AP Biology

Lecture #29Chromosomal Errors

2006-2007

Errors of MeiosisChromosomal Abnormalities

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

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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r of

repl

icati

oner

ror o

fcr

ossi

ng o

ver

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

Genetic counseling• Pedigree can help us understand the past &

predict the future• Thousands of genetic disorders are inherited

as simple recessive traits– from benign conditions to deadly diseases• albinism• cystic fibrosis• Tay sachs• sickle cell anemia• PKU