1. DNA and the Genome
Key Area 6c
Chromosome Mutations

2. (c) Chromosome structure mutations
Learning Intentions
By the end of this topic you should be able to:
(c) Chromosome structure mutations
Define ‘chromosome structure mutation’
Name 4 chromosome structure mutations
Describe 4 chromosome structure mutations
Describe the effects of each of the 4 chromosome structure mutations

3. Chromosome Mutations: Change in structure of the chromosome
….a change in the number or sequence of genes in a chromosome:
Deletion
Duplication
Translocation
Inversion

4. Deletion – Section of a chromosome is removed
The chromosome breaks in 2 places, the segment in between breaks off and the 2 ends then join back together
The chromosome is now shorter and lacks genes, which has a drastic effect on the organism (often lethal)
e.g. cri du chat syndrome in humans is caused by deletion of part of chromosome 5 and results in a small head, widely spaced eyes and metal retardation

5. Deletion
As one chromosome is now lacking the gene (or genes), the allele present on the homologous chromosome will show in the phenotype
This means, even if the allele still present is the recessive form, the organism will be affected as it is the only gene (allele) there is

6. Duplication – section of a chromosome is added from its homologus partner
A set of genes becomes repeated.
This usually happens when a section of one chromosome of a homologous pair is duplicated and is then inserted and attached into the other chromosome.

7. Duplication
Even though it is only some of the genes that are duplicate, it can result in symptoms seen in Down’s…
ie there will be 3 copies of one (or more genes) present in the organism (see genes C & D below)

8. Translocation – a section of a chromosome is added to another chromosome – not its homologous partner
A section of one chromosome becomes broken off and attaches to another chromosome that is NOT its homologous partner.

10. Inversion – a section of a chromosome is reversed
The chromosome breaks in 2 places, the section in between turns around and then joins up again.
This causes the sequence of genes to be reversed, which causes problems at meiosis (producing sex cells) and production of non-viable gametes.

12. Non-reciprocal translocation

13. Reciprocal translocation