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Around 1857, Mendel began Around 1857, Mendel began breeding garden peas to study breeding garden peas to study inheritanceinheritance. Because they are . Because they are

available in many varieties with available in many varieties with distinct heritable distinct heritable characterscharacters with with

different traits (different traits (genesgenes))..

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• Each pea plant has male (Each pea plant has male (stamensstamens) ) and female (and female (carpalcarpal) sexual organs.) sexual organs.

• In nature, pea plants typically In nature, pea plants typically self-fertilize, fertilizing ova with their self-fertilize, fertilizing ova with their own pollens.own pollens.

• However, Mendel could also move However, Mendel could also move pollens from one plant to another to pollens from one plant to another to cross-pollinate plants.cross-pollinate plants.

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PPppPPpp

P p P p

PPPPPPPP PPppPPpp PPppPPpp pppp

PPPP pp

100% 100% PurplePurple

F2 generation

3 3 PurplePurple : 1 White : 1 White

F1 generation

Recessive Recessive alleleallele

Dominant Dominant alleleallele

PPPPPPPP ppppXX PPppPPpp PPppPPppXXPea plantPea plant

HomozygousHomozygous HeterozygousHeterozygous

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GenotypeGenotype(Genetic make up)

PPPP

PhenotypePhenotype(Colour)(Colour)

PPPPPPPP ppppppppHomozygousHomozygous

PPppPPpp HeterozygousHeterozygous

An organism having a pair of An organism having a pair of identical allelesidentical alleles

An organism having a pair of two An organism having a pair of two different allelesdifferent alleles

PhenotypePhenotype: : Is t Is the organism’s appearance he organism’s appearance ..المظهرالمظهرGenotypeGenotype: : Is t Is the organism’s genetic makeup he organism’s genetic makeup

الـچينى الـچينى الطرز ..الطرز

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The two pairs of alleles segregate The two pairs of alleles segregate independentlyindependently of of each othereach other..

The presence of one specific allele for one trait has no The presence of one specific allele for one trait has no impactimpact تأثيرتأثير on the presence of a specific allele for the on the presence of a specific allele for the

second traitsecond trait..

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• When sperm and ova each with When sperm and ova each with four classes of alleles four classes of alleles combine, there would be 16 combine, there would be 16 equally probable ways in which equally probable ways in which the alleles can combine the alleles can combine in the in the FF22 generation.generation.

• These combinations produce These combinations produce four distinct phenotypes in a four distinct phenotypes in a 9:3:3:19:3:3:1 ratio. ratio.

• This was consistent with This was consistent with Mendel’s results.Mendel’s results.

• Each character appeared to be Each character appeared to be inherited independently.inherited independently.

Fig. 14.7b, Page 253

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It is a mating between two parent plants differing in two characters.

Y

R

y

r

Y R y r

RRYYyy rr

FF11 Yellow Round Yellow Round

YYYY RRRR rrrryyyyXX

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RYy r XX RYy r

yyrr

yyRR

YYrr

YYRRYRYR YYrr yyRR yr

YRYR

YYrr

yyRR

yryr

Yellow Round

Yellow Wrinkled

Green Round

Green Wrinkled

F2: % of Phenotype ?Page 253, Fig. 14.7

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It is not possible to predict the genotype of It is not possible to predict the genotype of an organism with a dominant phenotypean organism with a dominant phenotype..

The organism must have one The organism must have one dominantdominant allele, but it allele, but it could be could be homozygoushomozygous dominant or dominant or heterozygousheterozygous..

Test crossTest cross , , is is breeding a breeding a

homozygous recessive homozygous recessive with with dominant phenotypedominant phenotype, ,

but but unknown genotypeunknown genotype,,can determine the identity can determine the identity

of the unknown alleleof the unknown allele . .

31Fig. 14.6

Q:Q: What is the result of What is the result of Cross hybridization of Cross hybridization of purplepurple XX whitewhite colored colored flowers ? flowers ?

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Dominant character (allele)Dominant character (allele) السائدة الصفة

Is fully expressed in the organism’s appearanceIs fully expressed in the organism’s appearance..Recessive characterRecessive character (allele)(allele) الُم�تنحية الصفة

Has no noticeable effectHas no noticeable effect ملحوظ غير ملحوظ تأثير غير on the organism’s on the organism’s تأثيرappearanceappearance..

HomozygousHomozygous الجينات تماثل Cالجينات ُم تماثل Cُم

An organism with two An organism with two identicalidentical alleles for a character alleles for a character..HeterozygousHeterozygous الجينات الجينات ُمCختلف ُمCختلف

An organism with two An organism with two differentdifferent alleles for a character alleles for a character..KaryotypeKaryotype الكروُموسوُمى الكروُموسوُمى الطرز الطرز

The display of an organism’s chromosomal patternThe display of an organism’s chromosomal pattern PhenotypePhenotype المظهرى المظهرى الطرز الطرز

A description of an organism’s traitsA description of an organism’s traits( ( featurefeature ُمظهرُمظهر))..GenotypeGenotype الجينى الجينى الطرز الطرز

A description of an organism’s genetic makeupA description of an organism’s genetic makeup..41

Thousands of genetic disorders, including disabling or deadly Thousands of genetic disorders, including disabling or deadly hereditary diseases, are inherited as simple hereditary diseases, are inherited as simple recessiverecessive traits traits..

These range from the relatively mildThese range from the relatively mild( ( albinismalbinism هاق� الُب to to( ( األلُبينو،life-threatening (cystic fibrosis)life-threatening (cystic fibrosis)..

Heterozygotes have a normal phenotypeHeterozygotes have a normal phenotype because one because one “normal” allele produces enough of the required factors (“normal” allele produces enough of the required factors (for

normal trait))..A recessively inherited disorder shows up only in the A recessively inherited disorder shows up only in the

individuals who inherit individuals who inherit homozygous recessive allelehomozygous recessive allele from parentsfrom parents..

Thus, individuals who lack the disorder are either Thus, individuals who lack the disorder are either homozgyous dominanthomozgyous dominant or or heterozygousheterozygous..

Heterozygous member may have no clear phenotypic effects, Heterozygous member may have no clear phenotypic effects, but is a but is a carriercarrier who may transmit a recessive allele to their who may transmit a recessive allele to their

offspringoffspring..Most people with recessive disorders are born from carrier Most people with recessive disorders are born from carrier

parents with normal phenotypesparents with normal phenotypes..Two carriers have a 1/4 chance of having a child with the Two carriers have a 1/4 chance of having a child with the

disorder, 1/2 chance of a carrier, and 1/4 freedisorder, 1/2 chance of a carrier, and 1/4 free..42

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.1.1Cystic fibrosis:Cystic fibrosis: a lethal recessive disordera lethal recessive disorderOne in 25 person is a carrierOne in 25 person is a carrier..The normal allele codes for a membrane protein that The normal allele codes for a membrane protein that

transports transports ClCl-- between cells and the environment between cells and the environment..If these channels are absent, there are abnormally high If these channels are absent, there are abnormally high

extracellular levels of chlorideextracellular levels of chloride that causes the mucus that causes the mucus coats of certain cells to become thicker and stickier coats of certain cells to become thicker and stickier

than normalthan normal..This mucus build-up in the pancreas, lungs, digestive This mucus build-up in the pancreas, lungs, digestive

tract, and elsewhere favors bacterial infectionstract, and elsewhere favors bacterial infections..Without treatment, affected children die before five, Without treatment, affected children die before five,

but with treatment can live past their late 20’sbut with treatment can live past their late 20’s . .

2.2. Tay-Sachs disease Tay-Sachs disease a lethal recessive disorder.a lethal recessive disorder.

– It is caused by a dysfunctional enzyme that fails to break down It is caused by a dysfunctional enzyme that fails to break down specific brain lipids.specific brain lipids.

– The symptoms begin with seizures, blindness, and degeneration of The symptoms begin with seizures, blindness, and degeneration of motor and mental performance a few months after birth. motor and mental performance a few months after birth.

– Inevitably, the child dies after a few years.Inevitably, the child dies after a few years.

A- Recessively inherited disordersA- Recessively inherited disorders

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.3.3Sickle-cell diseaseSickle-cell disease..It is caused by the substitution of a single amino It is caused by the substitution of a single amino

acid in hemoglobinacid in hemoglobin..When oxygen levels in When oxygen levels in

the blood of an affected the blood of an affected individual are low, individual are low,

sickle-cell sickle-cell hemoglobin hemoglobin crystallizes into long crystallizes into long

rods rods..This deforms red blood This deforms red blood

cells into a sickle shape cells into a sickle shape..Doctors can use regular Doctors can use regular

blood transfusions to blood transfusions to prevent brain damage prevent brain damage and new drugs to and new drugs to

prevent or treat prevent or treat other other

problemsproblems..• The two alleles are The two alleles are codominantcodominant as both normal and abnormal as both normal and abnormal hemoglobins are synthesized.hemoglobins are synthesized.

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Many other disorders have a multifactorial basisMany other disorders have a multifactorial basis..

These have a genetic component plus a significant These have a genetic component plus a significant environmental influenceenvironmental influence..

Multifactorial disorders includeMultifactorial disorders include::

heart disease, diabetes, cancer, alcoholism, and certain heart disease, diabetes, cancer, alcoholism, and certain mental illnesses, such a schizophrenia and manic-depressive mental illnesses, such a schizophrenia and manic-depressive

disorderdisorder..

The genetic component is typically The genetic component is typically polygenicpolygenic..

At present, little is understood about the genetic At present, little is understood about the genetic contribution to most multifactorial diseasescontribution to most multifactorial diseases

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Section B: Sex ChromosomesSection B: Sex ChromosomesSection B: Sex ChromosomesSection B: Sex Chromosomes

CHAPTER 15 CHAPTER 15 THE CHROMOSOMAL BASIS THE CHROMOSOMAL BASIS

OF INHERITANCEOF INHERITANCE

1.1. The chromosomal basis of sex varies with the organismThe chromosomal basis of sex varies with the organism

2.2. Sex-linked genes have unique patterns of inheritanceSex-linked genes have unique patterns of inheritance

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In human and other mammals, In human and other mammals, there are two varieties of sex there are two varieties of sex

chromosomes, chromosomes, XX & & YY..An individual who inherits An individual who inherits two Xtwo X

chromosomes usually develops as a chromosomes usually develops as a femalefemale..

An individual who inherits an An individual who inherits an X and X and a Ya Y chromosome usually develops as chromosome usually develops as

a a malemale..• This X-Y system of mammals is not the This X-Y system of mammals is not the

only chromosomal mechanism of only chromosomal mechanism of determining sex.determining sex.

• Other options include the Other options include the X-0X-0 ( (in locustin locust) ) system, the system, the Z-W Z-W system (system (in birdsin birds), and the ), and the haplo-diploid system (haplo-diploid system (in beesin bees).).

• In Human, the In Human, the SRYSRY gene ( gene (SSex-determining ex-determining RRegion of the egion of the YY chromosome) modifies chromosome) modifies embryonic gonads into testes.embryonic gonads into testes.

• Females lack the Females lack the SRYSRY gene, thus, the gene, thus, the embryonic gonads develop into ovaries.embryonic gonads develop into ovaries.

Fig. 15.8, Page 276

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In the X-Y system, Y and X chromosomes In the X-Y system, Y and X chromosomes behave as homologous chromosomes during behave as homologous chromosomes during

meiosismeiosis..In reality, they are only partially homologous and In reality, they are only partially homologous and

rarely undergo crossing overrarely undergo crossing over

In both testes (XY) and ovaries (XX), the two In both testes (XY) and ovaries (XX), the two sex chromosomes segregate during meiosis sex chromosomes segregate during meiosis

and each gamete receives oneand each gamete receives one..Each egg receives an X chromosomeEach egg receives an X chromosome..Half the sperm receive an X chromosome and half Half the sperm receive an X chromosome and half

receive a Y chromosomereceive a Y chromosome..

Because of this, each conception has about a Because of this, each conception has about a fifty-fifty chance of producing a particular fifty-fifty chance of producing a particular

sexsex..

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The sex chromosomes, The sex chromosomes, especially the especially the XX

chromosome, have genes for chromosome, have genes for many characters unrelated to many characters unrelated to

sexsex..These These sex-linkedsex-linked genes follow genes follow

the same pattern of inheritance the same pattern of inheritance as the as the white-eye white-eye locus in locus in

DrosophilaDrosophila..If a sex-linked trait is due to a If a sex-linked trait is due to a

recessiverecessive allele, a female have allele, a female have this phenotype only if this phenotype only if

homozygoushomozygous..Heterozygous females will be Heterozygous females will be

carrierscarriers..

Fig. 15.9, Page 277

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• Because males have only one Because males have only one XX chromosome chromosome ((hemizygoushemizygous), any male receiving the recessive allele ), any male receiving the recessive allele from his mother will express the trait.from his mother will express the trait.

• Therefore, males are far more likely to inherit sex-Therefore, males are far more likely to inherit sex-linked recessive disorders than are females. linked recessive disorders than are females.

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Sex-linked disordersSex-linked disorders in humanin human..

1.1. Duchenne muscular dystrophy:Duchenne muscular dystrophy: affects one in 3,500 males affects one in 3,500 males born in the United States.born in the United States.

– Affected individuals rarely live past their early 20s.Affected individuals rarely live past their early 20s.

– This disorder is due to the absence of an X-linked gene for a key This disorder is due to the absence of an X-linked gene for a key

muscle protein, called muscle protein, called dystrophindystrophin. .

– The disease is characterized by a weakening The disease is characterized by a weakening ضعفضعف of the muscles of the muscles and loss of coordination and loss of coordination التوازن التوازن فـقْـد ..فـقْـد

2.2. Hemophilia:Hemophilia: is a sex-linked recessive trait defined by the is a sex-linked recessive trait defined by the absence of one or more absence of one or more clotting factorsclotting factors تجلط ..عوامل

– These proteins normally slow and then stop bleeding.These proteins normally slow and then stop bleeding.

– Individuals with hemophilia have prolonged bleeding because a firm Individuals with hemophilia have prolonged bleeding because a firm clot clot تجلط forms slowly.forms slowly.

– Individuals can be treated with intravenous injections of the missing Individuals can be treated with intravenous injections of the missing protein.protein.

– This gene is transmitted to offspring This gene is transmitted to offspring viavia the mothers. the mothers.

– Thus, Sons borne from hemophilic woman should be exempted from Thus, Sons borne from hemophilic woman should be exempted from circumcision.circumcision.

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.3.3Color blindnessColor blindness:: األلوان is a is a عُمىdisorder inherited as a disorder inherited as a

recessive sex-linked recessive sex-linked charactercharacter and affect both and affect both

males and femalesmales and females..A color blind female (A color blind female (XXaaXXaa) may ) may

be born to a color blind father be born to a color blind father ((XXaaYY) and a carrier mother () and a carrier mother (XXAAXXaa))

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• It is common It is common شائع in meiosis and includes:in meiosis and includes:a)a) Chromosomal deletions/translocationsChromosomal deletions/translocations

Homologous chromatids may Homologous chromatids may break break تَـنكسر andand rejoin rejoin at incorrect at incorrect places, thus, one chromatid will loose more genes than it receives.places, thus, one chromatid will loose more genes than it receives.

b)b) Chromosomal duplicationsChromosomal duplications (Polyploidy). (Polyploidy). result from nondisjunction during gamete production in one result from nondisjunction during gamete production in one

parent.parent.

• A diploid embryo that is homozygous for a large deletion A diploid embryo that is homozygous for a large deletion or male with a large deletion to its single or male with a large deletion to its single XX chromosome chromosome is usually missingis usually missing many essential genes and this leads many essential genes and this leads to a lethal outcome.to a lethal outcome.

– Duplications and translocations are very harmful.Duplications and translocations are very harmful.

• Translocation or inversion can alter phenotype because Translocation or inversion can alter phenotype because a gene’s expression is influenced by its location.a gene’s expression is influenced by its location.

Chromosomal aberrationChromosomal aberration..

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A)-A)- AneuploidyAneuploidy ( (Chromosomal duplication)Chromosomal duplication)

11 - -Down syndrome Down syndrome [[Polyploidy (2n + 1), trisomy in autosomes]:: Is due to Is due to

three copies of chromosome 21 (three copies of chromosome 21 (TrisomiesTrisomies)).. Although chromosome 21 is the smallest human Although chromosome 21 is the smallest human

chromosome, it severely alterschromosome, it severely alters غير� an individual’s an individual’s ُيphenotype in specific waysphenotype in specific ways..

Fig. 15.14

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Most cases of Down syndrome result from Most cases of Down syndrome result from nondisjunction during gamete production in nondisjunction during gamete production in

one parentone parent..The frequency of Down syndrome correlates with the The frequency of Down syndrome correlates with the

age of the motherage of the mother..This may be linked to some age-dependent abnormality in This may be linked to some age-dependent abnormality in

the spindle checkpoint during the spindle checkpoint during meiosis meiosis II, leading to , leading to nondisjunctionnondisjunction..

2- Klinefelter’s syndrome2- Klinefelter’s syndrome [Polyploidy (2n + 1), trisomy in sex chromosomes ],, (a)- An (a)- An XXYXXY male male, occurs , occurs once in every 2000 live births.once in every 2000 live births.

• These individuals have male sex organs, but are sterile.These individuals have male sex organs, but are sterile.• There may be feminine characteristics There may be feminine characteristics أنثوُية صفات but their intelligence is , but their intelligence is ,له

normal.normal.

(b)- An (b)- An XYYXYY male, male, tend to somewhat taller than average. tend to somewhat taller than average.

(c)- A trisomy female ((c)- A trisomy female (XXXXXX),), which occurs once in every 2000 live which occurs once in every 2000 live births, produces healthy females.births, produces healthy females.

3- Turner’s syndrome,3- Turner’s syndrome, a monosomy female a monosomy female ((X0X0)),, or, which occurs or, which occurs once in every 5000 births, produces phenotypic, but immature once in every 5000 births, produces phenotypic, but immature females females جنسيا ناضجة جنسيا غير ناضجة . .غير

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B)- ChromosomalB)- Chromosomal structure-alterationsstructure-alterations::

It can also cause human disordersIt can also cause human disorders..Deletions, even in a heterozygous state, cause severe Deletions, even in a heterozygous state, cause severe

physical and mental problemsphysical and mental problems..

.1.1Cri-du-chatCri-du-chat القط مواء , , عارضresults from a specific deletion in chromosome 5results from a specific deletion in chromosome 5..

These individuals are mentally retarded, have a small head These individuals are mentally retarded, have a small head with unusual facial features, and a cry like the mewing of a with unusual facial features, and a cry like the mewing of a

distressed catdistressed cat..This syndrome is fatal in infancy or early childhoodThis syndrome is fatal in infancy or early childhood..

2.2. Myelogenous,Myelogenous, [leukemia (CML)].[leukemia (CML)]. Caused by chromosomal translocations since a fragment of Caused by chromosomal translocations since a fragment of chromosome 22 switches places with a small fragment from the chromosome 22 switches places with a small fragment from the tip of chromosome 9.tip of chromosome 9.

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www.youtube.com/watch?v=prbAy_2gKRBiology - Laws of Heredity - Genetics

Plz open the video

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www.youtube.com/watch?v=-mRphwIVEcMGregor Mendel working with Punnett Squares.

Plz open the video

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Plz open the video

www.youtube.com/watch?v=-2YPAt8hOmEMendelian Genetics

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www.youtube.com/w

Mendel and Genetics atch?v=rU3CDi08-Hk

► 

3:34

► 

3:34

Plz open the video

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www.youtube.com/watch?v=ZnoxLqnb_QgMendel study genes and inherited traits using peas

Plz open the video

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www.youtube.com/watch?v=ioMhw85wI18

Mendel's Law of Heredity

Plz open the video

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Q1: Write whether each of the following statements is True (T) or False (F): 1-( ) Sickle-cell disease is a dominantly inherited disorder.2-( ) The 3 : 1 phenotypic ratio is characteristic of the F1 generation of a monohybrid cross.3-( )A mating between parents differing in two characters is called;Monohybrid 4-( )The separation of alleles into separate gametes is known as Mendel’s law of independent assortment .5-( ) An organism’s expressed traits is known as genotype .

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Q 2: Choose the correct answer (one answer only): 1-Which of the following is a genetic disorder that caused by genes and environmental factors (multi-factorial disorder):

a)- colour blindness b)- Tay-Sachs disease c)- cancer

d)- cystic fibrosis2-Which of the following is a dominant genetic disease?

a)- sickle-cell anaemia. b)- cystic fibrosis c)- Tay Sachs disease d)- none of the above

3-Huntington's disease is an example of a genetic disorder caused by: a)- lethal dominant allele b)- non-lethal dominant

allele c)- recessive allele d)- multiple alleles

4- An organism with two identical alleles for a character (trait) is:a)- homozygous b)- heterozygousc)- semizygous d)- multizygous

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5-The self-cross of dihybrid (RrYy) will result in which of the following phenotypic ratio:

a)- 1: 1: 1: 1 b)- 9: 3: 3: 1 c)- 1: 2: 2: 1 d)- 3: 3: 3: 1

6-Mendel's law of segregation means that the two members of an allelic pair are: a)- distributed to separate gametes. b)- distributed to the same gamete.

c)- assorted dependently. d)- segregated pairwise.7-Which of the following is represented by the appearance (such as colour)

a)- phenotype b)- genotype c)- kariotype d)- none of them

8-Different alleles are ---------------------

-Alternative version of genes (different molecular forms of a gene).ab-Different phenotypes.c-Self-fertilizing,true breeding homozygotes.d-Organism’s genotype.

9-Crosses between F1 individuals resulting from the cross AABB X aabb lead to F2 phenotypic ratios close to-------------

a-1:2:1 b-1:1:1:1c-3:1 d-9:3:3:1

10-A heterozygote has a---------------for the trait being studied.a-Pair of identical alleles.b-Pair of nonidentical alleles.c-Haploid condition,in genetic terms.

d- a and c .