Types of Chromosome Mutations

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Types of Chromosome Mutations
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Chromosome Mutations
A B C
D E F
A C
D E F
Deletion/ Deficiency
A B C
D E F
A B B C
D E F
Duplication
A B C
D E F
A E D
C B F
Inversion
A B C
D E F
A B C
D J K
Translocation
G H I
J K
G H I
E F
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Reciprocal Translocation
Reciprocal translocation heterozygotes are
semisterile. 50 of gametes are genetically
unbalanced. In plants, these gametes are not
viable. In animals, zygotes that are formed by
these gametes are not viable. Adjacent
segregation produces genetically unbalanced
gametes. Alternate segregation produces
genetically balanced gametes.
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Down Syndrome and Translocation Heterozygote

• Down syndrome is caused by trisomy 21 (3 copies
  of chromosome 21).
• 95 of Down syndrome cases are associated with
  nondisjunction and shows no familial recurrence.
  
• The other 5 (familial Down syndrome) is
  attributed to Robertsonian translocation between
  chromosome 21 and chromosome 14.

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Chromatin and Gene Expression

• Heterochromatin
• Contains methylated histones (H3)
• Associated with heterochromatin protein-1 (HP-1)
• Transcriptionally Active Euchromatin
• Contains hyperacetylated histones
• Prevention of Heterochromatin Formation
• DNA elements (barrier insulators) promote binding
  of histone acteyltransferase

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Gene Silencing is Caused by the Spread of
Heterochromatin
When a chromosome mutation places a gene next to
heterochromatin, the gene can become inactivated.

Inversion, deletion, duplication, and
translocation can place a gene next to
heterochromatin.
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Heterochromatin May Spread Farther in Some Cells
Than in Others
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Position-effect Variegation
A heterozygote for a gene and a translocation can
show variegated phenotype for that
gene. Position-effect variegation is exhibited
by this w/w heterozygote. Wild-type allele is
no longer wild-type in its expression in some of
the eye facets. Any chromosomal change that
places a locus next to heterochromatin can result
in inactivation of that gene. A tissue or organ
that is comprised of a mixture of cells that
express one or the other phenotype exhibit this
variegation.
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Types of Chromosome Mutations
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The fates of a million implanted human zygotes
50 of spontaneous abortions are associated with
chromosome abnormalities. Over half of these are
trisomics, with trisomy 21 accounting for 9 of
them. 18 of abnormalities are the Turner
condition.
0.6 of birthed infants have chromosome
abnormalities. 36 of birthed infants with
chromosome abnormalities are aneuploidy of sex
chromosome. 23 of birthed infants with
chromosome abnormalities are trisomics.
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Molecular Basis for Relationship between Genotype
and Phenotype
DNA
genotype
DNA sequence
transcription
RNA
translation
amino acid sequence
protein
function
organism
phenotype