenotype
intermediate between
of either allele
the two homozygous
phenotypes
Codominance
Both phenotypes
expressed in
heterozygotes
Example
PP
Pp
CRCR CRCW CWCW
IAIB
Multiple alleles
In the whole population, ABO blood group alleles
some genes have more
IA, IB, i
than two alleles
Pleiotropy
One gene is able to affect Sickle-cell disease
multiple phenotypic
characters
Figure 14.UN04
+
Relationship among
two or more genes
Epistasis
Description
The phenotypic
expression of one
gene affects that
of another
Example
BbEe
BE
BbEe
bE
Be
be
BE
bE
Be
be
9
Polygenic inheritance
A single phenotypic
character is affected
by two or more genes
AaBbCc
:3
:4
AaBbCc
Figure 14.UN05
+
Character
Dominant
Recessive
Flower position
Axial (A)
Terminal (a)
Stem length
Tall (T)
Dwarf (t)
Seed shape
Round (R)
Wrinkled (r)
Figure 14.UN06
+
Figure 14.UN07
+
George
Sandra
Tom
Sam
Arlene
Wilma
Ann
Michael
Carla
Daniel
Alan
Tina
Christopher
Figure 14.UN08
+
Figure 14.UN09
+
Figure 14.UN10
+
Figure 14.UN11
+
Figure 14.UN12
+
Figure 14.UN13
+