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Ch7. Chromosome Mutation Variation in Number and Arrangement

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.

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Ch7. Chromosome Mutation Variation in Number and Arrangement

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  1. Ch7. Chromosome MutationVariation in Number and Arrangement 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.

  2. Taken together, such changes are called chromosome mutations or chromosome aberrations, to distinguish them from gene mutations. • Because, according to Mendelian laws, the chromosome is the unit of genetic transmission, chromosome aberrations are passed on to offspring in a predictable manner, resulting in many unique genetic outcomes.

  3. 7.1 Variation in Chromosome Number :An Overview • 7.2 Nondisjunction :The origin of Aneupoidy • 7.3 Monosomy • 7.4 Trisomy • 7.5 Polypoidy and its Origin • 7.6 Variation in Chromosome Structure and Arrangement: An Overview • 7.7 Deletion • 7.8 Duplication • 7.9 Inversion • 7.10 Translocation • 7.11 Fragile Sites In Humans

  4. A change in Genetic material---mutation • Numbers of 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 • Rearrangement (Structural alterations) of chromosomes Deletion Duplication Translocation inversion

  5. 1.Variation in Chromosome Number:Overview • Changes in chromosome number can occur by the addition of or the loss of all or part of an entire set of chromosomes • Aneuploidy: the loss or the gain of one or more of normal set chromosomes. Each of these conditions is a variation on the normal diploid number of chromosomes. • EuploidyChanges by an entire set of chromosomes ,Euploidy - an entire set of chromosomes is duplicated once or several times

  6. Euploidy

  7. Species A 2n=AA Species B 2n=BB Species C 2n=CC F1 2n=AB Autotriploid 3n=AAA Autotetraploid 4n=AAAA Triploid 3n=ABB Triploid 3n=ABC Allotetraploid 4n=AABB Autohexaploid 6n=AAAAAA Autoallohexaploid 6n=AABBBB Autopetaploid 5n=AAAAA Allohexaploid 6n=AABBCC Autoallooctaploid 8n=AAAABBBB

  8. Aneuploidy

  9. In addition to these conditions, more than one pair of homologous chromosomes may be involved. For example, • a double monosomic is missing one chromosome from each of two pair of homologous chromosome (designated 2N-1-1), • a double tetrasomic contains an extra pair of two pairs of homologous chromosomes (2N+2+2).

  10. telocentrics which are chromosomes that have a terminal centromere. These structures represent chromosomes that are missing the genetic material beyond that centromere. (Stocks containing these types of chromosomes are called • monotelosomics or monotelos (单端体）for short. • isochromosome which is a chromosome that contains the same genetic material on both arms.

  11. 2.Nondisjunction（不分离） :The origin of Aneuploidy • The development of aneuploids is not well understood, but they may have arisen by a process called nondisjunction. Nondisjunction occurs when paired chromosomes do not separate either during meiosis I or meiosis II. The direct result of this event is that gametes develop that have too few or too many chromosomes. If this occurs during meiosis I normal gametes are not developed, and if it occurs during meiosis II half of the gametes will be normal and the other half will be abnormal.

  12. Non-disjunction can also occur during mitosis and the result is an individual that expresses chromosomal mosaicism(嵌合体). • If this occurs during the early stages of cell divisions different portions of the body which descended from the different altered cells will beformed. • (See figure 19.20.) Many sex chromosome mosaics have been detected for example X/XX, X/XY, XX/XY and XXX/XXXXY. Mild to severe phenotypic symptoms have been associated with these mosaics

  13. 7.3 Monosomy • We turn now to a consideration of variations in the number of autosomes and the genetic consequence of such changes. The most common examples of aneuploidy where an organism has a chromosome number other than an exact multiple of the haploid set, are cases in which a single chromosome is either added to, or lost from, a normal diploid set. The loss of one chromosome produces a 2n - 1 complement and is called monosomy. • Although monosomy for the X chromosome occurs in humans, as we have seen in 45,X Turner syndrome, monosomy for any of the autosomes is not usually tolerated in human or other animals.

  14. Monosomy： • 大多数单体动植物不能存活。象玉米、番茄等植物，尽管单倍体能存活，但单体却不能成活，说明遗传物质的平衡很重要。 • 多倍体植物中单体容易得到，并能存活。例如异源六倍体小麦2n=42，理论上可以得到21种单体，并且事实上也确实如此 • 单体自交可得缺体（2n-II）。小麦利用21种单体，获得了21种缺体 。 • 利用单体和缺体可以把基因定位在某个染色体上 • 理论上,单体2n-I减数分裂时，可形成n和n-1两种配子。自交产生二倍体、单体、缺体三种类型。。

  15. 实际上, • 单体丢失 (2)n-1 与n配子参与受精的能力 (3)2n-II 和2n-1的胚胎发育

  16. The failure of monosomic individuals to survive in many animal species is at first quite puzzling, since at least a single copy of every gene is present in the remaining homolog. • However, if just one of those genes is represented by a lethal allele, the unpaired chromosome condition leads to the death of the organism. • This occurs because monosomy unmasks recessive lethals that are tolerated in heterozygotes carrying the corresponding wild-type alleles.

  17. Aneuploidy is better tolerated in the plant kingdom. • Monosomy for autosomal chromosomes has been observed in maize, tobacco, the evening primrose Oenoihera, and the Jimson weed Datura, among other plants. • Nevertheless, such monosomic plants are usually less viable than their diploid derivatives. • Haploid pollen grains, which undergo extensive development before participating in fertilization, are particularly sensitive to the lack of one chromosome and are seldom viable.

  18. Cri-du-Chat Syndrome 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

  19. 7.4 Trisomy • 三倍体自交或与二倍体杂交，由于三倍体可产生n+1或n两种配子，两种配子结合则可产生三体。另外自然条件下染色体分裂异常也可以产生三体。 • 三体减数分裂理论上可产生n+1或n两种配子。由于多一条染色体的配子在减数分裂后期I常有落后的现象，n+1配子通常少于50%，而且n+1雄性配子成活率低，很少与雌配子结合。n+1 配子通常由卵细胞传递。 • 三体若为二显性，与正常二倍体隐性测交比例为5：1，而非1：1。

  20. Genetics of Aneuploids in fruit fly • Monosomics and trisomics are usually inviable in Drosophila. The exception are those aneuploids involving chromosome IV. Therefore the effects of aneuploidy on inheritance has been investigated using stocks with altered chromosome IV. • haplo-IV (monosomic for chromosome IV; 2n-1); • diplo-IV (normal; 2n) and • triplo- IV (trisomic for chromosome IV; 2n+1). Located on chromosome IV is the gene for eyeless (ey) that is recessive to normal eye. The following crosses illustrate several genetic principles of aneuploids. The first is a cross between a diplo-IV eyeless female (ey ey) and a haplo-IV normal eye male (ey+).

  21. A1A2a 3 (A1A2 a3 A1a3 A2 ) X 2 (a2A1 A2 ) x 2 Aaa

  22. 例如： P a/a x +/+/+ F1 a/+/+ 与 a/a测交 配子 2/6 a/+ 2/6 + 1/6 +/+ 1/6 a 5： 1

  23. eyeless female (ey ey) and a haplo-IV normal eye male (ey+).

  24. PATAU syndrome = trisomy 13 - 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

  25. DOWN syndrome = trisomy 21

  26. FIGURE 7-5 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.

  27. FIGURE 7—4 Incidence of Down syndrome births contrasted with maternal age.

  28. 7.5 Polyploidy and Its OriginsEuploidy: • 早在19世纪末，狄．弗里斯在月见草（Oenothera lamarckiana）中发现一种比普通月见草的组织器官大得多的变异型，并在1901年把他定为巨型月见草（O,新种） • 当时弗里斯在以为巨月件草是普通月见草通过基因突变而产生的。后来（1907，1909）细胞学的研究得知，月见草的核染色体数是28个（2n=28)，正好比普通的月见草 的染色体数（2n=14)多一倍。 • 这就启发人们开始认识染色体数目的变异可以导致遗传性状的变异，特点是按一个基本的染色体数目（基数）成倍的增加或减少。此后，细胞学的研究发现当初按形态分类所划定的不同属或不同种，有许多是和染色体数目的变异联系着的。

  29. 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: • A triploid has 3n chromosomes; • a tetraploid has 4n; • a pentaploid, 5n; and so forth. • Several general statements can be made about polyploidy. This condition is relatively infrequent in many animal species, but is well known in lizards, amphibians, and fish.

  30. It is much more common in plant species. • Odd numbers of chromosome sets are not usually maintained reliably from generation to generation, because a polyploid organism with an uneven number of homologs often does not produce genetically balanced gametes. • For this reason, triploids, pentaploids, and so on, are not usually found in plant species that depend solely on sexual reproduction for propagation.

  31. Polyploidy originates in two ways: • (1) The addition of one or more extra sets of chromosomes, identical to the normal haploid complement of the same species, resulting in autopolyploidy; and • (2) the combination of chromosome sets from different species occurring as a consequence of hybridization, resulting in allopolyploidy (from the Greek word allo, meaning other or different). The distinction between auto- and allopolyploidy is based on the genetic origin of the extra chromosome sets, as shown in Figure 7-6.

  32. Euploidy in Animals • A genome that contains three or more full copies of the haploid chromosome number are polyploid. As a general rule polyploids can be tolerated in plants, but are rarely found in animals. One reason is that the sex balance is important in animals and variation from the diploid number results in sterility. • Those few animals, such as brine shrimp, that avoid the hazards of polyploidy, utilize parthenogenesis, the development of the an individual from an egg without fertilization, to initiate embryo development.

  33. Euploidy in plants • Before we discuss polyploidy in plants in detail, first a distinction must be made between the two major classes of polyploids, autopolyploids and allopolyploids. • The following definitions will rely on these chromosomal descriptions. Two species will be considered, A and B. The chromosomal compositions of one species is: A = a1 + a2 + a3 . . . an • where a1, a2, etc. represent individual chromosomes and n is the haploid chromosome number.

  34. The chromosomal composition of the second species will be: • B = b1 + b2 + b3 . . . bn • Autopolyploid - an individual that has an additional set of chromosomes that are identical to parental species; • an autotriploid would have the chromosomal composition of AAA and an autotetraploid would be AAAA; both of these are in comparison to the diploid with the chromosomal composition of AA

  35. 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. • Triploids could also be produced by mating a diploid (gametes = n) with a tetraploid (gametes = 2n) to produce an individual that is 3n. The difficulty arises when autotriploids try to mate because unbalanced gametes are produced because of pairing problems with the additional chromosome set. Thus, these are invariably sterile.

  36. Allopolyploid - an individual that has an additional set of chromosomes derived from another species; these typically occur after chromosomal doubling and their chromosomal composition would be AABB; if both species have the same number of chromosomes then the derived species would be an allotetraploid

  37. Autotetraploids occur from a doubling of the chromosomal composition. This can occur naturally by doubling sometime during the life cycle or artificially through the application of heat, cold or the chemical colchicine秋水仙素. Because an additional set of chromosomes exists, autotetraploids can undergo normal meiosis.

  38. FIGURE 7-6 Contrasting chromosome origins of an autopolyploid versus an allopolyploid karyotype.

  39. 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.

  40. 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.

  41. Monoploidy • An individual that contains one half the normal number of chromosomes is a monoploid and exhibits monoploidy. Monoploids are very rare in nature because recessive lethal mutations become unmasked, and thus they die before they are detected. These alleles normally are not a problem in diploids because their effects are masked by dominant alleles in the genome. Some species such as bees, ants and male bees are normally monoploid because they develop from unfertilized eggs. • A stage in the life cycle of some fungal species can also be monoploid. • Consequently, these individuals will be sterile.

  42. 只含有一个染色体组，也就存在一套基因。它们可能是正常的也可能是异常的。只含有一个染色体组，也就存在一套基因。它们可能是正常的也可能是异常的。 • 正常的单倍体生物：低等生物的配子体，一些昆虫的雄体（蜜蜂等膜翅目），都是正常的的单倍体；有些动植物中也得到过单倍体，形成的配子也是单倍体。 • 异常的单倍体生物：二倍体生物成为单倍体后，情况则不同，通常形成的配子高度不育。因此单倍体通常是不育的，也就很难留下后代，得以保存。 • 其原因是形成正常具有完整的一套染色体组的配子的几率很低，取决于染色体数目，几率为（1/2）n

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