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Changes in Chromosome Number. Chapter 3. Central Points. Chromosomes are composed of DNA and proteins Most humans have 46 chromosomes Possible to test fetal chromosome number Extra chromosomes affect fetus Problems with genetic testing can result in lawsuits.

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Central points l.jpg
Central Points

  • Chromosomes are composed of DNA and proteins

  • Most humans have 46 chromosomes

  • Possible to test fetal chromosome number

  • Extra chromosomes affect fetus

  • Problems with genetic testing can result in lawsuits


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Case A: Results Worry Pregnant Woman

  • Martha, age 41, is 18-weeks pregnant

  • Increased risk of chromosomal abnormalities

  • Amniocentesis recommended

  • Test results:

    • NoDown syndrome

    • Fetus is XYY (Jacobs syndrome)



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3.1 Chromosomes

  • Thread-like structures in nucleus

  • Carry genetic information

  • Humans have 46

  • Parts

    • Centromere

    • p arm

    • q arm

    • Telomeres


Slide6 l.jpg

p arm

Centromere

q arm

Fig. 3-1, p. 43



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3.2 Changes in Chromosome Number organization)

  • Eggs and sperm are produced by meiosis

  • Begin with two copies of each chromosome (46)

  • Two divisions meiosis I and meiosis II

  • Homologous chromosome pairs separate

  • Produces haploid cells with one copy of each chromosome (23)



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Before cells begin meiosis, the chromosomes duplicate. As meiosis begins, chromosomes coil and shorten, and become visible in the microscope. Each chromosome has a matching partner and the two chromosomes may exchange parts (cross over) during this stage, called prophase I.

p. 44


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The chromosome pairs line up along the middle of the cell, and spindle fibers attach

to the centromere of each pair. This stage is called metaphase I.

p. 44


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Members of each homologous pair separate and move toward opposite sides of the cell. This stage is called anaphase I.

p. 44


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The chromosomes reach opposite poles of the cell, and the nuclei begin to re-form. This stage is called telophase I. The cytoplasm divides, and two cells are formed. These cells have half the number of chromosomes of the original cells and are called haploid cells.

p. 44


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MEIOSIS I nuclei begin to re-form. This stage is called telophase I. The cytoplasm divides, and two cells are formed. These cells have half the number of chromosomes of the original cells and are called haploid cells.

Before cells begin meiosis, the chromosomes duplicate. As meiosis begins, chromosomes coil and shorten, and become visible in the microscope. Each chromosome has a matching partner and the two chromosomes may exchange parts (cross over) during this stage, called prophase I.

The chromosome pairs line up along the middle of the cell, and spindle fibers attach to the centromere of each pair. This stage is called metaphase I.

Members of each homologous pair separate and move toward opposite sides of the cell. This stage is called anaphase I.

The chromosomes reach opposite poles of the cell, and the nuclei begin to re-form. This stage is called telophase I. The cytoplasm divides, and two cells are formed. These cells have half the number of chromosomes of the original cells and are called haploid cells.

Stepped Art

p. 44


Meiosis produces haploid cells15 l.jpg
Meiosis: Produces Haploid Cells nuclei begin to re-form. This stage is called telophase I. The cytoplasm divides, and two cells are formed. These cells have half the number of chromosomes of the original cells and are called haploid cells.


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Two cells formed during meiosis I. In prophase II, the chromosomes of these cells become coiled, and move toward the center of the cell.

p. 44


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The 23 chromosomes in each cell attach to spindle fibers at their centromeres. This stage is called metaphase II.

p. 44


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Each centromere divides, and the newly formed chromosomes (also called sister chromatids) move to opposite ends of the cell. This stage is called anaphase II.

p. 44


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Finally, the chromosomes uncoil and the nuclear membrane re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.

p. 44


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MEIOSIS II re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.

Two cells formed during meiosis I. In prophase II, the chromosomes of these cells become coiled, and move toward the center of the cell.

The 23 chromosomes in each cell attach to spindle fibers at their centromeres. This stage is called metaphase II.

Each centromere divides, and the newly formed chromosomes (also called sister chromatids) move to opposite ends of the cell. This stage is called anaphase II.

Finally, the chromosomes uncoil and the nuclear membrane re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.

Stepped Art

p. 44


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Events in Meiosis re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.


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Animation: Meiosis re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.


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Animation: Mitosis re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.


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Nondisjunction re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.

  • Chromosomes fail to separate

  • Results in gametes and zygote with an abnormal chromosome number

  • Aneuploidy is variations in chromosome number that involve one or more chromosomes

  • Most aneuploidy from errors in meiosis


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Nondisjunction re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.


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Chromosome re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.

number in gametes:

Extra chromosome

(n + 1)

Extra chromosome

(n + 1)

Missing chromosome

(n – 1)

Missing chromosome

(n – 1)

Chromosomes

align at metaphase I

Nondisjunction

at anaphase I

Alignments at metaphase II

Anaphase II

Fig. 3-2, p. 45


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Chromosome re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.

number in gametes:

Extra chromosome

(n + 1)

Extra chromosome

(n + 1)

Missing chromosome

(n – 1)

Missing chromosome

(n – 1)

Chromosomes

align at metaphase I

Nondisjunction

at anaphase I

Alignments at metaphase II

Anaphase II

Stepped Art

Fig. 3-2, p. 45


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Aneuploidy re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.

  • Effects vary by chromosomal condition

  • Many cause early miscarriages

  • Leading cause of mental retardation


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3.3 ID of Chromosomal Abnormalities re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.

Two tests:

  • Amniocentesis (> 16 weeks)

    • Collects amniotic fluid

    • Fetal cells grown and karyotype produced

  • Chorionic villus sampling (CVS) (10–12 weeks)

    • Rapidly dividing cells

    • Karyotype within few days


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Amniocentesis re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.


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Removal of about 20 ml of amniotic re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.fluid containing suspended cells that were sloughed off from the fetus

Biochemical analysis of the amniotic fluid after the fetal cells are separated out

Centrifugation

Fetal cells are removed from the solution

Analysis of fetal cells to determine sex

Cells are grown in an incubator

Karyotype analysis

p. 46


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Removal of about 20 ml of amniotic re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.fluid containing suspended cells that were sloughed off from the fetus

Biochemical analysis of the amniotic fluid after the fetal cells are separated out

Centrifugation

Fetal cells are removed from the solution

Analysis of fetal cells to determine sex

Cells are grown in an incubator

Karyotype analysis

Stepped Art

p. 46


Karyotype l.jpg
Karyotype re-forms. This stage is called telophase II. After the cytoplasm divides, the result is four cells, each with the haploid number of chromosomes. Meiosis is now completed.




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Chorionic villi preparation)

Ultrasound to monitor procedure

Developing placenta

Developing fetus

Bladder

Uterus

Chorion

Catheter

Amniotic cavity

Rectum

p. 47


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Amniocentesis Only Used in Certain Conditions preparation)

  • Risks for miscarriage; typically only done under one of following circumstances:

    • Mother > 35

    • History of child with chromosomal abnormalities

    • Parent has abnormal chromosomes

    • Mother carries a X-linked disorder

    • History of infertility or multiple miscarriages


Other chromosomal variations l.jpg
Other Chromosomal Variations preparation)

  • Polyploidy: multiple sets of chromosomes

  • Euploid: normal two copies of each chromosome

  • Trisomy: three copies of one chromosome

  • Monosomy: only one copy of a chromosome

  • Structural changes: duplication, deletion, inversion, translocation



Slide40 l.jpg

p. 47 preparation)


Slide41 l.jpg

Normal chromosome preparation)

One segment repeated three times

p. 47


Slide42 l.jpg

p. 47 preparation)


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Segment C deleted preparation)

p. 47


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p. 47 preparation)



Slide46 l.jpg

p. 47 preparation)


Slide47 l.jpg

Chromosome A preparation)

Chromosome B

Translocation

p. 47




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3.4 Effects of Changes in Chromosomes II)

  • Vary by chromosome and type of variation

  • May cause birth defects or fetal death

  • Monosomy of any autosome is fatal

  • Only a few trisomies result in live births





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Trisomy 13: Patau Syndrome (47,+13) II)

  • 1/15,000

  • Survival: 1–2 months

  • Facial, eye, finger, toe, brain, heart, and nervous system malformations



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Trisomy 13: Edwards Syndrome (47,+18) II)

  • 1/11,000, 80% females

  • Survival: 2–4 months

  • Small, mental disabilities, clenched fists, heart, finger, and foot malformations

  • Die from heart failure or pneumonia



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Trisomy 21: Down Syndrome (47,+21) II)

  • 1/800 (changes with age of mother)

  • Survival up to age 50

  • Leading cause of childhood mental retardation and heart defects

  • Wide, flat skulls; eyelid folds; large tongues; physical, mental, development retardation

  • May live rich, productive lives



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Leading Risk Factor for Trisomy II)

  • Maternal age

  • Unknown why, older eggs increase risk of nondisjunction

  • Eggs held in meiosis I from birth to ovulation

  • Possible changes in maternal selection



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Aneuploidy and Sex Chromosomes II)

  • More common than in autosomes

  • Turner syndrome(45,X): monosomy of X chromosome

  • Klinefelter syndrome(47,XXY)

  • Jacobs syndrome(47,XYY)





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Turner Syndrome (45,X) II)

  • Survival to adulthood

  • Female, short, wide-chested, undeveloped ovaries, possible narrowing of aorta

  • Normal intelligence

  • 1/10,000 female births, 95–99% of 45,X conceptions die before birth



Klinefelter syndrome 47 xxy l.jpg
Klinefelter Syndrome (47,XXY) II)

  • Survival to adulthood

  • Male

  • Features do not develop until puberty, usually sterile, may have learning disabilities

  • 1/1,000 males



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XYY or Jacobs Syndrome (47,XYY) II)

  • Survival to adulthood

  • Average height, thin, personality disorders, some form of mental disabilities, and adolescent acne

  • Some may have very mild symptoms

  • 1/1,000 male births



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3.5 Ways to Evaluate Risks II)

  • Genetic counselors are part of the health care team

  • In nondirective way, they assist understanding of:

    • Risks

    • Diagnosis

    • Progression

    • Possible treatments

    • Management of disorder

    • Possible recurrence


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Counseling Recommendations (1) II)

  • Pregnant women or those who are planning pregnancy

  • Women > age 35

  • Couples with a child with:

    • Mental retardation

    • A genetic disorder

    • A birth defect


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Counseling Recommendations (2) II)

  • Couples from certain ethic groups

  • Couples that are closely related

  • Individuals with jobs, lifestyles, or medical history that may pose a risk to a pregnancy

  • Women who have had two or more miscarriages or babies who died in infancy


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Genetic Counseling II)

  • Most see a genetic counselor:

    • After a prenatal test;

    • After the birth of a child; or

    • To determine their risk

  • Counselor

    • Constructs a detailed family history and pedigree

    • Shares information that allows an individual or a couple to make informed decisions


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Case A Questions II)

  • Child is XYY: What are the best options?

  • Would the options change if the child had a different condition?

  • Who should know?

  • See the textbook for further questions on this case


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Case B: Test Results Worry Doctor II)

  • 31-year-old woman gave birth to a child with serious abnormalities

  • Sued doctor for not performing amniocentesis

  • What legal issues should concern the doctor and what should she do?

  • See the textbook for further questions on this case


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Future of Genetic Counseling II)

  • Human Genome Project (HGP) changed medical care and genetic testing

  • Genetic counselor will become more important

  • Evaluate reproductive risks and other conditions

  • Allow at-risk individuals to make informed choices about lifestyle, children, and medical care


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3.6 Legal and Ethical Issues II)

  • Wrongful-birth suit

  • Wrongful-life suit

  • Based on:

  • Could a diagnosis of this condition have been made in time to have an abortion?

  • Was the condition serious enough that a reasonable person would have had an abortion?



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Issues with Wrongful-Birth and II)Wrongful-Life Suits

  • Wrongful-birth suit (most states allow):

    • Roe v. Wade gave a woman an alternative to birth

    • Doctors have extensive medical malpractice insurance

  • Wrongful-life suits (only 5 states allow):

    • Courts uncomfortable declaring someone should never have been born


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XYY Individuals (Jacobs Syndrome) II)

  • Early studies linking XYY with aggressive/ criminal behavior no longer supported by research

  • Should parents and or child know the condition?

  • What should the doctor do?


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Spotlight on Law: II)Becker v. Schwartz

  • Becker, age 37, was not informed about amniocentesis

  • Child born with Down syndrome, parents sued doctor for “wrongful life”

  • Parents won $2,500 and gave baby up for adoption

  • What is your opinion on this case?


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