MENDELIAN GENETICS
What is genetics?
The study of how traits are inherited
or how genetic information is passed
from one generation to the next.
It also explains biological variation
Gregor Mendel
• 1850’s Grew up in a farm wanting to garden
• Austrian monk (Flunked out of college twice) but
became a mathematician
• Experimented with garden pea plants
• Using pea plants looked at seven different
characters (height of plants, seed color, texture,
flower color) and found evidence of how
parents transmit genes to offspring
• Mendel’s statistical analysis provided a model
for predicting what the next generation would
be like
What was the prevalent believe about
inheritance before Mendel?
• People believed in “spontaneous generation” and
in the “blending of characters”
• Blending theory
– Problem:
• Would expect variation to disappear
• Variation in traits persists
Ex: Yellow and green parakeets should have all blue
babies. This is not what you observe.
The gene theory
• An alternative idea is the “gene” idea.
Parents pass on discrete individual heritable
units: genes
• Experimental genetics began in an abbey
garden
– Modern genetics
• Began with Gregor Mendel’s quantitative
experiments with pea plants
Petal
Stamen
Carpel
Figure 9.2 A
Figure 9.2 B
The Garden Pea Plant
• Mendel chose to work with the pea
plant because he could control which
plant mated with which. Pea plants are
• Self-pollinating
• True breeding (different alleles not
normally introduced)
• Can be experimentally cross-pollinated
– Mendel crossed pea plants that differed in certain
characteristics
• And traced traits from generation to generation
• Mendel started his experiments with plants that were “true
breeding”.
1 Removed stamens
from purple
flower
White
2 Transferred
pollen from stamens
of white flower to
carpel of purple flower
Stamens
Carpel
Parents
(P)
Purple
3 Pollinated carpel
matured into pod
4 Planted seeds
from pod
Offspring
(F1)
Figure 9.2 C
– Mendel hypothesized that there are alternative
forms of genes
• The units that determine heritable traits
Flower color
White
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Tall
Dwarf
Flower position
Stem length
Figure 9.2 D
Purple
Mendel’s Principles of Genetics
•
Mendel refuted the “blending theory” of
heredity and provided an explanation of how
inheritance works without knowing anything
about chromosomes or genes.
1. He figured that traits must be coded for by
some kind of inheritable particle which he
called “factors” and now we call “genes”.
2. He said that those genes were
transmitted as independent entities
from one generation to the next.
Mendel’s insight continued…
3. He figured that there must be different
versions of these “genes”
( we call them now “alleles”)and that
every individual has two genes for
each trait.
(Or we can say that: For each characteristic
an organism inherits two alleles, one from each parent)
He identified one as dominant, the other as
recessive.
4. He figured that the two alleles a parent has
are separated into different cells when
gametes (sex cells) are formed. This actually
happens during metaphase of meiosisI ( no one knew
about meiosis in those days).
This is known as the Law of Segregation
What are alleles?
Different versions of the same gene
Mendel’s Theory
of Segregation
• An individual inherits a unit of information
(allele) about a trait from each parent
• During gamete formation, the alleles
segregate from each other
– Mendel’s law of segregation
• Predicts that allele pairs separate from each other
during the production of gametes
P plants
Genetic makeup (alleles)
pp
PP
Gametes
All p
All P
F1 plants
(hybrids)
All Pp
1
P
2
Gametes
1
p
2
Sperm
P
F2 plants Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP: 2 Pp: 1 pp
Figure 9.3 B
p
P
PP
Pp
p
Pp
pp
Eggs
•Mendel’s law of segregation describes the
inheritance of a single characteristic
– From his experimental data
• Mendel deduced that an organism has two genes
(alleles) for each inherited characteristic
P generation
(true-breeding
parents)

Purple flowers
F1 generation
White flowers
All plants have
purple flowers
Fertilization
among F1 plants
(F1  F1)
F2 generation
3
4
of plants
have purple flowers
Figure 9.3 A
1
4
of plants
have white flowers
What is a dominant trait?
The trait that shows, the allele that is fully
expressed
What is a recessive trait?
The alleles that is masked, the gene is there
but it doesn’t show
What is the phenotype?
The observable traits
What is the genotype?
The genetic make up
– If the two alleles of an inherited pair differ
• Then one determines the organism’s appearance and is
called the dominant allele ( use capital letters)
– The other allele
• Has no noticeable effect on the organism’s appearance
and is called the recessive allele
Vocabulary
• When you mate two contrasting true
breeding plants you get a Hybrid.
• The true breeding parents are called the “P”
(parent) generation
• The hybrid offspring of the P generation are
called the F1 generation
• When two F1 individuals self pollinate you
get the F2 generation
F1 Results of One Monohybrid
Cross
F2 Results of
Monohybrid Cross
Mendel’s
Monohybrid
Cross Results
F2 plants showed
dominant-torecessive ratio that
averaged 3:1
5,474 round
1,850 wrinkled
6,022 yellow
2,001 green
882 inflated
299 wrinkled
428 green
152 yellow
705 purple
224 white
651 long stem
207 at tip
787 tall
277 dwarf
Punnett Square of a Monohybrid
Cross
Female gametes
A
Male
gametes
a
A
AA
Aa
a
Aa
aa
Dominant
phenotype can
arise 3 ways,
recessive only
one
A Test cross
• In a pea plant with purple flowers the
genotype is not obvious. Could be
homozygous or heterozygous
• Why do a test cross?
It allows us to determine the genotype of an
organism with a dominant phenotype but
unknown genotype
Test Cross
• You cross an individual that shows the dominant
phenotype with an individual with recessive
phenotype ( one who is homozygous recessive for
that trait)
• Examining offspring allows you to determine the
genotype of the dominant individual
Punnett Squares of
Test Crosses
Homozygous
recessive
a
a
Homozygous
recessive
a
a
A
Aa
Aa
A
Aa
Aa
a
aa
aa
A
Aa
Aa
Two phenotypes
All dominant phenotype
Geneticists use the testcross to determine unknown
genotypes
– The offspring of a testcross, a mating between an individual
of unknown genotype and a homozygous recessive individual
• Can reveal the unknown’s genotype

Testcross:
Genotypes
bb
B_
Two possibilities for the black dog:
BB
Gametes
Offspring
Bb
B
b
Figure 9.6
or
Bb
All black
b
B
b
Bb
bb
1 black : 1 chocolate
Homologous chromosomes bear the two
alleles for each characteristic
– Alternative forms of a gene
• Reside at the same locus on homologous
chromosomes
Dominant
allele
Gene loci
P
P
a
B
a
b
Recessive
allele
Genotype:
Figure 9.4
PP
aa
Homozygous
for the
dominant allele
Homozygous
for the
recessive allele
Bb
Heterozygous
Web sites to check
• http://gslc.genetics.utah.edu/units/basics/tou
r/inheritance.swf
• http://science.nhmccd.edu/biol/genetics.html
• http://library.thinkquest.org/20465/games.ht
ml
Mendel’s two Laws
• 1. Law of segregation
The two alleles for a trait segregate during gamete
formation and only one allele for a trait is
carried in a gamete. The gametes combine at
random
(In other words:A cell contains two copies of a particular
gene, they separate when a gamete is made).
• 2. Law of Independent Assortment
Alleles from one trait behave independently from
alleles for another trait. Traits are inherited
independently from one another
Independent Assortment
• Mendel concluded that the two “units” for
the first trait were to be assorted into
gametes independently of the two “units”
for the other trait
• Members of each pair of homologous
chromosomes are sorted into gametes at
random during meiosis
• The law of independent assortment is
revealed by tracking two characteristics at
once
– By looking at two characteristics at once
• Mendel tried to determine how two
characteristics were inherited
– Mendel’s law of independent assortment
• States that alleles of a pair segregate independently of
other allele pairs during gamete formation
P generation
Hypothesis: Dependent assortment
RRYY
rryy
Hypothesis: Independent assortment
RRYY
ry
Gametes RY

Gametes RY
ry
RrYy
RrYy
F1 generation
Sperm
Sperm
1
2 RY
1
1 ry
RY
4
4
1
2 ry
1
RY
2
F2 generation
Eggs
1
ry
2
Actual results
contradict hypothesis
Figure 9.5 A
rryy
1
RY
4
1
ry
4
Eggs
1
Ry
4
1
ry
4
1
RY
4
1 ry
4
RRYY
RrYY RRYy RrYy
RrYY
rrYY
RrYy
rrYy
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
Actual results
support hypothesis
9
16
3
16
3
16
1
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
– An example of independent assortment
Blind
Blind
Phenotypes
Genotypes
Black coat, normal vision
B_N_
Mating of heterozygotes
(black, normal vision)
Phenotypic ratio
of offspring
Figure 9.5 B
9 black coat,
normal vision
Black coat, blind (PRA) Chocolate coat, normal vision Chocolate coat, blind (PRA)
B_nn
bbN_
bbnn
BbNn
3 black coat,
blind (PRA)

BbNn
3 chocolate coat,
normal vision
1 chocolate coat,
blind (PRA)
A Dihybrid Cross - F1 Results
purple
flowers,
tall
TRUEBREEDING
PARENTS:
AABB
GAMETES:
AB
x
AB
white
flowers,
dwarf
aabb
ab
ab
AaBb
F1 HYBRID
OFFSPRING:
All purple-flowered, tall
16 Allele
Combinations in F2
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/16
1/16
1/16
1/16
AABB AABb AaBB AaBb
1/16
1/16
1/16
1/16
AABb AAbb AaBb Aabb
1/16
1/16
1/16
1/16
AaBB AaBb aaBB aaBb
1/16
1/16
1/16
1/16
AaBb Aabb aaBb aabb
Phenotypic Ratios in F2
AaBb
X
AaBb
Four Phenotypes:
– Tall, purple-flowered
(9/16)
– Tall, white-flowered (3/16)
– Dwarf, purple-flowered
(3/16)
– Dwarf, white-flowered
(1/16)
Explanation of Mendel’s
Dihybrid Results
If the two traits
are coded for by
genes
on separate
chromosomes,
sixteen gamete
combinations are
possible
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/16
1/16
1/16
1/16
AABB AABb AaBB AaBb
1/16
1/16
1/16
1/16
AABb AAbb AaBb Aabb
1/16
1/16
1/16
1/16
AaBB AaBb aaBB aaBb
1/16
1/16
1/16
1/16
AaBb Aabb aaBb aabb
• Mendel’s laws reflect the rules of
probability
– Inheritance follows the rules of probability
– The rule of multiplication
• Calculates the probability of two independent events
– The rule of addition
• Calculates the probability of an event that can occur in
alternate ways
F1 genotypes
Bb male
Formation of sperm
Bb female
Formation of eggs
1
2
1
2
B
1
2
b
B
B
b
1
4
B
b
1
4
b
B
1
4
F2 genotypes
Figure 9.7
1
2
B
b
b
1
4
Genetic traits in humans can be tracked
through family pedigrees
– The inheritance of many human traits
• Follows Mendel’s laws
Dominant Traits
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
Figure 9.8 A
– Family pedigrees
• Can be used to determine individual genotypes
Dd
Joshua
Lambert
D?
John
Eddy
Dd
Abigail
Linnell
dd
Jonathan
Lambert
D?
Abigail
Lambert
Dd
Dd
dd
D?
Hepzibah
Daggett
Dd
Elizabeth
Eddy
Dd
Dd
Dd
dd
Female Male
Deaf
Hearing
Figure 9.8 B
• Recessive Disorders
– Most human genetic disorders are recessive
Parents
Normal
Dd

Normal
Dd
Sperm
D
D
Offspring
Dd
Normal
(carrier)
Eggs
d
Figure 9.9 A
DD
Normal
d
Dd
Normal
(carrier)
dd
Deaf
VARIATIONS ON MENDEL’S
LAWS
The relationship of genotype to phenotype is rarely simple
– Mendel’s principles are valid for all sexually reproducing
species
• But genotype often does not dictate phenotype in the simple way his
laws describe
Genetics is not as simple as Gregor Mendel concluded,
(one gene, one trait).
We know now that there is a range of dominance and
that genes can work together and interact.
Incomplete dominance:
When the F1 generation have an appearance in between the
phenotypes of the parents.
Ex: pink snapdragons offspring of red and white ones.
Another way to say it is
In incomplete dominance
Heterozygote phenotype is somewhere between that of two
homozygotes
Flower Color in Snapdragons:
Incomplete Dominance
Red-flowered plant X White-flowered plant
(homozygote)
(homozygote)
Pink-flowered F1 plants
(heterozygotes)
Incomplete dominance in snapdragon color
Flower Color in Snapdragons:
Incomplete Dominance
• Red flowers - two alleles allow them to
make a red pigment
• White flowers - two mutant alleles; can’t
make red pigment
• Pink flowers have one normal and one
mutant allele; make a smaller amount of red
pigment
Flower Color in Snapdragons:
Incomplete Dominance
Pink-flowered plant X Pink-flowered plant
(heterozygote)
(heterozygote)
White-, pink-, and red-flowered plants
in a 1:2:1 ratio
Incomplete dominance in carnations
Co-Dominance or multiple alleles:
• Codominance
– Non-identical alleles specify two phenotypes
that are both expressed in heterozygotes
• Having more than 2 alleles for a given trait
and both alleles show in the phenotype. No
single one is dominant over the other.
•
Example: ABO blood types
Genetics of ABO Blood Types:
Three Alleles
• Gene that controls ABO type codes for
enzyme that dictates structure of a
glycolipid on blood cells
• Two alleles (IA and IB) are codominant
when paired
• Third allele (i) is recessive to others
ABO blood types
– The ABO blood type in humans
• Involves three alleles of a single gene
– The alleles for A and B blood types are codominant
• And both are expressed in the phenotype
Blood
Group
(Phenotype)
Figure 9.13
Genotypes
Antibodies
Present in
Blood
O
ii
Anti-A
Anti-B
A
IAIA
or
IAi
Anti-B
B
IBIB
or
IBi
Anti-A
AB
IAIB
—
Reaction When Blood from Groups Below Is Mixed with
Antibodies from Groups at Left
O
A
B
AB
Multiple alleles for the ABO blood groups
More exceptions to the dominant/recessive rule
Pleiotropy:
One genes having many effects. Only one gene affects an
organism in many ways.
Ex: sickle cell anemia and cystic fibrosis
Pleiotropy
• Alleles at a single locus may have effects on
two or more traits
• Classic example is the effects of the mutant
allele at the beta-globin locus that gives rise
to sickle-cell anemia
• A single gene may affect many phenotypic
characteristics
– In pleiotropy
• A single gene may affect phenotype in many ways
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Physical
weakness
Anemia
Impaired
mental
function
Figure 9.14
Heart
failure
Paralysis
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
Genetics of Sickle-Cell Anemia
• Two alleles
1) HbA
Encodes normal beta hemoglobin chain
2) HbS
Mutant allele encodes defective chain
• HbS homozygotes produce only the
defective hemoglobin; suffer from sicklecell anemia
Pleiotropic effects of the sickle-cell allele in a homozygote
Epistasis:
• Interaction between the products of gene
pairs
Interaction between two genes in which one
of the genes modifies the expression of
the other.
Ex: fur /hair color in mammals and albinism
Albinism
• Phenotype results when pathway for
melanin production is completely blocked
• Genotype - Homozygous recessive at the
gene locus that codes for tyrosinase, an
enzyme in the melanin-synthesizing
pathway
Genetics of Coat Color in
Labrador Retrievers
• Two genes involved
- One gene influences melanin production
• Two alleles - B (black) is dominant over b (brown)
- Other gene influences melanin deposition
• Two alleles - E promotes pigment deposition and is
dominant over e
Allele Combinations
and Coat Color
• Black coat - Must have at least one
dominant allele at both loci
– BBEE, BbEe, BBEe, or BbEE
• Brown coat - bbEE, bbEe
• Yellow coat - Bbee, BbEE, bbee
An example of epistasis
Human Variation
• Some human traits occur as a few discrete
types
– Attached or detached earlobes
– Many genetic disorders
• Other traits show continuous variation
– Height
– Weight
– Eye color
More modifications to Mendel’s rule
Polygenic Inheritance:
In this case many genes have an additive effect.
The characteristic or trait is the result of the
combined effect of several genes. Ex: human
skin color, height. Controlled by more than one
pair of genes
Continuous Variation
• Polygenic inheritance results in a
continuous range of small differences in a
given trait among individuals
• The greater the number of genes that affect
a trait, the more continuous the variation in
versions of that trait
A simplified model for polygenic inheritance of skin color
Environmental effects:
The degree to which an allele is expressed
depends on the environment
Ex: Siamese cat fur color ( enzyme for
melanin production inhibited by heat),
hydrangea flowers ( depends on acidity of
soil), height (nutrition)
Temperature Effects
on Phenotype
• Himalayan rabbits are
Homozygous for an allele that
specifies a heat-sensitive
version of an enzyme in
melanin-producing pathway
• Melanin is produced in cooler
areas of body
Environmental Effects on Plant
Phenotype
• Hydrangea macrophylla
• Action of gene responsible for floral color is
influenced by soil acidity
• Flower color ranges from pink to blue
The effect of environment of phenotype
Web sites to check
• http://gslc.genetics.utah.edu/units/basics/tou
r/inheritance.swf
• http://science.nhmccd.edu/biol/genetics.html
• http://library.thinkquest.org/20465/games.ht
ml
Thomas Hunt Morgan (1910) and Sex Linked
Inheritance
Morgan’s Experimental Evidence: Scientific Inquiry
• The first solid evidence associating a specific
gene with a a specific chromosome came from
Thomas Hunt Morgan
• Morgan’s experiments with fruit flies (Columbia
University, 1910) provided convincing evidence that
chromosomes are the location of Mendel’s
heritable factors. He provided confirmation of
the correctness of the chromosomal theory of
inheritance.
– Morgan’s experiments
• Demonstrated the role
of crossing over in
inheritance
Experiment
Black body,
vestigial
wings
Gray body,
long wings
(wild type)

GgLI
ggll
Male
Female
Offspring
Gray long
Black vestigial
Gray vestigial
Black long
965
944
206
185
Parental
phenotypes
Recombinant
phenotypes
391 recombinants
Recombination frequency =
Explanation
GgLI
(female)
G L
2,300 total offspring
G L
g l
g l
g l
g l
Gl
gL
Eggs
G L
g l
ggll
(male)
g l
Sperm
g l
g l
Offspring
Figure 9.20 C
= 0.17 or 17%
G l
g l
g L
g l
– Thomas Hunt Morgan
• Performed some of the early studies of crossing
over using the fruit fly Drosophila melanogaster
Figure 9.20 B
– In Drosophila
• White eye color is a sex-linked trait
Figure 9.23 A
SEX LINKED INHERITANCE
• CHROMOSOMES
• Humans have 22 pairs of AUTOSOMES and one
pair of SEX CHROMOSOMES : total=23 prs
• Thomas Morgan discovered SEX LINKED
INHERITANCE studying Drosophila (fruit fly)
• In fruit flies red eyes is the wild type and white
eyes is a mutant. He noticed the connection
between gender and certain traits. Only the male
flies had mutant white eyes.
SEX LINKED TRAITS ARE THOSE
CARRIED BY THE X CHROMOSOME
• Red-Green color blindness
Inability to see those colors. Red and green look all
the same ,like gray
• Hemophilia Blood clotting disorder.
The clotting factor VIII is not made, individual can
bleed to death.
• Muscular dystrophy
X linked recessive, gradual and progressive
destruction of skeletal muscles .
• Faulty teeth enamel
Extremely rare, X linked Dominant
•Sex-linked genes exhibit a unique pattern of
inheritance
– All genes on the sex chromosomes
• Are said to be sex-linked
– In many organisms
• The X chromosome carries many genes unrelated to sex
• new technologies can provide insight into
one’s genetic legacy
– New technologies
• Can provide insight for reproductive decisions
• Identifying Carriers
– For an increasing number of genetic disorders
• Tests are available that can distinguish carriers of genetic
disorders
• 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
• Fetal Testing
– Amniocentesis and chorionic villus sampling (CVS)
• Allow doctors to remove fetal cells that can be tested for genetic
abnormalities
Chorionic villus sampling (CVS)
Amniocentesis
Needle inserted
through abdomen to
extract amniotic fluid
Ultrasound
monitor
Fetus
Suction tube inserted
through cervix to extract
tissue from chorionic villi
Fetus
Placenta
Uterus
Ultrasound
monitor
Placenta
Chorionic
villi
Cervix
Cervix
Uterus
Amniotic
fluid
Centrifugation
Fetal
cells
Fetal
cells
Several
weeks
Figure 9.10 A
Biochemical
tests
Karyotyping
Several
hours
• Ethical Considerations
– New technologies such as fetal imaging and testing
• Raise new ethical questions
Mutations
• Mutations are permanent changes in DNA
Causes?
Errors in DNA replication that can be spontaneous.
Also caused by high energy radiation (X rays,
gamma rays),toxic chemicals in the environment
( pesticides,asbestos, tar) and viruses.
MUTATION: A PERMANENT CHANGE IN THE DNA.
When it happens in the gametes it is inheritable. Some
mutations are lethal but most are harmless. Mutations are
very important because it creates DIVERSITY
• WHAT CAUSES MUTATIONS?
• Most mutations are spontaneous, changes in DNA caused
by errors in replication ( the DNA is copied incorrectly
during cell division). The cell has mechanism to find and
correct mistakes but those that get through get passed
along.
• Some mutations can cause genetic disorders.
• Some environmental factors can cause molecular changes
in DNA.
• X rays, toxic chemicals (insecticides, fertilizers, dry
cleaning fluids, tar), some viruses, high energy radiation.
• Many inherited disorders in humans are
controlled by a single gene
– Some autosomal disorders in humans
Table 9.9
DISORDERS RESULTING FROM AUTOSOMAL
RECESSIVE INHERITANCE
• These are conditions in which the gene that is defective is
recessive.
• It is only expressed when the child receives both recessive genes
for the disorder (one from each parent)
• If a person is heterozygous, that is it has one dominant regular
gene and one recessive abnormal gene for the condition, he will
be a CARRIER but not have the disorder. The dominant allele
will mask the expression of the abnormal condition.
•
EXAMPLES:
•
•
•
•
•
•
ALBINISM:
SICKLE CELL ANEMIA:
CYSTIC FIBROSIS:
TAY- SACHS DISEASE;
PHENYLKETONURIA;
GALACTOSEMIA:
DISORDERS RESULTING FROM RECESSIVE
INHERITANCE
Many not life threatening traits are inherited this way.
widows peak, and attached earlobes.
• ALBINISM: No pigmentation in skin This is
also an example of “EPISTASIS”(one pair of
genes modifies the expression of another)
• SICKLE CELL ANEMIA: This is also an
example of “PLEIOTROPY”
Red blood cells curved shape. Decreased oxygen
to brain and muscles (offers resistance to
Malaria)
DISORDERS RESULTING FROM RECESSIVE
INHERITANCE
• CYSTIC FIBROSIS: Excessive mucus secretions.Impaired lung
function, lung infections. Protein channel that transport chloride
across cell membrane does not function. Protects against cholera.
This is also an example of “PLEIOTROPY”
• TAY –SACHS DISEASE: Nervous system degeneration in
infants. Enzyme fails to breakdown lipids which accumulate in
nerve cells and kills the cells. Progressive degeneration starting
with the brain cells.
DISORDERS RESULTING FROM RECESSIVE
INHERITANCE
• GALACTOSEMIA: Produces brain, liver,
eye damage. Enzyme that breaks down
lactose is lacking. It accumulates to toxic
levels. Death in infancy
• PHENYLKETONURIA: Results in mental
retardation
Disorders resulting from Autosomal
Dominant Inheritance
Dominant genes: Many are harmless for
example:freckles, dimples, cleft chin, free
earlobe, short big toe, tongue rollers, left
thumb on top, curly hair and dark hair
• Dominant traits appear in each generation
since the allele shows in the heterozygous
individual.
• Dominant Disorders
– Some human genetic disorders are dominant
Figure 9.9 B
Disorders resulting from Dominant
Inheritance
• Acondroplasia or dwarfism:
A condition where the bone does not grow properly and
can’t make proper cartilage. Person is less than 4 feet
with short arms and legs but a regular size trunk.
• Cholesterolemia:
High cholesterol levels in the blood causing arteries to
clog and high incidence of early heart attacks.
• Marfan Syndrome:
Abnormal connective tissue
Disorders resulting from Autosomal Dominant
Inheritance
• Huntington’s Disorder:
Progressive degeneration of nervous system and muscle
control. Affects motor and mental abilities and it is
irreversible. Late onset, usually late 30’s. Usually the
person already had children.
• Progeria:
Premature accelerated aging. Usually dead by 18. Genes that
bring about growth and development are abnormal.
• Polydactily:
Extra toes and fingers
Karyotype
• A karyotype is a visual display of an individual’s
chromosomes. A man made picture of a person’s 23
pairs of chromosomes. ( the photo is taken during
metaphase when the sister chromatids are lined up
together)
• It is useful in sex determination and diagnosis of
certain conditions.
INHERITED DISORDERS DUE TO
CHROMOSOMES CHANGES
•
Chromosome changes can cause a lot of
genetic disorders as well as a lot of variety
• WHEN AND HOW CAN A CHROMOSOME
CHANGE?
• Mistakes in replication. During the S phase of the
cell cycle segments of a chromosome could be
deleted, duplicated, inverted or moved to a new
location. Also during Metaphase I (meiosis) there
can be improper separation after duplication. This
can change the total number of chromosomes in
each gamete of the new individual.
• If during meiosis the paired chromatids fail to
separate correctly this is called NONDISJUNCTION
• ANEUPLOIDY means an abnormal number of
chromosomes.
• When an individual ends up with the wrong
number of chromosomes most of the time it is
miscarried ( spontaneous abortion).
• The wrong number of somatic chromosomes are
almost always lethal. Ex: trisomy 21(three chrom.
21): Down Syndrome
• You can live with the wrong number of sex pair
chromosomes.
CHANGES IN THE NUMBER OF SEX
CHROMOSOMES
• X Turner syndrome One X instead of a pair.
This happens because of non disjuction of sperm.
Most are aborted spontaneously. If they live, she is
very short, infertily and with reduced sex
characteristics.
• XXY Klinefelter syndrome One in 500 live
male births. Taller than average, infertile, some
low intelligence, some normal. Testosterone
injections help.
• XYY “super male” about 1 in 1000. taller,
mildly retarded but normal phenotype.
SEX CHROMOSOMES AND SEXLINKED GENES
• Chromosomes determine sex in many
species
– In mammals, a male has one X chromosome
and one Y chromosome
• And a female has two X chromosomes
(male)
44
Parents’
+
diploid
XY
cells
22
+
X
Figure 9.22 A
(female)
44
+
XX
22
+
Y
Sperm
22
+
X
44
+
XX
44
+
XY
Offspring
(diploid)
Egg
– Other systems of sex determination exist in
other animals and plants
22
+
XX
22
+
X
76
+
ZW
76
+
ZZ
32
16
Figure 9.22 B
Figure 9.22 C
Figure 9.22 D
– The Y chromosome
• Has genes for the development of testes
– The absence of a Y chromosome
• Allows ovaries to develop
Comb Shape in Poultry
Alleles at two loci (R and P) interact
•
•
•
•
Walnut comb - RRPP, RRPp, RrPP, RrPp
Rose comb - RRpp, Rrpp
Pea comb - rrPP, rrPp
Single comb - rrpp
Campodactyly:
Unexpected Phenotypes
• Effect of allele varies:
– Bent fingers on both hands
– Bent fingers on one hand
– No effect
• Many factors affect gene expression