Chromosome Structure & Recombination (CHAPTER 8- Brooker Text) April 4 & 9, 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 1
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 2
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 3
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 4
Inversions A chromosomal inversion is a segment that has been flipped to the opposite orientation Centromere lies within inverted region Centromere lies outside inverted region Figure 8.11 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 5
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 6
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 7
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 8
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 9
Figure 8.15 10
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