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 Centromere lies within inverted outside inverted region 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 •Inreciprocal translocations two non-homologous chromosomes exchange genetic material =Balanced translocations •Insimple 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 11.