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

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

• In human and other mammals, there are In human and other mammals, there are two varieties of sex chromosomes, two varieties of sex 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 a YX and a Y

chromosome usually develops as a chromosome usually develops as a malemale..

1. The chromosomal basis of sex 1. The chromosomal basis of sex varies with the organismvaries with the organism

• 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.

• In the X-Y system, Y and X chromosomes behave as In the X-Y system, Y and X chromosomes behave as homologous chromosomes during meiosis.homologous chromosomes during meiosis.– In reality, they are only partially homologous and rarely In reality, they are only partially homologous and rarely

undergo crossing overundergo crossing over

• In both testes (XY) and ovaries (XX), the two sex In both testes (XY) and ovaries (XX), the two sex chromosomes segregate during meiosis and each chromosomes segregate during meiosis and each gamete receives one.gamete receives one.– Each egg receives an X chromosome.Each egg receives an X chromosome.– Half the sperm receive an X chromosome and half receive a Y Half the sperm receive an X chromosome and half receive a Y

chromosome.chromosome.

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

1. The chromosomal basis of sex 1. The chromosomal basis of sex varies with the organismvaries with the organism

• The sex chromosomes, especially the The sex chromosomes, especially the XX chromosome, have genes for many chromosome, have genes for many characters unrelated to sex.characters unrelated to sex.

• These These sex-linkedsex-linked genes genes مرتبطة جينات follow the same pattern of follow the same pattern of بالجنسinheritance as the inheritance as the white-eye white-eye locus locus in 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 this phenotype allele, a female have this phenotype only if only if homozygoushomozygous..– Heterozygous females will be carriers.Heterozygous females will be carriers.

2. Sex-linked genes have unique 2. Sex-linked genes have unique patterns of inheritancepatterns of inheritance

• 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 linked recessive disorders مرض than are females. than are females.

تورث بالجنس المرتبطة تورث الصفات بالجنس المرتبطة الصفاتألنهم فقط األم من الذكور ألنهم لألبناء فقط األم من الذكور لألبناء

الكروموسوم منها الكروموسوم يرثون منها الحامل الحامل XXيرثونإن حيث الـچـينات، من النوع إن لهذا حيث الـچـينات، من النوع ال ال YYلهذا

شىء شىء يحمل يحمل

Sex-linked genesSex-linked genes

Sex-linked disordersSex-linked disorders المرتبطة المرتبطة األمراض األمراض

..in humanin humanبالجنسبالجنس1.1. Duchenne muscular dystrophy:Duchenne muscular dystrophy: العضالت َهَو ن

affects one in 3,500 males born in the United States.affects one in 3,500 males 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: الدمو يالنزيف is a sex-linked recessive trait defined is a sex-linked recessive trait defined by the absence of one or more by the 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 Individuals with hemophilia have prolonged bleeding مستمر نزيفbecause a firm clot because a firm 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 Thus, Sons borne from hemophilic woman should be exempted 0ستثنى ..الختان from circumcision from circumcision ي

3.3. Color blindness:Color blindness: األلوان is is عمىa disorder inherited as a a disorder inherited as a recessive recessive sex-linked charactersex-linked character and affect and affect both males and females.both males and females.

– A color blind female (A color blind female (XXaaXXaa) may be ) may be

born to a color blind father (born to a color blind father (XXaaYY) and ) and

a carrier mother (a carrier mother (XXAAXXaa))

• It is common It is common شائع in meiosis and includes:in meiosis and includes:a)a) Chromosomal deletions/translocationsChromosomal deletions/translocations جزء فقد أو من نقص

الكروموسوم Homologous chromatids may Homologous chromatids may break break تَـنكسر andand rejoin rejoin مرة تلتحم و

at incorrect places, thus, one chromatid will loose more at incorrect places, thus, one chromatid will loose more أخرىgenes than it receives.genes than it receives.

b)b) Chromosomal duplicationsChromosomal duplications تضاع0ف (Polyploidy (Polyploidy الكروموسومات .(.(تعددa)a) result from nondisjunction result from nondisjunction أثناء للكروموسومات الكامل اإلنفصال عدم

الميوز ياإلنقسام during gamete production in one parent.during gamete production in one 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 missing is usually missing يفتقد many essential genes and this many essential genes and this leads to a lethal leads to a lethal يتIم0م outcome.outcome.

– Duplications and translocations Duplications and translocations المقطوعة األجزاء أماكن are very are very تغييرharmful.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 aberration Chromosomal aberration الشذوذ

يالكروموسوم ..

Chapter 12Chromosomal MutationsChromosomal Mutations

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

الكروموسوم الكروموسوم التعدد التام ييالتعدد التام غير غير ,,

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

Is due to three copies of chromosome 21 (Is due to three copies of chromosome 21 (TrisomiesTrisomies كروموسومية مجموعات

..((ثالثية

Although chromosome 21 is the smallest human chromosome, it Although chromosome 21 is the smallest human chromosome, it

severely alters severely alters 0غير .an individual’s phenotype in specific ways.an individual’s phenotype in specific ways ي

Symptoms of Down SyndromeSymptoms of Down Syndrome

• Upward slant to eyes.Upward slant to eyes.

• Small ears that fold over at the top.Small ears that fold over at the top.

• Small, flattened nose.Small, flattened nose.

• Small mouth, making tongue appear large.Small mouth, making tongue appear large.

• Short neck.Short neck.

• Small hands with short fingers.Small hands with short fingers.

• In addition, down syndrome always involves some In addition, down syndrome always involves some degree of mental retardation, from mild to severe. In degree of mental retardation, from mild to severe. In most cases, the mental retardation is mild to moderate.most cases, the mental retardation is mild to moderate.

2- Klinefelter’s syndrome2- Klinefelter’s syndrome [Polyploidy (2n + 1), trisomytrisomy in sex chromosomes ],,• Disorder occurring due to nondisjunction of the X chromosome.Disorder occurring due to nondisjunction of the X chromosome.• The Sperm containing both X and Y combines with an egg containing the X, results The Sperm containing both X and Y combines with an egg containing the X, results

in a male child. in a male child. • The egg may contribute the extra X chromosome.The egg may contribute the extra X chromosome.

(a)- An (a)- An XXYXXY male male, occurs once in every 2000 live births., occurs 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 أنثوية صفات .but their intelligence is normal., but their intelligence is normal ,له

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

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

• Males with some development of breast tissue normally seen in females.

• Little body hair is present, and such person are typically tall, have small testes.

• Infertility results from absent sperm.

• Evidence of mental retardation may or may not be present.

• Turner syndrome is associated with underdeveloped ovaries, short stature.

• Bull neck, and broad chest. Individuals are sterile, and lack expected secondary sexual characteristics.

• Mental retardation typically not evident.

• Chromosomal or monogenic?

3- Turner’s syndrome,3- Turner’s syndrome, a monosomy female a monosomy female ((X0X0)),,

Occurs once in every 5000 births, produces phenotypic, but Occurs once in every 5000 births, produces phenotypic, but immature females immature females جنسيا ناضجة جنسيا غير ناضجة ..غير

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

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

physical and mental problems.physical and mental problems.

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

-- -- Is a rare genetic disorder due to a missing part of Is a rare genetic disorder due to a missing part of

chromosome 5chromosome 5 – These individuals are mentally retarded, have a These individuals are mentally retarded, have a

small head with unusual facial features, and a cry small head with unusual facial features, and a cry like the mewing of a distressed cat.like the mewing of a distressed cat.

– This syndrome is fatal in infancy This syndrome is fatal in infancy الطفوية or or early childhood. early childhood.

2.2. MyelogenousMyelogenous,, [leukemia (CML)].[leukemia (CML)]. Caused by chromosomal translocations since a fragment of chromosome Caused by chromosomal translocations since a fragment of chromosome 22 switches places with a small fragment from the tip of chromosome 9.22 switches places with a small fragment from the tip of chromosome 9.