Transcript Chromosome Number Changes

Changes in Chromosome
Number
Chromosome Number Mutations
Type of Mutation
Aneuploidy
Difference from wild type
chromosome number
Abnormal number of
individual chromosomes
(addition or loss of a small number
of chromosomes)
Polyploidy
Abnormal number of
chromosome sets
Euploid = organism with multiples of the number
of chromosomes in a basic set
Aneuploid Numbers
Nullisomic
2n - 2
Monosomic 2n - 1
Trisomic
2n + 1
Tetrasomic
2n + 2
Missing both copies of
a homolog
Missing one copy of a
homolog
Having an extra copy
of one homolog
Having two extra
copies of one homolog
n = haploid number of chromosomes
2n = diploid number of chromosomes
Causes of Aneuploidy
1. Loss of a chromosome that has a
centromeric deletion
2. Loss of the small chromosome produced
by Robertsonian translocation
3. Nondisjunction = failure of homologs to
separate during meiosis or chromatids to
separate during meiosis or mitosis
Nondisjunction
Aneuploidy in Plants: Seed
Capsule Changes Due to
Trisomy in Jimson Weed
Human Autosomal Aneuploidy
•Most common type is Down Syndrome: Trisomy 21
•Arises in two ways
•Primary Down Syndrome
•Due to nondisjunction in egg formation
• Egg with 2 copies of #21 (24 chromosomes)
+ Sperm with 1 copy of #21 (23 chromosomes)
= Embryo with 3 copies of #21 (47 chromosomes)
•Familial Down Syndrome
•Inherited from a carrier of a Robertsonian
Translocation involving chromosome 21
Down Syndrome
Karyotype and
Physical Features
Eye fold
Palm Crease
Incidence of Primary Down Syndrome
Increases with Maternal Age
Robertsonian Translocation Carrier
of Down Syndrome
Translocation of
21q to 15q
Chromosome Segregation
in a Translocation Carrier
Expected Result: 1/3 carrier + 1/3 normal + 1/3 Down Syndrome
Human Sex Chromosome Aneuploidy
Condition
Turner
Syndrome
Klinefelter
Syndrome
Triple-X
Female
Genotype
XO
XXY
XXX
XYY
Phenotype
Human Sex Chromosome
Aneuploidy
Turner Syndrome
One Copy of X chromosome
No second sex chromosome
Eg. Egg with 0 copies of X (22 chromosomes)
+Sperm with 1 copy of X (23 chromosomes)
= Embryo with 1 copy of X (45 chromosomes)
Turner Syndrome
Karyotype and
Physical Features
Non-functional Ovaries
From Adult Female with
Turner Syndrome
Child with
Turner Syndrome
Normal uterus, tubes
and ovaries
Human Sex Chromosome
Aneuploidy
Klinefelter’s Syndrome
Two Copies of the X chromosome
One Copy of the Y chromosome
Eg. Egg with 2 copies of X (24 chromosomes)
+ Sperm with 1 copy of Y (23 chromosomes)
= Embryo with XXY
(47 chromosomes)
Klinefelter Syndrome
Karyotype and
Physical Features
X-Chromosome Inactivation in Females
•Inactivation of one of the X
chromosomes in each cell of an
adult female balances the
sex chromosome/autosome ratio.
• Either the maternal or paternal
chromosome is inactivated.
•The arrow shows a Barr body, or
inactivated X.
•The number of Barr bodies
equals the number of
X chromosomes minus one.
female
male
Polyploid Numbers
Three chromosome sets
Triploid
Four chromosome sets
Tetraploid
Five chromosome sets
Pentaploid
Six chromosome sets
Hexaploid
Polyploid = euploid organism with more than
two sets of chromosomes
Types of Polyploids
Autopolyploid Multiple chromosome
sets from within one
species
Allopolyploid Multiple chromosome
sets from different
species
Polyploidy Arises by Nondisjunction
Chromosome Segregation in Polyploids
Leads to Unbalanced Gametes
Production of
modern wheat