Transcript Presentation

Mendelian Genetics and the
Inheritance of Genetic Traits
Gregor Mendel:
Father of Modern Genetics
IB Topic 4.3- Theoretical Genetics
Campbell: Ch. 14
Allott: Ch. 12
Theoretical Genetics Defined:
• Theoretical Genetics- concerned with the
probabilities associated with producing offspring
of a particular genotype or phenotype.
Experimental Genetics Began in an abbey
Garden
• Modern theoretical genetics began with Gregor
Mendel’s quantitative experiments with pea plants
Stamen
Carpel
Figure 9.2A, B
Gregor Mendel (Father of Genetics)
• Discovered the fundamentals of Genetics
in the 1860’s
• Lived in Austria and studied in Vienna
• Worked with Garden Peas (Pisum sativum)
• Gathered a huge amount of numerical data
• Discovered the frequency of how traits are
inherited
• Established basic principles of Genetics
MENDEL’S PRINCIPLES
• The science of heredity dates back to
ancient attempts at selective breeding
• Until the 20th century, however, many
biologists erroneously believed that
– characteristics acquired during lifetime could be
passed on
– characteristics of both parents blended
irreversibly in their offspring
Reason Mendel worked with
Garden Peas
•
•
•
•
Easy to grow
Many variations were available
Easy to control pollination (self vs cross)
Flower is protected from other pollen
sources
(reproductive structures are completely
enclosed by petals)
• Plastic bags can be used for extra
protection
• Mendel crossed
pea plants that
differed in certain
characteristics
and traced the
traits from
generation to
generation
• This illustration
shows his
technique for
cross-fertilization
Figure 9.2C
White
1
Removed
stamens
from purple
flower
Stamens
Carpel
PARENTS
(P)
2 Transferred
Purple
pollen from
stamens of white
flower to carpel
of purple flower
3 Pollinated carpel
matured into pod
4
OFFSPRING
(F1)
Planted
seeds
from pod
• Mendel studied
seven pea
characteristics
FLOWER
COLOR
Purple
White
Axial
Terminal
SEED
COLOR
Yellow
Green
SEED
SHAPE
Round
Wrinkled
POD
SHAPE
Inflated
Constricted
POD
COLOR
Green
Yellow
STEM
LENGTH
Tall
Dwarf
FLOWER
POSITION
• He hypothesized
that there are
alternative forms
of genes (although
he did not use that
term), the units
that determine
heredity
Figure 9.2D
Mendel’s Experiment
1. He set up true-breeding
plants (bred for many
generations) by
allowing them to selffertilize.
• He controlled
pollination, looking at 1
or 2 characteristics at a
time.
2. He crossed a true breeding plant with a
plant of the opposite trait (purple x white).
He called this the Parental (P1) generation.
3. He recorded data on the offspring of this cross,
calling it the First Filial, or F1 Generation.
4. He self pollinated the F1 offspring
5. He recorded data on the offspring of the second
generation, calling it the Second Filial generation
(F2)
Mendel’s Results
Analysis
• The F1 generation always displayed one trait (he
later called this the dominant trait)
• The F1 generation must have within it the trait
from the original parents - the white trait
• The F2 generation displayed the hidden trait, 1/4
of the F2 generation had it (he later called this
hidden trait the recessive trait)- 3:1 ratio.
• Each individual has two "factors" that determine
what external appearance the offspring will
have. (We now call these factors genes or
alleles)
Mendel established three principles (or
Laws) from his research:
1. The Principle of Dominance and
Recessiveness - one trait is masked or covered
up by another trait
2. Law of Segregation - the two factors (alleles)
for a trait separate during gamete formation
3. Law of Independent Assortment - factors of a
trait separate independently of one another
during gamete formation; another way to look at
this is, whether a flower is purple has nothing to
do with the length of the plants stems - each trait
is independently inherited
Genetic Crosses
1. Mendel's factors are now called
ALLELES. For every trait a person has,
two alleles determine how that trait is
expressed.
2. We use letters to denote alleles, since
every gene has two alleles, all genes can
be represented by a pair of letters.
PP = purple, Pp = purple, pp = white
Homologous chromosomes bear the two
alleles for each characteristic
• Alternative forms of a gene (alleles) reside
at the same locus on homologous
chromosomes
GENE LOCI
P
P
a
a
B
DOMINANT
allele
b
RECESSIVE
allele
GENOTYPE:
PP
aa
HOMOZYGOUS
for the
dominant allele
HOMOZYGOUS
for the
recessive allele
Bb
HETEROZYGOUS
Figure 9.4
Let’s do some definitions
• Genotype- the
alleles possessed
by an organism.
Ex: BB, or Bb
• Phenotype- the
characteristics of an
organism.
Ex: Brown hair
More Definitions
•
Homozygous- having
two identical alleles of a
gene. Ex: BB or bb
• Heterozygous- having
two different alleles of a
gene. Ex: Bb
When we cross-breed 2 things, looking at one
factor, we have a:
• Monohybrid cross = a cross involving one pair
of contrasting traits. Ex. Pp x Pp
We can figure the possibilities of offspring using a:
• Punnet Square: used to determine the
PROBABILITY of having a certain type of
offspring given the alleles of the parents
How to Solve a Punnett Square
1. Determine the genotypes (letters) of the
parents. Bb x Bb
2. Set up the punnett square with one
parent on each side.
3. Fill out the punnett square middle
4. Analyze the number of offspring of each
type.
An Example
• In pea plants, round seeds are dominant to
wrinkled. The genotypes and phenotypes are:
• RR = round
Rr = round
rr = wrinkled
• If a heteroyzous round seed is crossed with itself
(Rr x Rr) a punnett square can help you figure
out the ratios of the offspring.
Set up your square
Remember, it’s Rr x Rr
• Note that the letters get separated on the top and the
side. It DOES NOT MATTER which parent goes on top
or on the side.
Results
So,The Phenotypic Ratio is 3:1, Round to Wrinkled
The Genotypic Ratio is 1:2:1, and refers to the letters. It is
1 RR, 2 Rr, 1 rr.
Monohybrid cross: be able to
Predict Genotypes and Phenotypes
Try this: what are the genotypic and phenotypic
ratios of offspring from a cross between two
heterozygous brown-haired people?
(Brown is dominant to blond)
Now try some more from the worksheets provided.
Independent Assortment in Budgie
Birds
Geneticists use the testcross to
determine unknown genotypes
• testing a
suspected
heterozygote by
crossing it with a
known
homozygous
recessive.
Dihybrid
Crosses:
Crosses that
involve 2
traits.
For these
crosses your
punnett square
needs to be 4x4
(Note the
9:3:3:1 ratio)
Non-single Gene Genetics
Incomplete dominance:
-neither pair of alleles are completely
expressed when both are present.
-Typically, a third phenotype is produced, which
is a blend of the traits
Ex: snapdragons, roses, carnations (pink flowers)
Codominance: Two alleles are expressed in a
heterozygote condition.
Ex: Human Blood types
Incomplete dominance results in
intermediate phenotypes
• When an offspring’s
phenotype—such
as flower color— is
in between the
phenotypes of its
parents, it exhibits
incomplete
dominance
P GENERATION
White
rr
Red
RR
Gametes
R
r
Pink
Rr
F1 GENERATION
1/
1/
Eggs
1/
F2 GENERATION
2
2
2
R
1/
2
r
1/
R
R
Red
RR
r
Pink
Rr
Sperm
1/
Pink
rR
White
rr
Figure 9.12A
2
2
r
Many genes have more than two alleles
in the population
• In a population, multiple alleles often exist for a
characteristic
• This is called Codominance- When there are multiple
alleles, but both express themselves equally in phenotypic
expression.
Ex- White + Chestnut horse= Roan (white and red hairs
mixed together).
+
Codominance-Also Observed in
Blood Types- p. 140 (Allott)
• Both A and B
are dominant.
• Type O is
recessive
• Four phenotypes
• Six genotypes
• Blood types are caused by the presence
of a protein cell-surface marker. If an
antigen on the surface of the RBC plasma
membrane is mixed with the wrong blood
type, antigens are bound by antibodies=
clumping.
4 Types of Blood
• Type A with A antigens on the red cells and anti
B antibodies in the plasma.
• Type B with B antigens on the red cells and anti
A antibodies in the plasma.
• Type AB with both A and B antigens on the red
cells and no blood type antibodies in the plasma.
• Type O with no antigens on the red cells and
both anti A and anti B antibodies in the plasma
• ** Group O blood cannot be clumped by any human
blood, and therefore people with Group O are called
universal donors.
Blood Donor Chart
What is the + and - ?
• The Rh blood group (named for the rhesus
monkey in which it was discovered) is
made up of those Rh positive (Rh+)
individuals who can make the Rh antigen
and those Rh negative (Rh-) who cannot.
Rh factor, cont.
• Hemolytic disease of the newborn (HDN) results
from Rh incompatibility between an Rh- mother
and Rh+ fetus.
• Rh+ blood from the fetus enters the mother's
system during birth, causing her to produce Rh
antibodies. The first child is usually not affected,
however subsequent Rh+ fetuses will cause a
massive secondary reaction of the maternal
immune system. To prevent HDN, Rh- mothers
are given an Rh antibody during the first
pregnancy with an Rh+ fetus and all subsequent
Rh+ fetuses.
Blood Type Frequencies of different
Ethnic Groups
Non-single Gene Genetics
Pleiotropy: genes with multiple phenotypic effect.
Ex: sickle-cell anemia
combs in roosters
coat color in rabbits
Epistasis: a gene at one locus (chromosomal
location) affects the phenotypic expression of a
gene at a second locus.
Ex: mice coat color & Labrador coat color
Polygenic Inheritance: an additive effect of two or
more genes on a single phenotypic character
Ex: human skin pigmentation and height
A single gene may affect many
phenotypic characteristics
• A single gene may affect phenotype in
many ways
– This is called pleiotropy
– The allele for sickle-cell disease is an example
Pleiotropy – Sickle Cell anemia
Effects of Sickle Cell Anemia
Explain that polygenic inheritance can contribute
to continuous variation using two examples.
1) Human skin color- is thought to be
controlled by at least 3 independent
genes.
AABBCC x aabbcc
F1 = AaBbCc , then perform a dihybird
cross (AaBbCc), and there are many
possible outcomes, such as:
AABBCc, AABBcc, AABbcc,
AAbbcc, etc.
2) Human hair color- is also thought to
be controlled but multiple genes,
accounting for the large variety in
shade.
Polygenic Inheritance
P GENERATION
aabbcc
AABBCC
(very light) (very dark)
F1 GENERATION
Eggs
Sperm
Fraction of population
AaBbCc AaBbCc
Skin pigmentation
F2 GENERATION
Figure 9.16
Epistasis
• Epistasis: a gene at one locus (chromosomal location)
affects the phenotypic expression of a gene at a second
locus. Ex: mice and Labrador coat color
Epistasis
• Examples: Labrador’s coat color
Albino Koala
•
Two Genes Involved:
Allele Symbol
-Pigment- Black (Dominant)
B
b
E/e
Chocolate (recessive)
-Expression or deposition of the Pigment
Black
Yellow
BBEE
BbEE
BBEe
BbEe
BBee
Bbee
Chocolate
bbEE
bbEe
Which genotype is missing and what group should it be listed under?
Epistasis
Statistical Tools to Analyze results
• Chi-Square: Will tell you how much your
data is different from expected (calculated)
results. It is Non-Parametric and deals
with different categories.
Formula:
 2 = ∑ (o – e)2
e
2: what we are solving:
o: observed value
e: expected (calculated value)
Sample Problem using Chi square
• Two hybrid Tall plants are crossed. If the F2
generation produced 787 tall plants and 277
short plants. Does this confirm Mendel’s
explanation?
• What is the expected value?
This is your null hypothesis (HO)
• Total number of plants: 1064
• 3:1 Phenotypic ratio
• Expected value should be: 798 tall and 266 short
(75%)
(25%)
Calculation of Chi Square Value
2 =  (O – E)2
E
2 = (787 – 798)2
(277 – 266)2 = 0.61
798
266
There are two categories and therefore the degrees of
freedom would be 2-1 = 1 .
+
• Look up the critical value for 1 degree of freedom:
3.84 (next slide-always given)- next slide.
• 0.61 is less than 3.84 therefore we cannot reject the null
hypothesis. We must accept the null hypothesis (3:1 ratio)
as accurate.
Solving Question #3
Formula:
x2 = ∑ (O – E)2
E
Accepting or Rejecting your
hypothesis?
• p<0.05 is accepted as being significant
• Accepting the Null (H0) means that there is NO
SIGNIFICANT difference between the observed
and expected value (p<0.05). Chance alone can
explain the differences observed.
• Rejecting the Null (H0) means that the
observations are significantly different from
the expectations. (p>0.05). Evaluate the results.
Human Genome & Genetic
Disorders
Chapter 15
Information Gained by the Genome
Project (2003)
• Entire DNA (nucleus) composed of about
2.9 billion base pairs of nucleotides
• Six to Ten anonymous individuals were
used
• Estimated number of genes = under 30,000
• Only 1% to 2% of human DNA codes for a
protein or RNA
• On Chromosome 22: 545 genes have been
identified.
Genetic traits in humans can be tracked
through family pedigrees
• The inheritance of many
human traits follows
Mendel’s principles and
the rules of probability
Figure 9.8A
• Family pedigrees are used to determine
patterns of inheritance and individual
genotypes
Dd
Joshua
Lambert
Dd
Abigail
Linnell
D_?
Abigail
Lambert
D_?
John
Eddy
dd
Jonathan
Lambert
Dd
Dd
dd
D_?
Hepzibah
Daggett
Dd
Elizabeth
Eddy
Dd
Dd
Dd
dd
Female Male
Deaf
Figure 9.8B
Hearing
• A high incidence of hemophilia has plagued
the royal families of Europe
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.23B
Pedigree of Alkaptonuria
Table 9.9
SEX CHROMOSOMES AND
SEX-LINKED GENES
• A human male has one X chromosome and
one Y chromosome
• A human female has two X chromosomes
• Whether a sperm cell has an X or Y
chromosome determines the sex of the
offspring
Human sex-linkage
•
•
•
•
•
SRY gene: gene on Y chromosome that triggers the development of
testes
Fathers= pass X-linked alleles to all daughters only (but not to sons)
Mothers= pass X-linked alleles to both sons & daughters
Sex-Linked Disorders: Color-blindness; Duchenne muscular
dystropy (MD); hemophilia
Sex-linked disorders affect mostly
males
• Most sex-linked human
disorders are due to
recessive alleles
– Examples: hemophilia,
red-green color blindness
– These are mostly seen in males
Figure 9.23A
– A male receives a single X-linked allele from his
mother, and will have the disorder, while a
female has to receive the allele from both
parents to be affected
Sex Linked Trait: Colorblindness
Methods of Detecting Genetic
Disorders
•
•
•
•
•
•
Amniocentesis
Ultrasound
CVS (Chorionic Villus Sampling)
PGD (Preimplantation Genetic Diagnosis)
Fetuscopy
Genetic Couseling/Screening
Amniocentesis -Pg 281
• Karyotyping and biochemical tests of fetal
cells and molecules can help people make
reproductive decisions
– Fetal cells can be obtained through
amniocentesis
Amniotic
fluid
Amniotic
fluid
withdrawn
Centrifugation
Fluid
Fetal
cells
Fetus
(14-20
weeks)
Biochemical
tests
Placenta
Figure 9.10A
Uterus
Cervix
Several
weeks later
Cell culture
Karyotyping
Diagnostic Procedures to detect
Genetic Disorders in Babies
• Chorionic Villus Sampling (CVS) is another
procedure that obtains fetal cells for
karyotyping.
Pg.
Fetus
(10-12
weeks)
Several hours
later
Placenta
Suction
Chorionic villi
Fetal cells
(from chorionic villi)
Karyotyping
Some
biochemical
tests
Figure 9.10B
UltraSound
(Pg. )
• Examination of the fetus with ultrasound is
another helpful technique
Figure 9.10C, D
PGD: Preimplantion Genetic
Diagnosis
• Used for Couples who are carriers of an
abnormal allele.
• IVF Procedure is used
• Eggs are fertilized, grown in culture and
tested for the disorder
• Normal embryos are implanted into the
uterus.
Genetic testing can detect diseasecausing alleles
• Genetic testing can be of
value to those at risk of
developing a genetic
disorder or of passing it on
to offspring
• Dr. David Satcher, former U.S.
surgeon general, pioneered
screening for sickle-cell
disease
Figure 9.15B
Figure 9.15A
Table of Disorders
Name
Chromosome
Cellular effect
Overall
involvement or (#)
Phenotypic Result
_______________________________________________________________________________
Down Syndrome
Auto (47)
Many
Kleinfelter’s Syndrome
Sex (47)
Turner’s Syndrome
Sex (45)
Cri du Chat
Auto/Deletion #5
Fragile X
Auto & Sex
Phenylketonuria (PKU)
Auto rec.
Enzyme def.
Alkaptonuria
Auto rec.
Enzyme def.
Sickle Cell Anemia
Auto rec.
Hemoglobin Struct.
Cystic Fibrosis
Auto rec.
Tay Sachs
Auto rec.
Huntington’s Disorder
Auto Dom.
Achondroplasia
Auto Dom.
Albinism
Auto rec.
Color Blindness
Sex-linked
Muscular Dystrophy
Sex-linked
Hemophlia
Sex-linked
Alzheimer’s
Auto Dom.
Hypercholesterolemia
Auto Dom.
• A few are caused by Dominant alleles
– Examples: Achondroplasia, Huntington’s
disease
Figure 9.9B
Human Disorders
The Family Pedigree
Recessive disorders:
-Cystic fibrosis
-Tay-Sachs
-Sickle-cell
Dominant Disorders:
-Huntington’s
-Poydactaly
Diagnosing/Testing:
-Amniocentesis
-Chorionic villus
sampling (CVS)
Chapter 15:
The Chromosomal Theory of
Inheritance
•
•
•
•
•
•
Gene linkage (Drosophila)
Wild-types & mutants
Gene mapping
Non-Disjunction (anueploidy)
Barr bodies (inactive X)
Alterations of Chromosome
structure
• Genomic imprinting
Pgs. 274-291
Genes on the same chromosome tend to
be inherited together
• Certain genes are linked
– They tend to be inherited together because they
reside close together on the same chromosome
How to Determine if Two Genes
are linked.
Perform a Two Point Test Cross:
Parents: AaBb X aabb
Possible gametes: AB, Ab, aB, ab X ab
Following Mendelian principles of independent
assortment (not linked on the same chromosome)
then:
ab
AB
Ab
aB
ab
AaBb
Aabb
aaBb
aabb
(25%)
(25%)
(25%)
(25%)
If Genes are Linked
• More Parental types should be present in the
offspring and fewer recombinants.
Parental type
ab
recombinant
recombinant Parental type
AB
Ab
aB
ab
AaBb
Aabb
aaBb
aabb
(more)
40%
(less)
10%
(less)
10%
(more)
40%
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
– Fruit fly eye
color is a
sex-linked
characteristic
Figure 9.22A
Chromosomal Linkage
• Thomas Morgan
• Drosophilia melanogaster
• XX (female) vs. XY (male)
• Sex-linkage: genes located
on a sex chromosome
• Linked genes: genes located
on the same chromosome
that tend to be inherited
together
– Their inheritance pattern reflects the fact that
males have one X chromosome and females
have two
– These figures illustrate inheritance patterns for
white eye color (r) in the fruit fly, an X-linked
recessive trait
Female
XRXR
Male
Xr Y
XR
Female
XRXr
Xr
XRXr
Male
XRY
XRY
Xr
XRXR
XrXR
XRY
XrY
R = red-eye allele
r = white-eye allele
Male
XRXr
XR
XR
Y
Female
XrY
Xr
XR
Y
Xr
XRXr
Xr Xr
Y
XRY
XrY
Figure 9.22B-D
Figure 9.18
Generating Recombinant Offspring
Generating Recombinants in
Drosophila
Figure 9.19C
Crossing Over Developing
Genetic Maps
Pgs. 294-296
Crossing over produces new
combinations of alleles
• This produces gametes with recombinant
chromosomes
• The fruit fly Drosophila melanogaster was
used in the first experiments to demonstrate
the effects of crossing over
Genetic Recombination
•
•
•
Crossing over
Genes that DO NOT assort
independently of each other
Genetic maps
The further apart 2 genes
are, the higher the probability that
a crossover will occur between
them and therefore the higher the
recombination frequency
Linkage maps
Genetic map based on
recombination frequencies
Geneticists use crossover data to map
genes
• Crossing over is more likely to occur
between genes that are farther apart
– Recombination frequencies can be used to map
the relative positions of genes on chromosomes
Chromosome
g
c
l
17%
9%
9.5%
Figure 9.20B
A
B
a
b
a
B
A B
a
b
Tetrad
A
b
Crossing over
Gametes
Figure 9.19A, B
• A partial genetic map of a fruit fly
chromosome
Mutant phenotypes
Short
aristae
Black
body
(g)
Long aristae
(appendages
on head)
Gray
body
(G)
Cinnabar
eyes
(c)
Red
eyes
(C)
Vestigial
wings
(l)
Brown
eyes
Normal
wings
(L)
Red
eyes
Wild-type phenotypes
Figure 9.20C
Genetic Map of Drosophila
• Alfred H. Sturtevant, seen here at a party
with T. H. Morgan and his students, used
recombination data from Morgan’s fruit fly
crosses to map genes
Figure 9.20A
Sex-Linked Patterns of
Inheritance and Non-Disjunction
Sex-Linked Patterns of Inheritance
(male)
(female)
Parents’
diploid
cells
X
Y
Male
Sperm
Egg
Offspring
(diploid)
Figure 9.21A
• Other systems of sex determination exist in
other animals and plants
– The X-O system
– The Z-W system
– Chromosome number
Figure 9.21B-D
Connection: Abnormal numbers of sex
chromosomes do not usually affect
survival
• Nondisjunction can also produce gametes
with extra or missing sex chromosomes
– Unusual numbers of sex chromosomes upset
the genetic balance less than an unusual
number of autosomes
Accidents During Meiosis Can Alter
Chromosome Number
• Abnormal
chromosome count
is a result of
nondisjunction
– Either
homologous
pairs fail to
separate
during
meiosis I
Nondisjunction
in meiosis I
Normal
meiosis II
Gametes
n+1
n+1
n–1
n–1
Number of chromosomes
Figure 8.21A
Chromosomal Errors
Nondisjunction: members of
a pair of homologous
chromosomes do not
separate properly during
meiosis I or sister
chromatids fail to separate
during meiosis II
Aneuploidy: chromosome
number is abnormal
• Monosomy~ missing
chromosome
• Trisomy~ extra chromosome
(Down syndrome)
• Polyploidy~ extra sets of
chromosomes
• Fertilization after Non-disjunction in the
mother results in a zygote with an extra
chromosome
Egg
cell
n+1
Zygote
2n + 1
Sperm
cell
n (normal)
Figure 8.21C
ALTERATIONS OF CHROMOSOME NUMBER AND
STRUCTURE
• To study human chromosomes
microscopically, researchers stain and
display them as a karyotype
– A karyotype usually shows 22 pairs of
autosomes and one pair of sex chromosomes
• Preparation of a Karyotype
Blood
culture
Packed red
And white
blood cells
Hypotonic solution
Stain
White
Blood
cells
Centrifuge
3
2
1
Fixative
Fluid
Centromere
Sister
chromatids
Pair of homologous
chromosomes
4
5
Figure 8.19
An extra copy of chromosome 21 causes
Down syndrome
• This karyotype shows three number 21
chromosomes
• An extra copy of chromosome 21 causes
Down syndrome
Figure 8.20A, B
• Chromosomal changes in a somatic cell can
cause cancer
– A chromosomal translocation in the bone
marrow is associated with chronic myelogenous
leukemia
Chromosome 9
Chromosome 22
Reciprocal
translocation
“Philadelphia chromosome”
Activated cancer-causing gene
Figure 8.23C
• A man with Klinefelter syndrome has an
extra X chromosome
Poor beard
growth
Breast
development
Underdeveloped
testes
Figure 8.22A
• A woman with Turner syndrome lacks an X
chromosome
Characteristic
facial
features
Web of
skin
Constriction
of aorta
Poor
breast
development
Underdeveloped
ovaries
Figure 8.22B
• The chance of having a Down syndrome
child goes up with maternal age
Figure 8.20C
Table 8.22
Barr Bodies
• Inactive X Chromosome
Pg. 284
• Predominant in females
• Dark Region of chromatin is visible at the edge of
the nucleus within a cell during interphase.
(Please see Figure 15.11)
• A small fraction of the genes located on this X
chromosome usually are expressed.
• Inactivation is a random event among the somatic
cells.
• Heterozygous individuals: ½ cells alleles expressed
• Ex. Calico cat & Tortoise shell (Variegation)
Calico Kitten w/Barr Bodies
Example of Variegation
Barr Bodies
Connection: Alterations of chromosome
structure can cause birth defects and
cancer
• Chromosome breakage can lead to
rearrangements that can produce genetic
disorders or cancer
– Four types of rearrangement are:
deletion, duplication, inversion, and translocation
Chromosomal Errors
•
•
•
•
•
Alterations of chromosomal structure:
Pg. 327
Deletion: removal of a chromosomal segment
Duplication: repeats a chromosomal segment
Inversion: segment reversal in a chromosome
Translocation: movement of a chromosomal segment to
another
Example of a Chromosomal
Deletion
• Cri Du Chat: “Cat cry” syndrome
– Effects chromosome #5
– Altered facial Features “moon face”
– Severe mental retardation
Outbreeding vs. Inbreeding
• Inbreeding
-Increases homozygosity in the population.
-Increases frequency of genetic disorders
-Amplifies the homozygous phenotypes
• Outbreeding:
-Leads to better adapted offspring
-Heterozygous advantage & Hybrid Vigor become
evident and buffers out undesirable traits
Genomic Imprinting
• Def: a parental effect
on gene expression
• Identical alleles may
have different effects
on offspring, depending
on whether they arrive
in the zygote via the
ovum or via the sperm
Fragile X Syndrome
• More common in Males
• Common form of
Mental Retardation
• Thinned region on tips
of chromatids
• Triplicate “CGG” repeats over
200 to 1000 times
• Normal: repeat 50 X or less
• Commonly seen in Cancer cells
• Varies in severity:
Pg. 327-328
Mild learning disabilities ADD  Mental retardaton
Fragile X Syndrome