Loading in 2 Seconds...
Loading in 2 Seconds...
Ch7. Chromosome Mutation Variation in Number and Arrangement. Although most members of diploid species normally contain precisely two haploid chromosome sets, many known cases vary from this pattern.
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Although most members of diploid species normally contain precisely two haploid chromosome sets, many known cases vary from this pattern.
Modifications include a change in the total number of chromosomes, and changes of chromosomal structures, the deletion or duplication of genes or segments of a chromosome, and rearrangements of the genetic material either within or among chromosomes.
Euploidy(整倍体): monoploidy and polyploidy
polyploidy :Autopolyploidy(同源) Allopolyploids(异源)
Aneuploidy(非整倍体): loss 2n- 1 or more and add 2n+1 or more of the whole set of chromosomes
In humans, autosomal monosomy has not been reported beyond birth. There are, however, examples of survivors where only part of one chromosome is lost. These cases are sometimes referred to as segmental deletions.
One such case was first reported by Jerome LeJeune in 1963 when he described the clinical symptoms of the cri-du-chat (cry of the cat) syndrome. This syndrome is associated with the loss of part of the short arm of chromosome 5 (Figure 7-2). of
Thus, the genetic constitution may be designated as 46,5p-, meaning that such an individual has all 46 chromosomes but that some
F1 a/+/+ 与 a/a测交
配子 2/6 a/+ 2/6 + 1/6 +/+ 1/6 a
- facial defects, polydactyly, heart defects, die within a few months of birth
EDWARDS syndrome = trisomy 18
- small + muliple defects, usually die in first year of life
DOWN syndrome = trisomy 21
The karyotype and phenotypic depiction of an infantwith Patau syndrome, where three members of the D-group chromosome 13 are present, creating the 47,13+ condition.
FIGURE 7—4 Incidence of Down syndrome births contrasted with maternal age.
The term polyploidy describes instances in which more than two multiples of the haploid chromosome set are found. The naming of polyploids is based on the number of sets of chromosomes found:
An autotriploid could occur if a normal gamete (n) unites with a gamete that has not undergone a reduction and is thus 2n. The zygote would be 3n.
FIGURE 7-6 Contrasting chromosome origins of an autopolyploid versus an allopolyploid karyotype.
FIGURE 7-7 The potential involvement of colchicine(秋水仙素) in doubling the chromosome number, as occurs during the production of an autotetraploid. Two pairs of homologous chromosomes are followed. While each chromosome has replicated its DNA earlier during interphase, the chromosomes do not appear as double structures until late prophase. When anaphase fails to occur normally, the chromosome number doubles if the cell reenters interphase.
FIGURE 7-9 The origin and propagation of an amphidiploid(双二倍体).
Species I contains genome A consisting of three distinct chromosomes, a,, a2, and a3.
Species 2 contains genome 6 consisting of two distinct chromosomes, b1 and b2.
Following fertilization between members of the two species and chromosome doubling, a fertile amphidiploid containing two complete diploid genomes (AABB) is formed.
The plants that are derived from this tissue will be monoploid, and the genetics of these individuals can be studied or they can be treated with a chemical to double the chromosome number.
三显性 AAAa 1AA：1Aa all A
二显性 AAaa 1AA：4Aa：1aa 35A：1a
单显性 Aaaa 1Aa：1aa 3A：1a
种 学名 亚种 染色体数
二倍体小麦 滔氏麦草 T tauschii 14
斯氏麦草 T searsii 14
一粒小麦 T monocum 野生种 14
四倍体小麦 二粒小麦 T dicocum 28
提莫非维 T timopheari 28
茹科夫斯基 T zhukovskyi 42
普通小麦 T aestivum 许多种 42
To describe these species it is necessary to introduce the final symbol X.
Wild potato (2n=24)Cultivated Potato (2n=48)
Wild Cotton (2n=26)Cultivated Cotton (2n=52)
Dahlia (2n=32)Garden Dahlia (2n=64)
Wild Tobacco (2n=24)Cultivated Tobacco (2n=48)
For some plant species a series of successive ploidy levels are seen.
The New World cotton species Gossypium hirsutum has a 2n chromosome number of 52. The Old World species G. thurberiand G. herbaceum each have a 2n number of 26. Hybrids between these species show the following chromosome pairing arrangements at meiosis:
The second general class of chromosome aberrations includes structural changes that delete, add, or rearrange substantial portions of one or more chromosomes
FIGURE 7-11 The origin of duplicated and deficient regions of chromosomes as a result of unequal crossing over. The tetrad on the left is mispaired during synapsis. A single crossover between chromatids 2 and 3 results in deficient and duplicated chromosomal regions (see chromosomes 2 and 3, respectively, on the right).The two chromosomes uninvolved in the crossover event remain normal in their gene sequence and content
segments on homologue
Although many gene products are not needed in every cell of an organism, other gene products are known to be essential components of all cells. For example, ribosomal RNA mustbe present in abundance in order to support protein synthesis. The more metabolically active a cell is, the higher is the demand for this molecule. We might hypothesize that a single copy of the gene encoding rRNA is inadequate in many cells. Studies using the technique of molecular hybridization, which allows the determination of the percentage of the genome coding for specific RNA sequences, show that our hypothesis is correct! Indeed, multiple copies of genes code for rRNA. Such DNA is called rDNA, and the general phenomenon is called gene redundancy.
A B C D E F G H
A B F E D C G H
D / +
D+/++ × +Gl/++
++/++ D+/++ D+/+G l ++/+Gl
D+/D+ D+/+Gl +Gl/+Gl
Cy + Cy +
+ A + A
Cy + Cy + + A
Cy + + A + A
（Activator-Dis sociation system）
Cytogenetic studies of the parents and their offspring from these unusual cases explain the cause of familial Down syndrome.
During meiosis, one-fourth of the individual's gametes have two copies of chromosome 21: a normal chromosome and a second copy translocated to chromosome 14. When such a gamete is fertilized by a standard haploid gamete, the resulting zygote has 46 chromosomes but three copies of chromosome 21. These individuals exhibit Down syndrome. Other potential surviving offspring contain either the standard diploid genome (without a translocation) or the balanced translocation like the parent. Both cases result in normal individuals. Knowledge of translocations has allowed geneticists to resolve the seeming paradox of an inherited trisomic phenotype in an individual with an apparent diploid number of chromosomes.
This syndrome affects about 1 in 1250 males and 1 in 2500 females. Because it is a dominant trait, females carrying only one fragile X chromosome can be mentally retarded. Fortunately, the trait is not fully expressed, as only about 30 percent of fragile X females are retarded, whereas about 80 percent of fragile X males are mentally retarded.