Mendelian Genetics and the Inheritance of Genetic Traits
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Transcript of Mendelian Genetics and the Inheritance of Genetic Traits
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Mendelian Genetics and the Inheritance of Genetic Traits
IB Topic 4.3- Theoretical Genetics
Campbell: Ch. 14
Allott: Ch. 12
Gregor Mendel:
Father of Modern Genetics
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Theoretical Genetics Defined:
• Theoretical Genetics- concerned with the probabilities associated with producing offspring of a particular genotype or phenotype.
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• Modern theoretical genetics began with Gregor Mendel’s quantitative experiments with pea plants
Experimental Genetics Began in an abbey Garden
Figure 9.2A, B
Stamen
Carpel
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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
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• 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
MENDEL’S PRINCIPLES
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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
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• Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation
Figure 9.2C
• This illustration shows his technique for cross-fertilization
1 Removed stamensfrom purple flower
White
Stamens
Carpel
PurplePARENTS(P)
OFF-SPRING
(F1)
2 Transferred pollen from stamens of white flower to carpel of purple flower
3 Pollinated carpel matured into pod
4 Planted seeds from pod
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• Mendel studied seven pea characteristics
Figure 9.2D
• He hypothesized that there are alternative forms of genes (although he did not use that term), the units that determine heredity
FLOWER COLOR
FLOWER POSITION
SEED COLOR
SEED SHAPE
POD SHAPE
POD COLOR
STEM LENGTH
Purple White
Axial Terminal
Yellow Green
Round Wrinkled
Inflated Constricted
Green Yellow
Tall Dwarf
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Mendel’s Experiment
1. He set up true-breeding plants (bred for many generations) by allowing them to self-fertilize.
• He controlled pollination, looking at 1 or 2 characteristics at a time.
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2. He crossed a true breeding plant with a plant of the opposite trait (purple x white). He called this the Parental (P1) generation.
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3. He recorded data on the offspring of this cross, calling it the First Filial, or F1 Generation.
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4. He self pollinated the F1 offspring5. He recorded data on the offspring of the second generation, calling it the Second Filial generation (F2)
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Mendel’s Results
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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)
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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
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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
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• Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes
Homologous chromosomes bear the two alleles for each characteristic
GENE LOCI
Figure 9.4
P a B
DOMINANTallele
RECESSIVEallele
P a b
GENOTYPE: PP aa Bb
HOMOZYGOUSfor thedominant allele
HOMOZYGOUSfor therecessive allele
HETEROZYGOUS
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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
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More Definitions
• Homozygous- having two identical alleles of a gene. Ex: BB or bb
• Heterozygous- having two different alleles of a gene. Ex: Bb
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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
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How to Solve a Punnett Square
1. Determine the genotypes (letters) of the parents. Bb x Bb2. Set up the punnett square with one parent on each side.3. Fill out the punnett square middle4. Analyze the number of offspring of each type.
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An Example
• In pea plants, round seeds are dominant to wrinkled. The genotypes and phenotypes are:
• RR = roundRr = roundrr = 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.
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Set up your squareRemember, 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.
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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.
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Monohybrid cross: be able to Predict Genotypes and PhenotypesTry 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.
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Independent Assortment in Budgie Birds
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Geneticists use the testcross to determine unknown genotypes
• testing a suspected heterozygote by crossing it with a known homozygous recessive.
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Dihybrid Crosses:
Crosses that involve 2
traits.For these
crosses your punnett square needs to be 4x4
(Note the 9:3:3:1 ratio)
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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
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• When an offspring’s phenotype—such as flower color— is in between the phenotypes of its parents, it exhibits incomplete dominance
Incomplete dominance results in intermediate phenotypes
P GENERATION
F1 GENERATION
F2 GENERATION
RedRR
Gametes R r
Whiterr
PinkRr
R r
R R
r r
1/21/2
1/2
1/21/2
1/2 SpermEggs
PinkRr
PinkrR
Whiterr
RedRR
Figure 9.12A
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• 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).
Many genes have more than two alleles in the population
+
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Codominance-Also Observed in Blood Types- p. 140 (Allott)
• Both A and B are dominant.
• Type O is recessive
• Four phenotypes
• Six genotypes
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• 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.
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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.
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Blood Donor Chart
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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.
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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.
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Blood Type Frequencies of different Ethnic Groups
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Non-single Gene GeneticsPleiotropy: 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
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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
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Pleiotropy – Sickle Cell anemia
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Effects of Sickle Cell Anemia
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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.
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Polygenic Inheritance
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Figure 9.16
P GENERATION
F1 GENERATION
F2 GENERATION
aabbcc(very light)
AABBCC(very dark)
AaBbCc AaBbCc
Eggs Sperm
Fra
cti
on
of
po
pu
lati
on
Skin pigmentation
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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
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Epistasis• Examples: Labrador’s coat color
Albino Koala
• Two Genes Involved: Allele Symbol
-Pigment- Black (Dominant) B Chocolate (recessive) b
-Expression or deposition of the Pigment E/e
Black Yellow ChocolateBBEE BBee bbEE
BbEE Bbee bbEe BBEe BbEe
Which genotype is missing and what group should it be listed under?
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Epistasis
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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)
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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%)
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Calculation of Chi Square Value 2 = (O – E)2
E2 = (787 – 798)2 + (277 – 266)2 = 0.61
798 266There 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.
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Solving Question #3
Formula: x2 = ∑ (O – E)2
E
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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.
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Human Genome & Genetic Disorders
Chapter 15
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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.
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• The inheritance of many human traits follows Mendel’s principles and the rules of probability
Genetic traits in humans can be tracked through family pedigrees
Figure 9.8A
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• Family pedigrees are used to determine patterns of inheritance and individual genotypes
Figure 9.8B
DdJoshuaLambert
DdAbigailLinnell
D_Abigail
Lambert
Female
DdElizabeth
Eddy
D_JohnEddy
? D_HepzibahDaggett
?
?
ddDdDdDdddDdDd
MaleDeaf
Hearing
ddJonathanLambert
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• A high incidence of hemophilia has plagued the royal families of Europe
Figure 9.23B
QueenVictoria
Albert
Alice Louis
Alexandra CzarNicholas IIof Russia
Alexis
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Pedigree of Alkaptonuria
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Table 9.9
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• 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
SEX CHROMOSOMES AND SEX-LINKED GENES
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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•
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• Most sex-linked human disorders are due to recessive alleles– Examples: hemophilia,
red-green color blindness
– These are mostly seen in males
– 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 disorders affect mostly males
Figure 9.23A
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Sex Linked Trait: Colorblindness
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Methods of Detecting Genetic Disorders
• Amniocentesis
• Ultrasound
• CVS (Chorionic Villus Sampling)
• PGD (Preimplantation Genetic Diagnosis)
• Fetuscopy
• Genetic Couseling/Screening
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• Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions– Fetal cells can be obtained through
amniocentesis
Amniocentesis -Pg 281
Figure 9.10A
Amnioticfluid
Fetus(14-20weeks)
Placenta
Amnioticfluidwithdrawn
Centrifugation
Fetalcells
Fluid
Uterus Cervix Cell culture
Severalweeks later Karyotyping
Biochemicaltests
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Diagnostic Procedures to detect Genetic Disorders in Babies
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• Chorionic Villus Sampling (CVS) is another procedure that obtains fetal cells for karyotyping. Pg.
Figure 9.10B
Fetus(10-12weeks)
Placenta
Chorionic villi
Suction
Several hourslater
Fetal cells(from chorionic villi)
Karyotyping
Some biochemical
tests
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UltraSound (Pg. )
• Examination of the fetus with ultrasound is another helpful technique
Figure 9.10C, D
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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.
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• Genetic testing can be of value to those at risk of developing a genetic disorder or of passing it on to offspring
Genetic testing can detect disease-causing alleles
Figure 9.15B
Figure 9.15A
• Dr. David Satcher, former U.S. surgeon general, pioneered screening for sickle-cell disease
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Table of Disorders Name Chromosome Cellular effect Overall involvement or (#) Phenotypic Result _______________________________________________________________________________Down Syndrome Auto (47) ManyKleinfelter’s Syndrome Sex (47)Turner’s Syndrome Sex (45) Cri du Chat Auto/Deletion #5Fragile X Auto & SexPhenylketonuria (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-linkedMuscular Dystrophy Sex-linkedHemophlia Sex-linkedAlzheimer’s Auto Dom.Hypercholesterolemia Auto Dom.
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• A few are caused by Dominant alleles
Figure 9.9B
– Examples: Achondroplasia, Huntington’s disease
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Human Disorders
The Family PedigreeThe Family PedigreeRecessive disorders:
-Cystic fibrosis-Tay-Sachs-Sickle-cell
Dominant Disorders:-Huntington’s-Poydactaly
Diagnosing/Testing:-Amniocentesis-Chorionic villus sampling (CVS)
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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
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• Certain genes are linked– They tend to be inherited together because they
reside close together on the same chromosome
Genes on the same chromosome tend to be inherited together
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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 (25%)
Aabb(25%)
aaBb(25%)
aabb(25%)
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If Genes are Linked
• More Parental types should be present in the offspring and fewer recombinants.
Parental type recombinant recombinant Parental type
AB Ab aB ab
ab AaBb(more)
40%
Aabb(less)
10%
aaBb(less)
10%
aabb(more)
40%
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• 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
Sex-linked genes exhibit a unique pattern of inheritance
Figure 9.22A
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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
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– Their inheritance pattern reflects the fact that males have one X chromosome and females have two
Figure 9.22B-D
– These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait
Female Male Female Male Female Male
XrYXRXR
XRXr
XRY
XR Xr
Y
XRXr
XR
Xr XRXR
XR
Y
XRY
XrXR XRY
XrY
XRXr
XR
Xr
Xr
YXRXr
XrXr XRY
XrY
XrY
R = red-eye alleler = white-eye allele
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Figure 9.18
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Generating Recombinant Offspring
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Generating Recombinants in Drosophila
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Figure 9.19C
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Crossing Over Developing Genetic Maps
Pgs. 294-296
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• This produces gametes with recombinant chromosomes
• The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over
Crossing over produces new combinations of alleles
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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 mapsGenetic map based on
recombination frequencies
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• 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
Geneticists use crossover data to map genes
g
Figure 9.20B
Chromosome
c l
17%
9% 9.5%
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A B
a b
Tetrad Crossing over
A B
a
ba
BA b
Gametes
Figure 9.19A, B
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• A partial genetic map of a fruit fly chromosome
Figure 9.20C
Shortaristae
Blackbody(g)
Cinnabareyes(c)
Vestigialwings(l)
Browneyes
Long aristae(appendageson head)
Graybody(G)
Redeyes(C)
Normalwings(L)
Redeyes
Mutant phenotypes
Wild-type phenotypes
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Genetic Map of Drosophila
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• 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
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Sex-Linked Patterns of Inheritance and Non-Disjunction
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Figure 9.21A
X Y
Male
(male)
Parents’diploidcells
(female)
Sperm
Offspring(diploid)
Egg
Sex-Linked Patterns of Inheritance
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• Other systems of sex determination exist in other animals and plants
Figure 9.21B-D
– The X-O system
– The Z-W system
– Chromosome number
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• 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
Connection: Abnormal numbers of sex chromosomes do not usually affect
survival
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• Abnormal chromosome count is a result of nondisjunction– Either
homologous pairs fail to separate during meiosis I
Accidents During Meiosis Can Alter Chromosome Number
Figure 8.21A
Nondisjunctionin meiosis I
Normalmeiosis II
Gametes
n + 1 n + 1 n – 1 n – 1
Number of chromosomes
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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
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• Fertilization after Non-disjunction in the mother results in a zygote with an extra chromosome
Figure 8.21C
Eggcell
Spermcell
n + 1
n (normal)
Zygote2n + 1
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• 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
ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE
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• Preparation of a Karyotype
Figure 8.19
Blood culture
1
Centrifuge
Packed redAnd white blood cells
Fluid
2
Hypotonic solution
3
Fixative
WhiteBloodcells
Stain
4 5
Centromere
Sisterchromatids
Pair of homologouschromosomes
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• This karyotype shows three number 21 chromosomes
• An extra copy of chromosome 21 causes Down syndrome
An extra copy of chromosome 21 causes Down syndrome
Figure 8.20A, B
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• Chromosomal changes in a somatic cell can cause cancer
Figure 8.23C
Chromosome 9
– A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia
Chromosome 22Reciprocaltranslocation
“Philadelphia chromosome”
Activated cancer-causing gene
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• A man with Klinefelter syndrome has an extra X chromosome
Figure 8.22A
Poor beardgrowth
Under-developedtestes
Breastdevelopment
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• A woman with Turner syndrome lacks an X chromosome
Figure 8.22B
Characteristicfacialfeatures
Web ofskin
Constrictionof aorta
Poorbreastdevelopment
Under-developedovaries
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• The chance of having a Down syndrome child goes up with maternal age
Figure 8.20C
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Table 8.22
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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)
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Calico Kitten w/Barr BodiesExample of Variegation
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Barr Bodies
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• Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer– Four types of rearrangement are:
deletion, duplication, inversion, and translocation
Connection: Alterations of chromosome structure can cause birth defects and
cancer
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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
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Example of a Chromosomal Deletion
• Cri Du Chat: “Cat cry” syndrome– Effects chromosome #5– Altered facial Features “moon face”– Severe mental retardation
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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
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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
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Fragile X Syndrome• More common in Males Pg. 327-328• 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:
Mild learning disabilities ADD Mental retardaton
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Fragile X Syndrome