ap biology lecture #29 chromosomal errors 2006-2007 errors of meiosis chromosomal abnormalities
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Chromosomal abnormalities
• Incorrect number of chromosomes– nondisjunction • chromosomes don’t separate properly during meiosis
– breakage of chromosomes• deletion• duplication• inversion• translocation
Nondisjunction • Problems with meiotic spindle cause errors in daughter
cells– homologous chromosomes do not separate properly during
Meiosis 1– sister chromatids fail to separate during Meiosis 2– too many or too few chromosomes
2n n
n
n-1
n+1
Alteration of chromosome number
all with incorrect number 1/2 with incorrect number
error in Meiosis 1
error in Meiosis 2
trisomy2n+1
Nondisjunction • Baby has wrong chromosome number~
aneuploidy– trisomy • cells have 3 copies of a chromosome
– monosomy • cells have only 1 copy of a chromosome
n+1 n
monosomy2n-1
n-1 n
Human chromosome disorders • High frequency in humans– most embryos are spontaneously aborted– alterations are too disastrous– developmental problems result from biochemical imbalance
• imbalance in regulatory molecules?– hormones?– transcription factors?
• Certain conditions are tolerated– upset the balance less = survivable– but characteristic set of symptoms = syndrome
Down syndrome• Trisomy 21– 3 copies of chromosome 21– 1 in 700 children born in U.S.
• Chromosome 21 is the smallest human chromosome– but still severe effects
• Frequency of Down syndrome correlates with the age of the mother
Sex chromosomes abnormalities• Human development more tolerant of wrong
numbers in sex chromosome• But produces a variety of distinct syndromes
in humans– XXY = Klinefelter’s syndrome male – XXX = Trisomy X female– XYY = Jacob’s syndrome male– XO = Turner syndrome female
• XXY male– one in every 2000 live births– have male sex organs, but are
sterile– feminine characteristics• some breast development• lack of facial hair
– tall– normal intelligence
Klinefelter’s syndrome
Jacob’s syndrome male• XYY Males – 1 in 1000 live male
births– extra Y chromosome– slightly taller than
average– more active– normal intelligence, slight learning disabilities– delayed emotional maturity– normal sexual development
Trisomy X• XXX– 1 in every 2000 live births– produces healthy females• Why?• Barr bodies
– all but one X chromosome is inactivated
Turner syndrome• Monosomy X or X0– 1 in every 5000 births– varied degree of effects – webbed neck– short stature– sterile
Changes in chromosome structure• deletion– loss of a chromosomal segment
• duplication– repeat a segment
• inversion– reverses a segment
• translocation– move segment from one chromosome to
another
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Chromosomal errors VI
Deletion
Duplication
Inversion
Reciprocaltranslocation
Nonhomologouschromosomes
Homologouschromosomes
Genomic imprinting• Def: a parental effect on gene
expression• Identical alleles may have
different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.
• Fragile X syndrome: higher prevalence of disorder and retardation in males
Human disorders• The family
pedigree• Recessive
disorders: • •Cystic
fibrosis •Tay-Sachs•Sickle-cell
• Dominant disorders:•Huntington’s
• achondroplasia
Recessive diseases• The diseases are recessive because the allele
codes for either a malfunctioning protein or no protein at all– Heterozygotes (Aa)
• carriers
• have a normal phenotype because one “normal” allele produces enough of the required protein
Heterozygote crosses
Aa x Aa
A amale / sperm
A
afem
ale
/ eg
gs
AA
Aa aa
Aa
Aa
A
a
Aa
A
a
AA
Aa aa
Aa
• Heterozygotes as carriers of recessive alleles
carrier
carrier disease
Fig. 14-16
Parents
Normal Normal
Sperm
Eggs
Normal Normal(carrier)
Normal(carrier) Albino
Aa Aa
A
AAA
Aa
a
Aaaa
a
Cystic Fibrosis
• Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent
• The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes
• Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Pedigree Analysis
• A pedigree is a family tree that describes the interrelationships of parents and children across generations
• Inheritance patterns of particular traits can be traced and described using pedigrees
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
KeyMale
Female
AffectedmaleAffectedfemale
Mating
Offspring, inbirth order(first-born on left)
Pedigree analysis• Pedigree analysis reveals Mendelian
patterns in human inheritance– data mapped on a family tree
= male = female = male w/ trait = female w/ trait
Fig. 14-15b
1st generation(grandparents)
2nd generation(parents, aunts,and uncles)
3rd generation(two sisters)
Widow’s peak No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Ww ww
Ww Wwww ww
ww
wwWw
Ww
wwWW
Wwor
Human disorders
• Testing:•amniocentesis•chorionic
villus sampling (CVS)• Examination of the fetus
with ultrasound is another helpful technique
Fig. 14-18
Amniotic fluidwithdrawn
Fetus
Placenta
Uterus Cervix
Centrifugation
Fluid
Fetalcells
Severalhours
Severalweeks
Severalweeks
(a) Amniocentesis (b) Chorionic villus sampling (CVS)
Severalhours
Severalhours
Fetalcells
Bio-chemical
tests
Karyotyping
Placenta Chorionicvilli
Fetus
Suction tubeinsertedthroughcervix
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