PowerLecture: Chapter 21 Chromosomes and Human Genetics

PowerLecture:PowerLecture:Chapter 21Chapter 21

Chromosomes and Chromosomes and Human GeneticsHuman Genetics

Learning ObjectivesLearning Objectives

Describe how an understanding of Describe how an understanding of chromosomes helps to account for events chromosomes helps to account for events that compose mitosis and meiosis.that compose mitosis and meiosis.

Name some ordinary and extraordinary Name some ordinary and extraordinary chromosomal events that can create new chromosomal events that can create new phenotypes (outward appearances).phenotypes (outward appearances).

Understand how changes in chromosome Understand how changes in chromosome structure and number can affect the structure and number can affect the outward appear ance of organisms.outward appear ance of organisms.

Distinguish autosomal recessive inheritance Distinguish autosomal recessive inheritance from sex-linked recessive inheritance.from sex-linked recessive inheritance.

Learning Objectives (cont’d)Learning Objectives (cont’d)

Explain how changes in chromosomal Explain how changes in chromosomal number can occur and present an example number can occur and present an example of such a change.of such a change.

List examples of phenotypic defects and List examples of phenotypic defects and describe how each can be treated.describe how each can be treated.

Explain how knowing about modern Explain how knowing about modern methods of genetic screening can minimize methods of genetic screening can minimize potentially tragic events. potentially tragic events.

Impacts/IssuesImpacts/Issues

Menacing MucusMenacing Mucus

Menacing Mucus Menacing Mucus

Cystic fibrosis (CF) is a debilitating genetic Cystic fibrosis (CF) is a debilitating genetic disorder.disorder.

Persons with two recessive genes Persons with two recessive genes will suffer from excessive will suffer from excessive accumulations of mucus in their accumulations of mucus in their lungs.lungs.

The defective gene is located on The defective gene is located on chromosome 7 and codes for a membrane chromosome 7 and codes for a membrane transport protein called CFTR.transport protein called CFTR.

Many thousands of prospective parents Many thousands of prospective parents have been screened for CF; genetic testing, have been screened for CF; genetic testing, however, is not without controversy.however, is not without controversy.

Useful References for Impacts/IssuesUseful References for Impacts/Issues

The latest references for topics covered in this section can be found at The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articles.www.thomsonedu.com to access InfoTrac articles.

Cystic Fibrosis FoundationCystic Fibrosis Foundation Cystic Fibrosis Foundation: Gene Therapy Cystic Fibrosis Foundation: Gene Therapy

and CFand CF InfoTrac: Constant Battle: Kerri Marks Is One InfoTrac: Constant Battle: Kerri Marks Is One

of the Survivors, Living Under a Disease’s of the Survivors, Living Under a Disease’s Invisible Clock. Jennifer Becknell. Invisible Clock. Jennifer Becknell. HeraldHerald, , July 10, 2006.July 10, 2006.

How Would You Vote?How Would You Vote?To conduct an instant in-class survey using a classroom response To conduct an instant in-class survey using a classroom response system, access “JoinIn Clicker Content” from the PowerLecture main system, access “JoinIn Clicker Content” from the PowerLecture main menu. menu.

Do we as a society want to encourage Do we as a society want to encourage women to give birth only to offspring who will women to give birth only to offspring who will not develop serious gene-based medical not develop serious gene-based medical problems?problems? a. Yes, in order to prevent needless suffering and a. Yes, in order to prevent needless suffering and

expense.expense. b. No, the diversity represented by special-needs b. No, the diversity represented by special-needs

children is important to a society. children is important to a society.

Useful References for Useful References for How Would You Vote?How Would You Vote?

The latest references for topics covered in this section can be found at The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articleswww.thomsonedu.com to access InfoTrac articles. .

InfoTrac: Ohio Court Limits Relief for Fetal Testing InfoTrac: Ohio Court Limits Relief for Fetal Testing Error; Compensation Denied for Costs of Raising Error; Compensation Denied for Costs of Raising Disabled Child. Judy Greenwald. Disabled Child. Judy Greenwald. Business Business InsuranceInsurance, Mar. 13, 2006. , Mar. 13, 2006.

InfoTrac: In New Tests for Fetal Defects, Agonizing InfoTrac: In New Tests for Fetal Defects, Agonizing Choices for Parents. Amy Harmon. Choices for Parents. Amy Harmon. The New York The New York TimesTimes, June 20, 2004., June 20, 2004.

InfoTrac: Offer All Pregnant Women Fetal Genetic InfoTrac: Offer All Pregnant Women Fetal Genetic Testing. Sherry Boschert. Testing. Sherry Boschert. OB GYN NewsOB GYN News, Dec. 15, , Dec. 15, 1999.1999.

Section 1Section 1

Genes and Genes and ChromosomesChromosomes

Genes and Chromosomes Genes and Chromosomes

Understanding inheritance starts with gene-Understanding inheritance starts with gene-chromosome connections.chromosome connections.

Genes, the units of instruction for heritable Genes, the units of instruction for heritable traits, are segments of DNA arranged along traits, are segments of DNA arranged along chromosomes in linear order; each gene thus chromosomes in linear order; each gene thus has its own has its own locuslocus..

Diploid cells have pairs of homologous Diploid cells have pairs of homologous chromosomes that are very much alike; chromosomes that are very much alike; homologues interact and segregate during homologues interact and segregate during meiosis.meiosis.

Alleles are different forms of a gene; they often Alleles are different forms of a gene; they often arise by mutation.arise by mutation.


Independent assortment dictates that genes Independent assortment dictates that genes generally move into gametes independently of generally move into gametes independently of one another.one another.

Crossing over leads to genetic recombination Crossing over leads to genetic recombination during meiosis.during meiosis.


Closely linked genes tend to stay together Closely linked genes tend to stay together when gametes form.when gametes form.

When the distance between two genes on the When the distance between two genes on the same chromosome is very short, the genes are same chromosome is very short, the genes are said to show said to show linkagelinkage (they are “linked”); close (they are “linked”); close genes generally travel together and are not genes generally travel together and are not segregated independently.segregated independently.

Genes that are far apart on a chromosome will Genes that are far apart on a chromosome will segregate independently as a result of crossing segregate independently as a result of crossing over between them.over between them.

x

AB ab

50% AB 50% ab

meiosis, gametes form

Closely linked genes in parents:

Genes stay together in gametes.

AB

ab

AB

ab

ab

AB

Fig. 21.1a, p. 388

Genes less likely to stay together as gametes form.

x

meiosis, gamete formation

AC ac

A

C A

C

AC

a

c

ac

Ac

aC

a

c

Most gametes have parent’s

genotype.

A few gametes have recombinant genotypes.

Fig. 21.1b, p.388

Weaker linkage in parents:

x

AB ab

50% AB 50% ab

Closely linked genes in parents:

Genes stay together in gametes.

AB

AB

ab

ab

meiosis, gametes form

ab

AB

Fig. 21.1a, p. 388

Stepped Art

Genes less likely to stay together as gametes form.

x

AC ac

A

C A

C

a

c a

c

meiosis, gamete formation

AC

ac

Ac

aC

Most gametes have parent’s

genotype.

A few gametes have recombinant genotypes.

Fig. 21.1b, p.388

Weaker linkage in parents:

Stepped Art


The X and Y chromosomes are quite The X and Y chromosomes are quite different genetically. different genetically.

Sex chromosomes determine gender; males Sex chromosomes determine gender; males have one X and one Y chromosome while have one X and one Y chromosome while females have two X chromosomes.females have two X chromosomes.

The X and Y chromosomes can synapse in a The X and Y chromosomes can synapse in a small region along their length, allowing them to small region along their length, allowing them to behave as homologues during meiosis.behave as homologues during meiosis.

Useful References for Section 1Useful References for Section 1


Rocklin and Roseville TodayRocklin and Roseville Today: Genetic : Genetic Behaviors Go Beyond X and Y Behaviors Go Beyond X and Y ChromosomesChromosomes

InfoTrac: Cord-Blood Storage Is Big InfoTrac: Cord-Blood Storage Is Big Business, But Is It Worth It? Blythe Bernhard. Business, But Is It Worth It? Blythe Bernhard. Orange County Register (Santa Ana, CA)Orange County Register (Santa Ana, CA), , June 2, 2006.June 2, 2006.

Section 2Section 2

Picturing Chromosomes Picturing Chromosomes with Karyotypeswith Karyotypes

A karyotype is a “picture” of a person’s A karyotype is a “picture” of a person’s chromosomes captured when the chromosomes captured when the chromosomes have condensed to their chromosomes have condensed to their metaphase (mitosis) state.metaphase (mitosis) state.

Picturing Chromosomes with KaryotypesPicturing Chromosomes with Karyotypes

Figure 21.2fFigure 21.2f

Cells are harvested from a patient, grown in Cells are harvested from a patient, grown in the lab, and arrested in metaphase of mitosis the lab, and arrested in metaphase of mitosis (using colchicines) before the cells are (using colchicines) before the cells are disrupted and the chromosomes recovered.disrupted and the chromosomes recovered.

Once photographed, the chromosomes are Once photographed, the chromosomes are arranged in their homologous pairs and arranged in their homologous pairs and analyzed.analyzed.

Picturing Chromosomes with KaryotypesPicturing Chromosomes with Karyotypes

© 2007 Thomson Higher Education

b. Centrifugation

Fig. 21.2(1), p.389

a. Add cells from a small blood sample

c. Prepare cells

d. Put cells on microscope

e. Photograph cell



InfoTrac: Microarray Analysis of Cell-Free InfoTrac: Microarray Analysis of Cell-Free Fetal DNA in Amniotic Fluid: A Prenatal Fetal DNA in Amniotic Fluid: A Prenatal Molecular Karyotype. Paige B. Larrabee et al. Molecular Karyotype. Paige B. Larrabee et al. American Journal of Human GeneticsAmerican Journal of Human Genetics, Sept. , Sept. 2004.2004.

Section 3Section 3

How Sex Is DeterminedHow Sex Is Determined

How Sex Is Determined How Sex Is Determined

Sex is a question of X or Y.Sex is a question of X or Y. Gender of the human offspring is determined by Gender of the human offspring is determined by

the father’s sperm.the father’s sperm.• If an X-bearing If an X-bearing

sperm fertilizes an sperm fertilizes an

egg, the offspring egg, the offspring

will be female.will be female.• If a Y-bearing If a Y-bearing

sperm fertilizes an sperm fertilizes an

egg, the offspring egg, the offspring

will be male.will be male.

Figure 21.3aFigure 21.3a

X

X Y

X

XX

XY

XX

XY

X X

Y

X

x

x

eggs sperm

female(XX)

male(XY)

Fig. 21.3a, p.390

diploid germ cells in female

meiosis, gamete formation in both female and male

sex chromosome combinations possible in new individual


The Y chromosome has a “male-determining The Y chromosome has a “male-determining gene” (gene” (SRYSRY) that codes for proteins that cause ) that codes for proteins that cause testes to form; in the absence of testes to form; in the absence of SRYSRY, a female , a female forms automatically.forms automatically.

Nonsexual traits are also coded for on the sex Nonsexual traits are also coded for on the sex chromosomes, mostly on the X.chromosomes, mostly on the X.

Genes that are specific to the X and Y Genes that are specific to the X and Y chromosomes are called chromosomes are called X-linked genesX-linked genes and and Y-linked genesY-linked genes, respectively., respectively.

Figure 21.3bFigure 21.3b

appearance of “uncommitted” duct

system of embryoat 7 weeks

Y chromosome present

Y chromosome absent

penis

testis

ovaryuterus

vagina

testis ovary

Fig. 21.3b, p.390

Y chromosome present

Y chromosome absent

testis

penis

uterus

vagina

Fig. 21.3b, p.390

testis ovary

ovary

appearance of “uncommitted” duct

system of embryoat 7 weeks

Stepped Art


In females, one X is inactivated.In females, one X is inactivated. Most or all of the genes on one of the X Most or all of the genes on one of the X

chromosomes are switched off in early chromosomes are switched off in early development, a process called development, a process called X inactivationX inactivation; ; which X becomes inactivated (maternal or which X becomes inactivated (maternal or paternal) is random.paternal) is random.

• The inactivated X chromosome becomes condensed The inactivated X chromosome becomes condensed into a into a Barr bodyBarr body..

• The female body is a mosaic of cells in which one or The female body is a mosaic of cells in which one or the other of the original pair of X chromosomes the other of the original pair of X chromosomes inherited from her parents is active.inherited from her parents is active.


Anhidrotic ectodermal dysplasiaAnhidrotic ectodermal dysplasia is a is a condition in females in which the active X condition in females in which the active X chromosome in certain tissues carries a chromosome in certain tissues carries a mutated gene that blocks the formation of mutated gene that blocks the formation of sweat glands in patches of tissue over the sweat glands in patches of tissue over the surface of the body. surface of the body.

Fig. 21.4a, p.391



InfoTrac: The Genetic Legacy of the InfoTrac: The Genetic Legacy of the Mongols. Tatiana Zerjal et alMongols. Tatiana Zerjal et al. American . American Journal of Human GeneticsJournal of Human Genetics, Mar. 2003., Mar. 2003.

Section 4Section 4

Human Genetic AnalysisHuman Genetic Analysis


A pedigree A pedigree shows genetic shows genetic connections.connections.

The analysis of The analysis of family family pedigree pedigree chartscharts provides provides data on data on inheritance inheritance patterns through patterns through several several generations.generations.

Figure 21.5Figure 21.5


A person who is heterozygous for a recessive A person who is heterozygous for a recessive trait (trait (carriercarrier) may show the dominant ) may show the dominant phenotype but is still capable of passing the phenotype but is still capable of passing the recessive gene on.recessive gene on.

• The term The term genetic abnormalitygenetic abnormality is applied to a is applied to a genetic condition that is a deviation from the usual, genetic condition that is a deviation from the usual, or average, but is not life threatening.or average, but is not life threatening.

• A A genetic disordergenetic disorder is more appropriately used to is more appropriately used to describe conditions that cause medical problems.describe conditions that cause medical problems.

• SyndromeSyndrome refers to a set of symptoms that refers to a set of symptoms that characterize a disorder by appearing together.characterize a disorder by appearing together.


Genetic analysis may predict disorders.Genetic analysis may predict disorders. Genetic analysis, beginning with determination Genetic analysis, beginning with determination

of the parental genotypes, is the first step to of the parental genotypes, is the first step to identifying any risks a couple may have in identifying any risks a couple may have in producing a child with a genetic disorder.producing a child with a genetic disorder.



Not all factors leading to disorders can be Not all factors leading to disorders can be readily quantified; it is important for prospective readily quantified; it is important for prospective parents to recognize that each pregnancy will parents to recognize that each pregnancy will hold the same risks.hold the same risks.



GeneTests: Amish Lethal MicrocephalyGeneTests: Amish Lethal Microcephaly InfoTrac: LCT Announces Results for InfoTrac: LCT Announces Results for

Treating Huntington’s Disease. Treating Huntington’s Disease. AsiaPulse AsiaPulse NewsNews, Aug. 2, 2005., Aug. 2, 2005.

Section 5Section 5

Inheritance of Genes Inheritance of Genes

on Autosomeson Autosomes

Inheritance of Genes on Autosomes Inheritance of Genes on Autosomes

Inherited recessive traits cause a variety of Inherited recessive traits cause a variety of disorders.disorders.

Recessive inheritance is characterized by the Recessive inheritance is characterized by the following:following:

• Either parent can carry the recessive allele on an Either parent can carry the recessive allele on an autosome.autosome.

• Heterozygotes are symptom free; homozygotes are Heterozygotes are symptom free; homozygotes are affected.affected.

• Two heterozygous parents have a 50% chance of Two heterozygous parents have a 50% chance of producing heterozygous children and a 25% chance producing heterozygous children and a 25% chance of a homozygous recessive child. When both parents of a homozygous recessive child. When both parents are homozygous, all children will be affected.are homozygous, all children will be affected.


Examples of autosomal recessive inheritance Examples of autosomal recessive inheritance include:include:

• Cystic fibrosisCystic fibrosis..• PhenylketonuriaPhenylketonuria (PKU), resulting from the abnormal (PKU), resulting from the abnormal

buildup of phenylalanine due to the lack of an buildup of phenylalanine due to the lack of an enzyme that normally enzyme that normally breaks it down.breaks it down.

• Tay-Sachs diseaseTay-Sachs disease, , which affects primarily which affects primarily infants, is characterized infants, is characterized by lack of an enzyme to by lack of an enzyme to break down lipids in the break down lipids in the brain.brain.



Some disorders are due to dominant genes.Some disorders are due to dominant genes. Inheritance of dominant alleles demonstrates Inheritance of dominant alleles demonstrates

the characteristics below:the characteristics below:• Because such alleles are usually expressed (even in Because such alleles are usually expressed (even in

heterozygotes), the trait appears in each generation.heterozygotes), the trait appears in each generation.• If one parent is heterozygous and the other If one parent is heterozygous and the other

homozygous recessive, there is a 50% chance that homozygous recessive, there is a 50% chance that any one child will be heterozygous.any one child will be heterozygous.

• Dominant alleles, even if they cause severe genetic Dominant alleles, even if they cause severe genetic disorders, persist in the population due to mutation, disorders, persist in the population due to mutation, nonreproductive effects, or post-reproductive onset.nonreproductive effects, or post-reproductive onset.


Examples of autosomal Examples of autosomal

dominant inheritance dominant inheritance

include:include:• Marfan syndromeMarfan syndrome results results

from a defective form of from a defective form of

fibrillin, found in connective fibrillin, found in connective

tissue; one effect is to tissue; one effect is to

disrupt both structure and disrupt both structure and

function of smooth muscle function of smooth muscle

cells of the aorta.cells of the aorta.

Figures 21.8 and 21.9Figures 21.8 and 21.9


• AchondroplasiaAchondroplasia (dwarfism) (dwarfism)

results in heights of about 4 results in heights of about 4

feet, but has no other serious feet, but has no other serious

effects; homozygotes, though, effects; homozygotes, though,

usually are stillborn.usually are stillborn.• Familial hypercholesterolemiaFamilial hypercholesterolemia

results in elevated levels of cholesterol due to few results in elevated levels of cholesterol due to few cell receptors for low-density lipoproteins.cell receptors for low-density lipoproteins.

• Huntington diseaseHuntington disease, a serious degenerative , a serious degenerative disease of the nervous system with an onset from disease of the nervous system with an onset from age 30 onward; homozygotes always die, thus adults age 30 onward; homozygotes always die, thus adults are always heterozygous.are always heterozygous.




InfoTrac: Geographic Distribution of Disease InfoTrac: Geographic Distribution of Disease Mutations in the Ashkenazi Jewish Mutations in the Ashkenazi Jewish Population Supports Genetic Drift over Population Supports Genetic Drift over Selection. Neil Risch et al. Selection. Neil Risch et al. American Journal American Journal of Human Geneticsof Human Genetics, April 2003., April 2003.

Section 6Section 6

Inheritance of Genes on Inheritance of Genes on the X Chromosomethe X Chromosome

Inheritance of Genes on Inheritance of Genes on the X Chromosome the X Chromosome

X-linked recessive inheritance.X-linked recessive inheritance. X-linked recessive inheritance is demonstrated X-linked recessive inheritance is demonstrated

by the following:by the following:• The mutated gene occurs only on the X The mutated gene occurs only on the X

chromosome.chromosome.• Heterozygous females are phenotypically normal Heterozygous females are phenotypically normal

because a dominant gene on the other X because a dominant gene on the other X chromosome masks the recessive’s effects; a male chromosome masks the recessive’s effects; a male will be affected if he inherits a recessive gene on his will be affected if he inherits a recessive gene on his sole X chromosome.sole X chromosome.


• A normal male mated with a female heterozygote A normal male mated with a female heterozygote together have a 50% chance of producing carrier together have a 50% chance of producing carrier daughters and a 50% chance of producing affected daughters and a 50% chance of producing affected sons. In the case of a homozygous female and a sons. In the case of a homozygous female and a normal male, all daughters will be carriers and all normal male, all daughters will be carriers and all sons affected.sons affected.



Examples of X-linked recessive inheritance Examples of X-linked recessive inheritance include:include:

• Duchenne muscular dystrophyDuchenne muscular dystrophy is a condition in is a condition in which the protein which the protein dystrophindystrophin is missing, causing is missing, causing muscle fibers to weaken.muscle fibers to weaken.

• Red/green color blindnessRed/green color blindness is an inconvenience but is an inconvenience but is not life threatening.is not life threatening.


• Hemophilia AHemophilia A,, where the inability of the blood to clot where the inability of the blood to clot because the genes do not code for the necessary because the genes do not code for the necessary clotting agent (factor VIII) can lead to death from any clotting agent (factor VIII) can lead to death from any cut or internal bleeding.cut or internal bleeding.



Some types of X-linked abnormalities are quite Some types of X-linked abnormalities are quite rare.rare.

Faulty enamel traitFaulty enamel trait is one of very few is one of very few known examples of a trait caused by a known examples of a trait caused by a dominant mutant allele that is X-linked; dominant mutant allele that is X-linked; it is expressed in heterozygous females it is expressed in heterozygous females but is less pronounced than in males.but is less pronounced than in males.

Testicular feminizing syndromeTesticular feminizing syndrome (androgen (androgen insensitivity) is an abnormality of an XY individual in insensitivity) is an abnormality of an XY individual in which a mutation in the X chromosome results in which a mutation in the X chromosome results in defective receptors for the male sex hormones; defective receptors for the male sex hormones; individuals have external female features, but no uterus individuals have external female features, but no uterus or ovaries.or ovaries.



Many factors complicate genetic analysis.Many factors complicate genetic analysis. Before diagnosing a case, geneticists often Before diagnosing a case, geneticists often

must pool many pedigrees and make detailed must pool many pedigrees and make detailed analyses of clinical data to keep track of analyses of clinical data to keep track of instances where multiple mutations can lead to instances where multiple mutations can lead to the same phenotype.the same phenotype.

As an example, some conditions can occur As an example, some conditions can occur because of changes to autosomes because of changes to autosomes oror to the X to the X chromosome.chromosome.

Useful References for Section 6 Useful References for Section 6


InfoTrac: Significant Improvement in InfoTrac: Significant Improvement in Spirometry after Stem Cell Transplantation in Spirometry after Stem Cell Transplantation in One Duchenne Muscular Dystrophy Patient. One Duchenne Muscular Dystrophy Patient. Zhiping Li et al. Zhiping Li et al. ChestChest, Oct. 2005., Oct. 2005.

Section 7Section 7

Sex-Influenced Sex-Influenced InheritanceInheritance

Sex-Influenced Inheritance Sex-Influenced Inheritance

Sex-influenced traitsSex-influenced traits either appear more either appear more frequently in one sex than the other or the frequently in one sex than the other or the phenotype differs depending on whether the phenotype differs depending on whether the person is male or female.person is male or female.

Genes for such traits appear on the autosomes.Genes for such traits appear on the autosomes. Appearance of the trait may be due to the Appearance of the trait may be due to the

influence of sex hormones on gene expression.influence of sex hormones on gene expression.

Sex-Influenced Inheritance Sex-Influenced Inheritance

A male will develop A male will develop pattern baldnesspattern baldness if if he is homozygous or he is homozygous or heterozygous for a heterozygous for a particular gene, but a particular gene, but a female will develop female will develop the condition only if the condition only if she is homozygous she is homozygous and then only late in and then only late in life.life.




InfoTrac: Genetic Variation in the Human InfoTrac: Genetic Variation in the Human Androgen Receptor Gene Is the Major Androgen Receptor Gene Is the Major Determinant of Common Early-Onset Determinant of Common Early-Onset Androgenetic Alopecia. Axel M. Hillmer et al. Androgenetic Alopecia. Axel M. Hillmer et al. American Journal of Human GeneticsAmerican Journal of Human Genetics, July , July 2005.2005.

Section 8Section 8

Changes in a Changes in a Chromosome or Its Chromosome or Its

GenesGenes

Changes in a Chromosome or Its Genes Changes in a Chromosome or Its Genes

A A gene mutationgene mutation is a change to one or is a change to one or more of the nucleotides that composite a more of the nucleotides that composite a given gene.given gene.

Various changes in a chromosome’s Various changes in a chromosome’s structure may cause a genetic disorder.structure may cause a genetic disorder.


A A deletiondeletion is the loss of part of a chromosome is the loss of part of a chromosome due to breaks caused by viruses, chemicals, or due to breaks caused by viruses, chemicals, or irradiation.irradiation.

• Loss of a portion of chromosome Loss of a portion of chromosome 5, for example, causes the 5, for example, causes the disorder disorder cri-du-chatcri-du-chat..

• Normal genes on the Normal genes on the homologue can homologue can compensate for deleted compensate for deleted genes.genes.

DuplicationDuplication occurs when a gene sequence is occurs when a gene sequence is repeated thousands of times.repeated thousands of times.



A A translocationtranslocation occurs when a part of one occurs when a part of one chromosome is transferred to a nonhomologous chromosome is transferred to a nonhomologous chromosome.chromosome.

• It is seen in some forms of cancer, such as when It is seen in some forms of cancer, such as when

a segment of chromosome 8 is translocated to a segment of chromosome 8 is translocated to

chromosome 14.chromosome 14.


• A chronic type of leukemia is caused by an A chronic type of leukemia is caused by an abnormally long chromosome 9 (Philadelphia abnormally long chromosome 9 (Philadelphia chromosome), which is due to a piece of chromosome), which is due to a piece of chromosome 22 that has become attached.chromosome 22 that has become attached.




Five P Minus Society: Family Support Group Five P Minus Society: Family Support Group for Children with Cri du Chat Syndromefor Children with Cri du Chat Syndrome

BBC: TranslocationBBC: Translocation InfoTrac: Brazilian Researchers Connect InfoTrac: Brazilian Researchers Connect

Gene Mutation to Blood Conditions. Gene Mutation to Blood Conditions. Xinhua Xinhua News AgencyNews Agency, June 20, 2006., June 20, 2006.

Section 9Section 9

Changes in Changes in Chromosome NumberChromosome Number

Changes in Chromosome Number Changes in Chromosome Number

Several kinds of events can change the Several kinds of events can change the number of chromosomes in gametes.number of chromosomes in gametes.

AneuploidyAneuploidy is a condition in which the cells of is a condition in which the cells of an affected individual end up with one extra or an affected individual end up with one extra or one less chromosome than is the normal one less chromosome than is the normal number.number.

PolyploidyPolyploidy is a condition in which new is a condition in which new individuals have three or more of each individuals have three or more of each chromosome; it is lethal in humans.chromosome; it is lethal in humans.


Nondisjunction is a common cause of Nondisjunction is a common cause of abnormal numbers of autosomes.abnormal numbers of autosomes.

NondisjunctionNondisjunction during mitosis or meiosis during mitosis or meiosis results in a change in chromosome number.results in a change in chromosome number.

• If a gamete with an extra chromosome (n + 1) joins a If a gamete with an extra chromosome (n + 1) joins a normal gamete at fertilization, the diploid cell will be normal gamete at fertilization, the diploid cell will be 2n + 1; this condition is called 2n + 1; this condition is called trisomytrisomy..

• If an abnormal gamete is missing a chromosome, the If an abnormal gamete is missing a chromosome, the zygote will be 2n – 1: zygote will be 2n – 1: monosomymonosomy..

Animation: NondisjunctionAnimation: Nondisjunction

CLICKTO PLAY

n + 1

n + 1

n - 1

n - 1

chromosome alignments at metaphase I

nondisjunction at anaphase

I

alignments at metaphase II

anaphase II chromosomenumber in gametes

Fig. 21.18, p.400

n + 1

n + 1

n - 1

n - 1

chromosome alignments at metaphase I

nondisjunction at anaphase

I

alignments at metaphase II

anaphase II chromosomenumber in gametes

Fig. 21.18, p.400

Stepped Art


Down syndromeDown syndrome results from trisomy 21. results from trisomy 21.• Trisomy 21 occurs in 1 out Trisomy 21 occurs in 1 out

of 1,000 live newborns in of 1,000 live newborns in North America; children will North America; children will show some form of mental show some form of mental retardation, and 40% have retardation, and 40% have heart defects.heart defects.

• There is an increased There is an increased probability that a woman probability that a woman over age 35 will conceive an over age 35 will conceive an embryo with Down syndrome, yet 80% of trisomic embryo with Down syndrome, yet 80% of trisomic infants are born to younger mothers simply because infants are born to younger mothers simply because women ages 18-35 have more babies.women ages 18-35 have more babies.



Nondisjunction also can change the number Nondisjunction also can change the number of sex chromosomes.of sex chromosomes.

Turner syndromeTurner syndrome involves females involves females whose cells have only one X whose cells have only one X chromosome (designated XO).chromosome (designated XO).

• Turner’s individuals are sterile and have Turner’s individuals are sterile and have other phenotypic problems such as other phenotypic problems such as premature aging and shorter life premature aging and shorter life expectancy.expectancy.

• Approximately 1 in 1,000 females are Approximately 1 in 1,000 females are XXXXXX; two of ; two of the Xs are condensed into Barr bodies, allowing the Xs are condensed into Barr bodies, allowing normal development.normal development.



In In Klinefelter syndromeKlinefelter syndrome, nondisjunction results , nondisjunction results in an extra X chromosome in the cells (XXY) of in an extra X chromosome in the cells (XXY) of affected males.affected males.

• This occurs in about 1 out This occurs in about 1 out of 500 live-born males and of 500 live-born males and results in mild mental results in mild mental retardation and low fertility.retardation and low fertility.

XYY condition: XYY condition: XYY XYY males males result from nondisjunction result from nondisjunction of duplicated Y chromosomes during meiosis.of duplicated Y chromosomes during meiosis.

• Affected individuals are taller than average with Affected individuals are taller than average with normal phenotype.normal phenotype.


Victims of Neurobiological DisordersVictims of Neurobiological Disorders

John Nash; Virginia WoolfJohn Nash; Virginia Woolf




InfoTrac: A Very Special Wedding. Claudia InfoTrac: A Very Special Wedding. Claudia Wallis. Wallis. Time, Time, July 24, 2006.July 24, 2006.

InfoTrac: Common Age Misconception about InfoTrac: Common Age Misconception about Down Syndrome. Down Syndrome. PR NewswirePR Newswire, May 2, , May 2, 2006.2006.

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PowerLecture: Chapter 21 Chromosomes and Human Genetics