Transcript Brooker Chapter 8

Chromosome Structure
& Recombination
(CHAPTER 8- Brooker Text)
Nov 1 & 6, 2007
BIO 184
Dr. Tom Peavy
• Genetic variation refers to differences
between members of the same species or
those of different species
– Allelic variations are due to mutations in
particular genes
– Chromosomal aberrations are substantial
changes in chromosome structure
• These typically affect more than one gene
• They are also called chromosomal mutations
• The banding pattern is useful in several
ways:
– 1. It distinguishes Individual chromosomes
from each other
– 2. It detects changes in chromosome structure
– 3. It reveals evolutionary relationships among
the chromosomes of closely-related species
Structural Mutations of Chromosomes
• Deficiency (or deletion)
– The loss of a chromosomal segment
• Duplication
– The repetition of a chromosomal segment compared to
the normal parent chromosome
• Inversion
– A change in the direction of the genetic material along a
single chromosome
• Translocation
– A segment of one chromosome becomes attached to a
different chromosome
– Simple translocations
• One way transfer
– Reciprocal translocations
• Two way transfer
Deficiencies
• A chromosomal deficiency occurs when a
chromosome breaks and a fragment is lost
Figure 8.3
• Chromosomal deletions can be detected by a variety
of experimental techniques
– Cytological, Molecular (probes) & Genetic analysis
Genetic
• Deletions can be revealed by a phenomenon
known as pseudodominance
– One copy of a gene is deleted
– So the recessive allele on the other chromosome is now
expressed
Duplications
• A chromosomal duplication is usually caused by
abnormal events during recombination
Figure 8.5
Genes derived
from a single
ancestral gene
Figure 8.9
Inversions
• A chromosomal inversion is a segment that has
been flipped to the opposite orientation
Centromere lies
within inverted
region
Figure 8.11
Centromere lies
outside inverted
region
Inversion Heterozygotes
• Individuals with one copy of a normal chromosome and one
copy of an inverted chromosome
• Such individuals may be phenotypically normal
– They also may have a high probability of producing
gametes that are abnormal in their genetic content
• The abnormality is due to crossing-over in the inverted
segment
Translocations
• A chromosomal translocation occurs when a
segment of one chromosome becomes attached to
another
• In reciprocal translocations two non-homologous
chromosomes exchange genetic material
=Balanced translocations
• In simple translocations the transfer of genetic
material occurs in only one direction
=Unbalanced translocations
• Unbalanced translocations are associated with
phenotypic abnormalities or even lethality
• Example: Familial Down Syndrome
– In this condition, the majority of chromosome 21 is
attached to chromosome 14
– The individual would have three copies of genes found
on a large segment of chromosome 21
• Therefore, they exhibit the characteristics of Down
syndrome
• Familial Down Syndrome is an example of
Robertsonian translocation
• This translocation occurs as such
– Breaks occur at the extreme ends of the short arms of
two non-homologous acrocentric chromosomes
– The small acentric fragments are lost
– The larger fragments fuse at their centromeic regions to
form a single chromosome
• This type of translocation is the most common type
of chromosomal rearrangement in humans
Balanced Translocations and
Gamete Production
• Individuals carrying balanced translocations have a
greater risk of producing gametes with unbalanced
combinations of chromosomes
– This depends on the segregation pattern during meiosis I
• During meiosis I, homologous chromosomes
synapse with each other
– For the translocated chromosome to synapse properly, a
translocation cross must form
– Refer to Figure 8.15
• Meiotic segregation can occur in one of three ways
– 1. Balanced segregation (Alternate)
• Chromosomes on opposite sides of the translocation cross
segregate into the same cell
• Leads to balanced gametes
– Both contain a complete set of genes and are thus viable
– 2. Non-homologous segregation (Adjacent-1)
• Adjacent non-homologous chromosomes segregate into the
same cell
• Leads to unbalanced gametes
– Both have duplications and deletions and are thus inviable
– 3. Homologous segregation (Adjacent-2)
• Adjacent homologous chromosomes segregate into the same
cell
• Leads to unbalanced gametes
– Both have duplications and deletions and are thus inviable
Figure 8.15
Balanced
Non-Homologous
Homologous
• Balanced and Non-homologous type segregations
are the likely outcomes when an individual carries a
reciprocal translocation
– Indeed, these occur at about the same frequency
• Homologous type segregation is very rare
• Therefore, an individual with a reciprocal
translocation usually produces four types of
gametes
– Half of which are viable (due to balanced types) and the
other half non-viable (due to non-homologous types)
– This condition is termed semisterility
Figure 8.15