mendelian genetics and the inheritance of genetic traits

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

Theoretical Genetics Defined:

• Theoretical Genetics- concerned with the probabilities associated with producing offspring of a particular genotype or phenotype.

• 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

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

• 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

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

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

• 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

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.

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 offspring5. 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

• 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

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 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.

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.

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.

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 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.

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

• 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

• 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

+

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 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

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

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

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 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?

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

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.

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.

• 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

• 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

• A high incidence of hemophilia has plagued the royal families of Europe

Figure 9.23B

QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis

Pedigree of Alkaptonuria

Table 9.9

• 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

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•

• 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

Sex Linked Trait: Colorblindness

Methods of Detecting Genetic Disorders

• Amniocentesis

• Ultrasound

• CVS (Chorionic Villus Sampling)

• PGD (Preimplantation Genetic Diagnosis)

• Fetuscopy

• Genetic Couseling/Screening

• 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

Diagnostic Procedures to detect Genetic Disorders in Babies

• 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

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 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

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.

• A few are caused by Dominant alleles

Figure 9.9B

– Examples: Achondroplasia, Huntington’s disease

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)

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

• 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

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%)

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%

• 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

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

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

Figure 9.18

Generating Recombinant Offspring

Generating Recombinants in Drosophila

Figure 9.19C

Crossing Over Developing Genetic Maps

Pgs. 294-296

• 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

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

• 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%

A B

a b

Tetrad Crossing over

A B

a

ba

BA b

Gametes

Figure 9.19A, B

• 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

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

Figure 9.21A

X Y

Male

(male)

Parents’diploidcells

(female)

Sperm

Offspring(diploid)

Egg

Sex-Linked Patterns of Inheritance

• 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

• 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

• 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

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

Figure 8.21C

Eggcell

Spermcell

n + 1

n (normal)

Zygote2n + 1

• 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

• 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

• 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

• 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

• A man with Klinefelter syndrome has an extra X chromosome

Figure 8.22A

Poor beardgrowth

Under-developedtestes

Breastdevelopment

• A woman with Turner syndrome lacks an X chromosome

Figure 8.22B

Characteristicfacialfeatures

Web ofskin

Constrictionof aorta

Poorbreastdevelopment

Under-developedovaries

• 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 BodiesExample of Variegation

Barr Bodies

• 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

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 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

Fragile X Syndrome