Types of Chromosome Mutations Chromosome Mutations Deletion Deficiency

Types of Chromosome Mutations

Chromosome Mutations Deletion/ Deficiency Duplication Inversion A B C D E F A B B C A B C D E F A E D C B F A B C D E F A B C D J K G H E F D E F Translocation I I

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.

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.

Chromatin and Gene Expression Heterochromatin • Contains methylated histones (H 3) • 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

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.

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.

Types of Chromosome Mutations

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.

Molecular Basis for Relationship between Genotype and Phenotype genotype DNA sequence transcription RNA translation protein function phenotype organism amino acid sequence