Types of Chromosome Mutations

Chromosome Mutations

A B C D E F A C D E F

A B C D E F A B B C D E F

A B C D E F A E D C B F

A B C D E F

G H I J K

A B C D J K

G H I E F

Deletion/Deficiency

Duplication

Inversion

Translocation

Inversions

A B C D E F G H

A B C D E F G H

A B F E D C G H

A B C D E G F H

pericentric inversion

paracentric inversion

A heterozygote for a normal chromosome and an inversion will form an inversion loop during meiosis.

The number of recombinant products is reduced in inversion heterozygotes by:

1) elimination of crossing over products within the inversion loop, and

2) inhibition of pairing between homologues in the region of the inversion.

Pairing in paracentric inversion heterozygotes and resulting meiotic products.

Anaphase bridge results in random breakage of chromosomal material.

2 of 4 meiotic products are not genetically balanced and will not produce viable gametes.

Pairing in pericentric inversion heterozygotes

and resulting meiotic products.

Possible effects of inversion at the molecular level

No disruption of any gene.

Chromosomal rearrangement is the only result.

Disruption of one gene by chromosomal breakage.

Disruption of two genes and fusion of those two genes.

Chromosome Mutations

A B C D E F A C D E F

A B C D E F A B B C D E F

A B C D E F A E D C B F

A B C D E F

G H I J K

A B C D J K

G H I E F

Deletion/Deficiency

Duplication

Inversion

Translocation

Translocation

A B C D E F G H G C D E F A B H

In reciprocal translocation, exchange of chromosomal segments between two nonhomologous chromosomes establishes new linkage groups.

In Robertsonian translocation, long arms of two acrocentric chromosomes are combined to form one large chromosome and one small chromosome.

If the short metacentric chromosome does not contain essential genetic information, it could be lost without any consequence to viability.

Reciprocal Translocation

Reciprocal translocationheterozygotes 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.

• 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.

Down Syndrome and Translocation Heterozygote

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

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.

Gene Silencing is Caused by the Spread of Heterochromatin

Heterochromatin May Spread Farther in Some Cells Than in Others

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.