Changes in Chromosome Number Chapter 3. Central Points Chromosomes are composed of DNA and proteins...
-
Upload
shannon-thomas -
Category
Documents
-
view
215 -
download
0
Transcript of Changes in Chromosome Number Chapter 3. Central Points Chromosomes are composed of DNA and proteins...
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
3.1 Chromosomes
Thread-like structures in nucleus
Carry genetic information Humans have 46 Parts• Centromere • p arm• q arm• Telomeres
3.2 Changes in Chromosome Number
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)
Meiosis: Produces Haploid Cells
Meiosis: Produces Haploid Cells
Animation: Meiosis
Nondisjunction
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
Meiosis: The Creations of Gametes
Meiosis 1
Meiosis 2
Non-Disjunction During Meiosis
Non-disjunction in Meiosis 1 Non-disjunction in Meiosis 2
Trisomy zygoteMonosomy zygote
Aneuploidy
Effects vary by chromosomal condition
Many cause early miscarriages
Leading cause of mental retardation
3.3 ID of Chromosomal Abnormalities
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
p. 46
Removal of about 20 ml of amniotic 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
Amniocentesis Only Used in Certain Conditions
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
Chorionic Villus Sampling (CVS)
Karyotype
Animation: Chromosomes and Human Inheritance (karyotype preparation)
Non-Invasive Prenatal Diagnosis
In 1997 it was determined the “cell-free” fetal DNA is found in maternal plasma
Has been used to determine the sex and blood group of the fetus
Harder to detect fetal chromosomal aneuploidies, because fetal DNA is only 3% of cell-free DNA in maternal plasma
Other Chromosomal Variations
Haploid: one copy of each chromosome Diploid: normal two copies of each chromosome Polyploidy: multiple sets of chromosomes
Aneuploid: A variation in chromosome number, but not involving all of the chromosomes
Trisomy: three copies of one chromosome Monosomy: only one copy of a chromosome
Structural changes: duplication, deletion, inversion, translocation
Duplication
Deletion
Karyotype of Deletion on Chromosome 16
Inversion
Translocation
Translocation Karyotype
3.4 Effects of Changes in Chromosomes
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
Patau Syndrome
Trisomy 13: Patau Syndrome (47,+13)
1/15,000
Survival: 1–2 months
Facial, eye, finger, toe, brain, heart, and nervous system malformations
Edwards Syndrome
Trisomy 18: Edwards Syndrome (47,+18)
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
Down Syndrome
Trisomy 21: Down Syndrome (47,+21) 1/800 (changes with age of mother)
Survival up to age 50
Leading cause of childhood mental retardation and heart defects
Wide, flat skulls; large tongues; physical, mental, development retardation
Maternal Age and Down Syndrome
Aneuploidy and Sex Chromosomes
More common than in autosomes
Turner syndrome (45,X): monosomy of X chromosome
Klinefelter syndrome (47,XXY)
Jacobs syndrome (47,XYY)
Turner Syndrome
Turner Syndrome (45,X)
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
Klinefelter Syndrome (47,XXY)
Survival to adulthood
Features do not develop until puberty, usually sterile, may have learning disabilities
1/1,000 males
XYY Syndrome
XYY or Jacobs Syndrome (47,XYY)
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
3.5 Ways to Evaluate Risks
Genetic counselors are part of the health care team
They assist understanding of: • Risks • Diagnosis• Progression• Possible treatments• Management of disorder• Possible recurrence
Counseling Recommendations (1)
Pregnant women or those who are planning pregnancy, after age 35
Couples with a child with: • Mental retardation• A genetic disorder• A birth defect
Counseling Recommendations (2)
Couples from certain ethnic 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
Genetic Counseling
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
Future of Genetic Counseling
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