1. Changes in Chromosome Structure

2. Chromosome Structure Mutations
Description
Duplication
Portion of the chromosome has been doubled
Deletion
Loss of chromosome segment
Inversion
A chromosomal segment is reversed (rotated 180o)
Translocation
A chromosomal segment is moved to another non-homologous chromosome

3. Mechanisms of Chromosome Mutation
Breakage and Rejoining
Unequal crossing-over

4. Duplications Type of Duplication Description/Example Adjacent-Tandem
Duplicated region is immediately adjacent to the original segment
A B C B C D
Adjacent-Reverse
Duplicated region is adjacent but the sequence is reversed
A B C C B D
Displaced
Duplication is located on a different part of the chromosome or on a different chromosome.

5. Meiotic Structures Type of Mutation Meiotic Structure in heterozygotes
Duplication
Duplication Loop
(Duplicated homolog forms loop, or both chromosomes form loops )

6. Consequences of Duplications
Altered phenotypes as a result of imbalances in amounts of gene products

7. Consequences of Duplications
Duplication can lead to other genetic changes
Alleles with new functions

8. Consequences of Duplications
Duplication can lead to other genetic changes
Higher degrees of duplication

9. Deletions Type of Deletion Description Terminal Interstitial
Loss of the end of a chromosome
Interstitial
Loss of internal portion of chromosome
Intragenic
Deletion within a gene, acts like a null mutation
Multigenic
Deletion of multiple genes, often harmful even in heterozygous condition

10. Meiotic Structures Type of Mutation Meiotic Structure in heterozygotes
Deletion
Deletion Loop
(Normal homolog forms loop)

11. Consequences of Deletions
Chromosome is lost if centromere is deleted.
Chromosomes with deletions do not revert to the wild type state.
Recombination frequencies between genes flanking the deletion are reduced.
Deletions are lethal in the homozygous state.

12. Consequences of Deletions
5. Multiple defects occur in heterozygotes due to
imbalances in the amounts of gene products
Cri du chat syndrome results from a deletion in chromosome 5
Solid tumors can accumulate deletions

13. Consequences of Deletions
5. Multiple defects occur in heterozygotes due to
pseudodominance = recessive alleles on the non-deleted homolog are expressed
haploinsufficiency = certain genes must be present in two copies for normal function
Wild Type Wing
Notch Wing

14. Inversions Type of Inversion Description Paracentric Pericentric
Centromere is outside inversion
Pericentric
Centromere is within inversion

15. Meiotic Structures Type of Mutation Meiotic Structure in heterozygotes
Inversion
Inversion Loop
(Both homologs form loop)

16. Consequences of Inversions
Generally viable since amount of genetic material is unchanged.
Leads to lethal mutation if breakpoint is within an essential gene.

17. Consequences of Inversions
Position Effect = Gene regulation can be altered by a difference in chromosome location
Recombination frequency is reduced due to inhibition of pairing and since crossover in heterozygotes yields additional structural mutations.

18. Crossing Over in a Paracentric Inversion Heterozygote

19. Crossing Over in a Pericentric Inversion Heterozygote
Anaphase II

20. Crossing Over in Inversion Heterozygotes
Type of Inversion
Crossover products
Paracentric
Dicentric bridge – breakage causes deletions
Acentric fragment – lost
Pericentric
Chromosomes with both duplications and deletions

21. Inversions in Evolutionary History

22. Translocations Type of Translocation Description Reciprocal
Exchange of chromosomal segments between non-homologous chromosomes
Non-reciprocal
Segment from one chromosome joins a non-homologous chromosome

23. Robertsonian Translocation
Long arms of two acrocentric chromosomes are joined to the same centromere
Results in one large chromosome and one small chromosome
Smaller chromosome may be lost

24. Inheritance of Down Syndrome from the carrier of a Robertsonian translocation, joining the long arms of chromosomes 14 and 21.

25. Consequences of Translocations
Behave as point mutations if the breakpoint disrupts an essential gene.
Show new linkage arrangements.
Position Effect = Gene expression is altered as a result of the new environment for translocated material.
Burkitt’s lymphoma is caused by a position effect.

26. Consequences of Translocations
Position-effect variegation of eye color in fruit flies due to translocation of w+ next to heterochromatin

27. Translocations found in cells of solid tumors

28. Consequences of Translocations
Leads to semisterility, where approximately half of the gametes carry structural mutations.
Meiotic Structures
Type of Mutation
Meiotic Structure in heterozygotes
Translocation
Cross
(Involves both homologs)

29. Meiotic Segregation in Translocation Heterozygotes

30. Meiotic Segregation in Translocation Heterozygotes