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Chapter 9

Chromosomal Variation

- Chromosomal mutations can arise in any cell of any
tissue. Remember that in order to be passed on (inheritied)
the mutation must be in germinal tissue.

- The result of changes in chromosomal structure is a
heterogeneous karyotype....one normal, one rearranged.

- Types of mutations:
> Deletion
> Duplication
> Inversion
> Translocation

- Changes due to:
1. “breaking” and rejoining
2. “illegitimate” recombination
Deletions:

1. Interstitial deletion- result of two breaks
2. Terminal deletion*- result of single break

A. Intragenic deletion- small deletion within gene
B. Multigenic deletion- large deletion involving two to
several thousand genes

Recognized by:
1. Reduced recombination frequency
2. Pseudodominance
3. Recessive lethality
4. Lack of reverse mutation
5. visible deletions
Pseudodominance - the sudden appearance of a recessive
phenotype in a pedigree, owing to a deletion of a masking
dominant gene.

Pseudodominance can be used to deletion map.
Higher order duplications
Bar Flies! ☺
Inversions: generally viable and usually show no particular
abnormalities. A break within a gene can result in lethal
or other abnormal phenotypes.

1. Paracentric inversion - centromere is outside inversion
2. Pericentric inversion - inversion spanning the centromere

Dicentric bridge - homologous centromeres are connected
Acentric fragment - fragment without a centromere

Diagnostic Features:
1. Reduction of recombination frequency
2. Reduced fertility due to unbalanced or
deleted meiotic products
3. Directly observed inverted chromosomal landmarks
4. Presence of inversion loops
Inversion loop:
Shown here is a
Paracentric inversion
that results in a
Dicentric bridge & an
Acentric fragment
Reciprocal translocations are diagnosed by:

1. Pseudolinkage of genes known to be on
separate chromosomes.

2. Semisterility.
Translocations:
Translocation:

1. Adjacent-1 segregation
T1 + N2 & T2 + N1

2. Alternate segregation
N1 + N2 & T1 + T2

3. Adjacent-2 segregation
N1 + T1 & N2 + T2

Homologous chromosomes
migrate to same pole
- very rare
Semisterility:
Polyploidy:

autopolyploidy - chromosome sets from same species

allopolyploidy - 2 or more chromosome sets from related,
but distinct species.

amphidiploid - organism produced by chromosomal doubling
(usually in allopolyploid)
Autopolyploidy:
Effect of genome doubling:
It is presumed that the amount
of gene product doubles as the
genome does.








Plants tolerate this!
Allopolyploid: 1928 G. Karpechenko
Wanted to make a hybrid that had the leaves of a
cabbage and the root of a radish…

He got the opposite but did
create a fertile plant by
spontaneous doubling.
Somatic allopolyploids from cell hybridization:
Plus other manipulations….
Modern innovative method of plant breeding…

1. enzymatically digest cell wall
stripped cell is called protoplast
2. cell suspensions are combined with
polyethylene glycol - fusion occurs.
3. fused cells proliferate on agar plates.
colonies are called calluses

Amphidiploidy:
A productive natural occurrence important in
speciation.
Amphidiploidy: Triticale Wheat x Rye
Wheat: success as a
hexaploid is ensured
by an allele called,
Ph. It was inherited
from the B set and
is located on the
long arm of Ch 5.
This allele suppresses
homeologous pairing.
Therefore diploid-like
behavior ensues.
Aneuploidy:
Somatic aneuploids: arise spontaneously in somatic
cells…. and result in mosaicism.
Gynandromorph: