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CHAPTER 11 & 12
Mendel & the Gene
Idea & the
Chromosomal Basis
of Inheritance
Genetics
• study of science of heredity
• began w/the use of wild type traits –
traits most commonly found in nature
• desirable traits were then bred
Con. 11.1
Mendel’s Methods
• used true breeding plant – made by self-
fertilization
• created hybrids by cross-fertilization
(crossing 2 different true breeding plants)
- P generation is parent generation
- F1 generation (1st filial) is offspring of P
generation
- F2 generation (2nd filial) is offspring made
by F1 x F1
Mendel’s Principles
(Principle of
Segregation)
1. alternative forms for genes called
alleles (DNA sequence variations)
2. an organism has 2 genes (alleles):
1 inherited from each parent
- sperm & egg each carry only 1 allele
for each inherited characteristic
3. when the alleles of the pair
are different, 1 is fully
expressed, the other is masked
- dominant allele is expressed
- recessive allele is masked
4. Law of Segregation states that the allele
pairs separate during gamete formation
(meiosis) & restored during fertilization
Punnett Square – diagram used to predict
results of a genetic cross
Homozygous – identical alleles for a trait
ex: G = green GG g = yellow gg
Heterozygous – 2 different alleles for a trait
Gg
Phenotype – the expressed trait (physical
appearance green or yellow)
Genotype – organism’s genetic makeup
(GG, Gg, gg)
Mendel’s Principles
(Independent Assortment)
• Monohybrid Cross –
parents differ only in a
single trait
Pod Color G = green
g = yellow
Genotype: 50% Gg &
50% gg
Phenotype: 50% green &
50% yellow
•Dihybrid Cross – parents differ in 2
different traits
- it follows the law of independent
assortment
- each allele pair separates
independently during gamete formation
P generation: RRYY x rryy
Pea Shape
R=round
r=wrinkled
Pea Color
Y=yellow
y=green
Gametes
RY x ry
RrYy
F 1:
RrYy x RrYy
Gametes:
F2
Genotype
Phenotype
•Testcross – a breeding of the recessive
homozygote w/an organism of
unknown genotype
Practice a testcross
Con. 11.3
Complications of Genotypes
to Phenotypes
Incomplete Dominance –
when 1 allele is not dominant over the
other (snapdragon)
Multiple Alleles – some genes exist in
more than 2 allele forms: blood types
- A, B, AB, O (phenotypes)
- A & B are codominant
Incomplete Dominance
RR=red
WW=white
RW=pink
RR x WW
F2 RW x RW
G
P
Pleiotropy – when a gene has multiple
effects
- affects phenotypic characteristics
Ex: sickle-cell anemia (single recessive
allele on both homologues) causes
formation of abnormal hemoglobin
which in turn causes: breakdown of red
blood cells, clumping of cells & clogging
of small blood vessels, accumulation of
sickle cells in spleen
NOTE: each of these causes additional
effects on an individual
-individuals who are heterozygous are
called carriers because they “carry” the
disease-causing allele & may transmit it
to their offspring
Epistasis (p. 217) – one gene dictates
the phenotypic expression of another
Polygenic Inheritance – an additive of 2
or more genes on a single phenotypic
characteristic (skin color controlled by
at least 3 genes) p. 218
Phenotypic range – norm of reaction
(diet vs. genetics): multifactional
Quantitative Characters – gradations
along a continuum
Chromosomal Theory of Inheritance
• Mendelian genes
are located on
chromosomes
• Chromosomes
undergo
segregation &
independent
assortment
Ch. 12
Linked Genes
• Discovered in 1908 by William
Bateson & Reginald Punnett
• Found on same chromosome
• The principle of independent
assortment does not apply because
the genes are part of a single
chromosome…p. 236
Chromosomal Basis of
Recombination
• Genetic Recombination – production
of offspring that combine the traits of
2 parents
• In unlinked genes independent
assortment will take place
- parental types – offspring w/same
phenotype as one or the other of the
parents
Con. 12.2
- Recombinants – offspring having
different combinations than either
parent
Linked Genes – independent
assortment does not take place
- crossing over can occur so new
combinations are passed on
- recombination does occur
Mapping Chromosomes
Cross Over Data
Relative Distance
Between Genes
• Use recombination • Determined by
crossover
data to assign a
frequency
position to genes
•
The
greater
the
• A map unit is equal
distance between
to 1%
genes, the greater
recombination
the chance for
frequency
crossing over to
occur
Chromosomal Basis of Sex
Determination
• Humans & other mammals have XX & XY
• Most insects have XX (female) & XO
(male)
• Birds, fish, butterflies, moths have a ZW
system: ZW (female and determines sex)
& ZZ (male)
• Most bees & ants are haplo-diploid:
female from fertilized eggs (diploid), male
from unfertilized eggs (haploid) –
parthenogenesis – virgin birth
NOTE: not all organisms have
separate sexes
-plants are monoecious (one
house), ex: corn
- animals are hermaphroditic –
all individuals of a species have
the same compliment of
chromosomes ex: earthworms,
garden snails
Morgan: Sex Linkage
• Worked w/fruit flies –
Drosophlia
- found that the gene for eye
color is on the X chromosome:
R = red r = white
- mated white eyed male w/red
eyed female (wild)
* all F1 have red eyes, then
mated F1 x F1
p. 230
XRXR x XrY
Xr
Y
G
P
G
P
XR
XR
F2
XR
Xr
XR
Y
The human genome project is:
a.The main character in Travelocity
commercials
b.Yard art
c. Aimed at sequencing all the DNA
on the human chromosomes
Genome
• One complete haploid set of chromosomes
of an organism
• in humans, 23 chromosomes
w/approximately 3 billion nucleotide pairs
of DNA that carry between 50,000 &
100,000 genes
•If genome’s chromosomes were uncoiled
and laid end to end, they would make a
very thin thread that would be
approximately 3 meters long
Karyotype
• A photographic overview of a person’s
genome
• cells from a person are fixed in
metaphase, stained, & photographed to
display all of a cell’s chromosomes
• Individual chromosomes are cut out,
paired w/their homologue, & arranged from
largest to smallest pairs for the 22
autosomes w/the sex chromosomes placed
last
• the karyotype
is used to
screen for
abnormal
numbers of
chromosomes
or defective
chromosomes
p. 240
Major Chromosomal
Alterations & Their Effects
Chromosome Numbers
• nondisjunction - when chromosomes
fail to separate during Meiosis I and II
• can cause aneuploidy - abnormal
chromosome numbers:
* monosomy (1 less chromosome)
* trisomy (1 extra chromosome)
Human Disorders
(nondisjunction/aneuploidy)
1. Down Syndrome
- trisomy on
chromosome #21
*occurs in 1 of
every 700 births
*rounded facial
features, varying
degrees of mental
delays p. 242
2. Patau Syndrome - trisomy on
chromosome #13
*occurs in 1 of
every 5000 births
*causes cleft palate,
harelip, brain
defects
#13
3. Edwards Syndrome - trisomy on
chromosome #18
*occurs in 1 of every 10,000 births
*affects almost every organ system
4. Klinefelter Syndrome - trisomy in male
(XXY)
*occurs in 1 of every 2000 births
*has male sex organs but are sterile
5. Metafemale - trisomy in female (XXX)
*occurs in 1 of every 1000 births
*limited fertility but otherwise appear
normal
6. Turner Syndrome - monosomy in
female (XO)
*occurs in 1 of every 5000 births
*no mature sex organs, sterile
Chromosome Structure
• Breakage of a chromosome can cause a
variety of rearrangements
• fragments are usually lost when a cell
divides in 1 of 4 ways:
p. 241
1-DELETION = a fragment of the
chromosome breaks off and is lost
(only dealing with one homologue)
For example, in
this picture gene
3 has broken off
and been lost.
becomes
(Where did gene 3
run off to?)
2-DUPLICATION = chromosome fragment
attaches to a homologue
now one
homologue has 2 sets of (same) info.
and the other is missing info.
(Old Homologue 2)
OLD
{12}345678
(Homologue 1 is left 12345678
without genes 1 & 2.
Homologue 2 ends NEW
up with both copies 345678
of genes #1 & 2.)
12{12}345678
(New Homologue 2)
3-INVERSION = chromosome breaks off
and reattaches in reverse order (only
dealing with one homologue)
{234}
Becomes
{432}
4-TRANSLOCATION = a fragment
breaks off and attaches to a nonhomologue (Example – chromosome
1 has a piece break off
Chromo.#1
and attach to
chromosome number
2 which is a
Chromo. #2
non-homologue)
(What will the new
chromosome #1
look like?)
345678
New #2
Example of deletion:
Williams Syndrome – deletion
of about 15 genes on 1 of the
homologous chromosomes in
chromosome #7
*occurs in 1 of every 20,000
births
*mild retardation, problems in
grasping spatial relationships;
possess extraordinary musical
talent
*thought to be elves/pixies in
medieval folklore
Inherited Disorders Due to Gene
p. 220-222
Mutations
Human Pedigree - a pedigree shows
the occurrence of a trait, seen in a
family tree type of style
Recessively Inherited Disorders carrier - a heterozygote (Xx) that is
phenotypically normal but transmits the
recessive allele to the offspring
1. Deafness - severely or totally deaf
Dd = carrier (normal)
DD = normal
dd = deaf
2. Cystic Fibrosis - excessive mucus
secretions clog airways of lungs &
passages of the liver and pancreas
3. Albinism - lack of (skin)
pigmentation
4. Tay-Sachs - an incurable disorder in
which the brain deteriorates due to
lipid build-up (p. 216)
5. Sickle Cell Anemia - red blood cells
are defective so they don’t
transport O2 tissues
properly (caused by
point mutation)
Dominantly Inherited Disorders
1. Dwarfism (Achondroplasia)homozygous dominant results
in spontaneous abortion
2. Alzheimer’s Disease-causes
mental deterioration (normally
no obvious effect until late in
life and effects are irreversible
and lethal)
3. Huntington’s Disease - degenerative
disorder of the brain cells
*no obvious effect until after age 30
*effects are irreversible and lethal
Why are Alzheimer’s and
Huntington’s becoming
so common?
Sex Linked Traits - fathers pass X linked
traits on to all of their daughters and
mothers can pass sex linked traits on to
both sons and daughters
Examples:
Hemophilia - blood disorder passed
from generation to generation
p. 232-233
Color Blindness - inability to see certain
colors due to malfunctioning lightsensitive cells in the eyes
Duchenne Muscular Dystrophy progressive weakening and loss of
muscle tissue
X inactivation in female mammals due
to DNA methylation
p. 233
Risk Assessment and Therapy for
Genetic Disorders
• Fetal Testing
Amniocentesis - needle obtains small
sample of amniotic fluid
*culture cells are taken from sloughed off
cell floating in amniotic fluid
p. 223
*done around 14-16
weeks
of pregnancy
*karyotype performed
*results in several weeks
(risk to pregnancy - 1%)
Chorionic Villus Sampling (CVS) - small
tube suctions off a small amount of
tissue from the villi of the embryonic
membrane (this tissue forms part of the
placenta)
*cells are rapidly undergoing mitosis
*done around 8-10 weeks of pregnancy
*perform a karyotype
*results in 1 day (risk to pregnancy 2%)
Ultrasound Imaging - high frequency
sound waves (sonar beyond the range
of hearing)
*produces a colorenhanced image of
fetus - age 18 weeks on
*results are immediate (noninvasive
and no known risk)
*used during amniocentesis and CVS
to determine position of fetus and
needle or tube
Fetoscopy - needle thin tube w/viewing
lens & light source
*produces direct view of fetus
*results are immediate (risk to
pregnancy - 10%)
- risks to pregnancy can be
complications that can result in
maternal bleeding, miscarriage, or
premature birth
• Carrier Recognition Counseling
Problem: parents are concerned they
are carrier of a recessive genetic
disorder; they do not wish to pass the
disorder onto their prospective children
Solution: physicians and genetic
counselors now have a growing list of
relatively simple biochemical tests that
can check a couple’s genotype for
genetic disorders
• Identification of Defective Genes and
Gene Therapy
- work by Dr. Nancy Wexler on Huntington’s
Disease as well as ongoing research making
progress in locating defective genes
- her work in Venezuela produced a
pedigree linking almost 10,000 people
- this allowed her to find a genetic
marker (a DNA strand signaling the
presence of a specific allele) and a test
to identify for HD in 1983
- she located the HD allele in 1993 and
identified the allele’s operation
- set up gene therapy
Problems w/gene therapy:
Technical - new gene must work at the
right time and throughout life, and
gene therapy works only with cells that
currently multiply (nerve cells do not)
Ethical - who will have access to it,
treat only serious diseases, enhance
athletic ability/physical appearance, and
treatment of germ cells (makes
gametes)
www.biology.ewu.edu/.
../ GeneTherapyTargeted.jpg
Human Genome Project
• Purpose: map all 3 billion nucleotides
(international, multi-billion, multidecade long successful effort)
• Potential: insight & understanding
into embryonic development &
evolution, aid in diagnosis, treatment,
prevention of many diseases
Yeast & Fly Genomes
• Reproduces by
budding and doubles
every 90 minutes
• sequenced in 1996
• 12 million base pairs
of DNA
• 6000 genes, at least
31% have human
equivalents
• Lifespan 2-3 months,
new generation every
10 days
• sequenced in March
2000
• 165 million base pairs
of DNA
• 13,600 genes, 50%
have human
equivalents
Mouse & Human Genome
• Lifespan 2 years, new
generation every 9
weeks
• sequenced in 2001
• 3 billion base pairs of
DNA
• 40,000 genes
• equivalents to human
and some blocks
proved impossible to
tell apart from human
• Lifespan in U.S.
70-80 years, new
generation every
20-25 years
• preliminary draft
in June 2000
• Close to final
draft in 2004
• 3 billion base
pairs of DNA
• 50,000 genes
Ethical Issues
• Who has access to your
genome?
• How far, if at all, do we go to
re-engineer someone?
• How do we prevent genetic
discrimination in the workplace,
insurance companies, social
settings, etc.?