How many duplications in meiosis

The first reason is that the complex pattern of colinearity—in which one region of a maize chromosome shares colinearity with several different chromosomes Fig. The multicopy proportion of the genome can be estimated, just as the duplicated proportion of the genome was estimated see above. With this approach, 8. With the Nadeau and Taylor correction 25 , these numbers increase to Thus, roughly one-tenth to one-third of the maize genome is multicopy.

This is not the first study to suggest that maize genomic regions are triplicated Helentjaris et al. Multicopy regions can be produced by a tetraploid event between diploid progenitors that contain duplicated regions Wilson et al.

The second reason is that small, streamlined genomes such as those of rice and Arabidopsis contain duplicated regions. More recent studies suggest that a far greater amount of the Arabidopsis genome is duplicated Blanc et al. Given the prevalence of multicopy regions in maize and recent information about Arabidopsis , it seems likely that the two diploid progenitors of maize contained extensive duplications. It is difficult to assess the effect of these duplications on the estimated rate of chromosomal rearrangement in maize.

On the one hand, this study has probably underestimated the number of rearrangements, due to conservative assumptions. On the other hand, rearrangements could have occurred in the diploid progenitors, far before the tetraploid event, and thus the rate of rearrangement could be overestimated.

In the end, more accurate inferences about rates of chromosomal rearrangement and the extent of multicopy regions will require additional data, such as detailed physical maps, extensive DNA sequence data, or genetic maps based on moderate-copy as opposed to low-copy markers. Nonetheless, this work provides a rough estimate of the rate of chromosomal rearrangement in the maize genome, and it also has shown that the maize genome has a complex organization typified by a substantial proportion of multicopy regions.

Two important questions remain. First, what mechanisms have acted to disrupt colinearity in the maize genome? Asymmetric colinearity between chromosomes—for example, asymmetry between chromosomes 3 and 9 Fig.

Nevertheless, the pattern of colinearity suggests that large chromosomal segments have been translocated, but the mechanisms underlying translocation are presently unclear. Second, the extent of chromosomal duplication raises questions about functional differentiation of duplicated genes. More specifically, what proportion of duplicated genes is lost and what proportion remains functional? This question has received much attention in the evolution literature.

For example, theoretical models predict that most duplicated genes will be lost Nei and Roychoudhury ; Takahata and Maruyama ; Walsh , but empirical studies suggest more duplicate genes retain function than predicted by theory Force et al. Alternative fates for duplicated genes include retention of original function Ohno , evolution of new or altered expression patterns Force et al.

Additional insight into this question requires detailed functional studies of duplicate gene pairs. Note, however, that maize could be a useful system for studying on a broad scale the evolutionary fate of duplicated genes.

Inferences about the maize genome have been based on the colinearity test, which has both advantages and disadvantages. One advantage is that the method requires few assumptions about either genome or marker evolution. Another advantage is objectivity, in that the method does not rely on an ad hoc number of markers to ascertain evidence for chromosomal duplications.

A third advantage is that the method uses both centimorgan distances and the number of markers in a run as criteria to evaluate colinearity, although physical rather than genetic distances are more desirable when available. The disadvantages include a potential lack of statistical power, but the fact that the method identifies all but one of the duplications noted in comparative maps suggests it is reasonably powerful.

A second weakness is the emphasis on nonoverlapping runs, which could make the method overly conservative. The general applicability of the colinearity test has yet to be determined, but a similar approach can be applied to other mapped plant genomes that contain extensive chromosomal duplications, such as soybean Grant et al. The approach can also be applied across species—for example, a rice chromosome could be used as a standard to compare with all 10 maize chromosomes.

Note that the availability of full genome sequences and dense genetic maps does not obviate the need for objective statistical approaches to detect colinear regions.

For example, Grant et al. The current evolutionary paradigm for grasses, based on comparative map data, asserts that: 1 Gross chromosomal organization has remained largely conserved during 60 million years of grass evolution, 2 30 rice linkage blocks adequately represent extant grass genomes, and 3 homologous blocks will prove useful for predicting the position of genes conferring key agronomic traits Devos and Gale The present study suggests that this paradigm needs to be modified somewhat for maize.

Second, the extent of multicopy regions within the maize genome suggests that accurate recognition of block homologies between maize and other grasses may be a more daunting task than previously appreciated. The question remains as to the best way to unravel grass genome relationships, particularly given the complexity of the maize genome. At present, two separate and sometimes complementary approaches are used to study grass genomes.

The first is comparative mapping. Despite the limitations of marker-based maps Bennetzen , marker-based mapping is still the most accessible way to gain a broad overview of whole-genome or nearly whole-genome organization. However, comparative maps often ignore species-specific data in favor of cross-species markers. A useful and efficient alternative may be to focus first on chromosomal relationships within a species—as I have done here in maize—and then to build within-species information into cross-species comparisons.

At the very least, however, a within-species first approach will use existing map data more efficiently. The second approach used to study grass genomes is the microsynteny, or DNA sequencing, approach for review, see Bennetzen This approach is invaluable because it provides detailed insights into rearrangement at the molecular level. The corresponding drawback is that microsynteny studies fail to provide a whole-genome view. Until whole-genome sequences and physical maps are available from multiple grass species, additional analyses of marker-based maps may be the best source for additional insights into grass genome organization and evolution.

I am grateful to S. Muse for discussion and to M. Le Thierry d'Ennequin, L. Eguiarte, P. Tiffin, L. Zhang, M. Clegg, J. Wendel and an anonymous reviewer for comments. The publication costs of this article were defrayed in part by payment of page charges.

Article and publication are at www. View all Previous Section Next Section. Figure 1. Figure 2. Figure 3. View this table: In this window In a new window. Table 1. Figure 4. Table 2. Figure 5. View larger version: In this window In a new window. Previous Section. Ahn S. CrossRef Medline Google Scholar. Anderson E. A report of progress. Google Scholar.

Bennetzen J. Plant Cell 12 : — Trends Genet. Genome Res. Blanc G. Brubaker C. Genome 42 : — CrossRef Google Scholar. Davis G. Genetics : — Devos K. Plant Mol. Plant Cell 12 : — , ibid. Ehrlich J.

Force A. Gale M. Science : 65 — , ibid. Galitski T. Science : — Gaut B. Goodman M. Genetics 96 : — Numbering begins from the centromere and continues outward to the end of each arm. Conventionally, the arms are divided into a number of regions by means of easily recognisable "land-mark" bands, and bands numbered sequentially within each.

Sub-bands are catered for by using a decimal system. The chromosome error was already present in the embryo. It could have occurred before fertilisation, being present in one of the 2 gametes, or possibly in the fertilised zygote. If the anomaly is unbalanced i. The patient has a cancer of the affected organ. Note: many of the descriptions in this paper, particularly the references to behaviour at Meiosis, cover the general field of structural changes.

It is important to realise that relatively few aberrations that occur lead directly to cancer, although some of them will introduce conditions within the cell that may trigger other events that can cause malignant transformation.

The terms "constitutional" and "acquired" are really quite general terms, and can be applied to any persistent change encountered in clinical practice. Within the context of this paper, the term Acquired anomalies will apply exclusively to malignant situations. Note: In practice, when an acquired anomaly is said homogeneous, it only means that no normal cell was karyotyped within the scored sample.

Thus one may find clones of cells carrying a particular change, obviously all derived from the original cell where the anomaly first arose.

Only a percentage of mitoses carry the anomalies, while other cells are normal. XO if a gonosome is lost, or - 5. Note that the karyotype is always unbalanced in case of a numerical anomaly. The change is balanced, if there is no loss or gain of genetic material.

Autosomes non disjunction in first meiotic division produces 4 unbalanced gametes. Gametes with an extra autosome produce trisomic zygotes. The majority of trisomies are non-viable e. A few trisomies are more or less compatible with life, e. Nullosomic gametes missing one chromosome produce monosomies. Monosomies are more deleterious than trisomies and almost all lead to early miscarriage. The only autosomal monosomy in humans which might be compatible with life is monosomy 21, but this is still a debatable situation.

Non-disjunction can affect each pair of chromosomes and rarely more than one pair may be involved in the same meiotic cell multiple non-disjunction most often involves a gonosome. Non-disjunction is not a rare event, but its occurrence is generally underestimated due to the early spontaneous elimination of most unbalanced conceptuses.

With reference to the drawings, remember that, in the male, all 4 types of gametes will be effectively present in the sperm. In the female, only 1 of the types presumably randomly selected on each occasion? Gonosomes Gonosome unbalance is much less deleterious, and various trisomies can occur, as well as monosomy X There must always be at least one X for viability. Table: Zygotes produced for each type gamete: Empty boxes indicate a non-viable conceptus. A live birth can occur, but the baby dies shortly afterwards.

Mechanisms of formation of triploidies: digyny: non-expulsion of the 2nd polar body. Diandry is 4 times more frequent than digyny. Literature records very few live births, but with death soon after. Hydatidiform moles are usually polyploid. These cell populations, however, come from 1, and only 1, zygote When recording, a mosaic is denoted by a slash between the various clones observed, e. Numerical anomaly is usually due to a mitotic non-disjunction: 1 daughter cell will get both chromatids of one of the homologues, the other none; so the former will be trisomic, the latter monosomic.

Note: Viability of the two daughter cells may differ. In the above-mentioned trisomy 21 example, the clone monosomic for 21 is non-viable and has disappeared. The phenotype of surviving individuals is more or less affected, according to the proportion of the various clones. Variability of clone proportions is affected by various factors: The precocity of the event e.

If 46, XX cells are the most numerous, the anomaly must have occurred late in development; if it occurred at the cell, or cell stage, all, none or part of the embryo could be affected, since by this stage, the cells destined for the primitive streak, and hence the embryo proper have been segregated, and the aberration might be confined to the membranes or placenta.

The distribution of the cell populations during embryogenesis. In this case, the proportions of the various clones will vary from one organ to another. A mosaic must not be confused with a chimaera. In a chimaera, the cells originate from two or more zygotes.

They are produced by: mixture, or exchange of cells, from different zygotes e. Note: Mosaicism is frequent in malignancies, either because normal cells can still be karyotyped, or because the malignant clone produces sub-clones with additional anomalies clonal evolution.

Initial breaks are thought to be at the level of the DNA, and are probably frequent events. DNA repair then occurs. For various reasons, DNA repair is insufficient in chromosome instability syndromes.

Most often, the break occurs in a non-coding sequence, and does not result in a mutation. Initial breaks can occur anywhere, short arms of acrocentics included. Ultimately, what is important for the individual, is to retain 2 normal copies of each gene, no more, no less.

This is particularly true for the embryo, where a full balanced genetic complement is vital for normal development. Embryos with unbalanced constitutional anomalies have 1 or 3 copies of a whole set of genes, and abnormal development results. Note: a full balanced complement is not absolutely necessary for the functioning of many differentiated tissue cells, particularly if they are not called upon to divide.

Nevertheless, relatively small imbalances can have dire consequences, even in somatic cells. A good example is the case of the Rb gene, implicated in the formation of retinoblastoma. Normal individuals carry 2 functional copies, but one of these can be inactivated by mutation or removal loss of heterozygosity and the cell continues normal function through the normal allele which is now acting as a tumour suppressor gene.

Loss of the second allele by removal or mutation leads to the formation of the tumour. Note: Many of the structural aberrations formed are cell lethal, and are soon eliminated from the cell population. Of those that survive and are transmitted, the most frequent are translocations, small inversions and deletions. Note: Rearranged chromosomes that are transmitted are called derivative chromosomes der and they are numbered according to the centromere they carry.

Thus a reciprocal translocation between chromosome 7 and chromosome 14 will result in a der 7 and a der B - Main structural anomalies Figure 1 - Reciprocal translocation A mutual exchange between terminal segments from the arms of 2 chromosomes. Provided that there is no loss or alteration at the points of exchange, the new arrangement is genetically balanced, and called a: Balanced rearrangement. Genomic disorders: Molecular mechanisms for rearrangements and conveyed phenotypes.

PLoS Genetics 1 , — Munns, C. SHOX -related haploinsufficiency disorders. Gene Reviews Perez-Jurado, L. A duplicated gene in the breakpoint regions of the 7q Human Molecular Genetics 7 , — Peoples, R.

American Journal of Human Genetics 66 , 47—68 Ptak, S. Absence of the TAP2 human recombination hotspot in chimpanzees. PLoS Biology 2 , — Shaw, C. Implications of human genome architecture for rearrangement-based disorders: The genomic basis of disease. Human Molecular Genetics 13 , R57—R64 Stankiewicz, P. Genome architecture catalyzes nonrecurrent chromosomal rearrangements.

American Journal of Human Genetics 72 , — Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer.

Karyotyping for Chromosomal Abnormalities. Prenatal Screen Detects Fetal Abnormalities. Synteny: Inferring Ancestral Genomes. Telomeres of Human Chromosomes. Chromosomal Abnormalities: Aneuploidies. Chromosome Abnormalities and Cancer Cytogenetics. Copy Number Variation and Human Disease. Genetic Recombination. Human Chromosome Number. Trisomy 21 Causes Down Syndrome. X Chromosome: X Inactivation. Chromosome Theory and the Castle and Morgan Debate. Developing the Chromosome Theory. Meiosis, Genetic Recombination, and Sexual Reproduction.

Mitosis and Cell Division. Genetic Mechanisms of Sex Determination. Sex Chromosomes and Sex Determination. Sex Chromosomes in Mammals: X Inactivation. Sex Determination in Honeybees. Shaw, Ph. Citation: Clancy, S. Nature Education 1 1 Deletions and duplications of single-base pairs typically arise during homologous recombination and cause diseases. But what happens when a mutation occurs over multiple genes?

Aa Aa Aa. Chromosomal Duplications. Bar gene in fruit flies results in decreased eye sizes. B A fly with a heterozygous Bar mutation has an extra copy of the gene on one chromosome, resulting in an eye size about half the size of normal eyes. C A fly with a homozygous Bar mutation has an extra copy of the gene on both chromosomes, resulting in an eye size about one-fourth the size of normal eyes.

D A fly with a heterozygous double Bar mutation has three Bar genes on one chromosome, resulting in an eye size about one-eighth the size of normal eyes. This results in a large, round red eye. This results in a vertical, oblong eye about half the size of the normal eye.

This results in an oblong eye about one-fourth the size of the wild-type eye. This results in an oblong eye about one-eighth the size of the wild-type eye. Figure 1. Chromosomal Deletions. All amphibians from this speciation event carry the beta-globin gene.

Approximately MYA, a globin gene duplication event occurred in the reptile and mammal lineages, resulting in a gene with the regions 5-prime-betaprime-omega. This caused a split into two genetic lineages; one carried the omega region, and one did not.

Approximately MYA, two speciation events took place. In the omega lineage, the speciation event led to one unknown species and mammalian species with the omega region of the globin gene. In the non-omega lineage, the sauropsid species was split from the rest of the remaining species. All sauropsids from this speciation event carry the beta-globin gene. Approximately MYA, two more speciation events took place.

In both the omega and non-omega lineages, the speciation event separated the monotremes from other mammals. All monotremes from the omega lineage speciation event carry the omega-globin gene. Approximately MYA, a globin gene duplication event in the non-omega lineage resulted in a globin gene with regions 5-prime-epsilon-betaprime-omega. The epsilon-globin and beta-globin genes arose via duplication of a proto beta-globin gene in the ancestor of therian animals.

Approximately MYA, three speciation events occurred. In the omega lineage, marsupials were separated from an unknown species. All marsupials resulting from this speciation event carry the omega-globin gene. In the epsilon and non-epsilon lineages, the marsupials were separated from the eutherians. In the epsilon lineage, the speciation event resulted in marsupials carrying the epsilon-globin gene. In the non-epsilon lineage, the speciation event resulted in marsupials carrying the beta-globin gene.

Approximately MYA, two globin gene duplication events occurred. A gene duplication event in the non-omega monotremes resulted in a new lineage with the gene 5-prime-epsilon-P-beta-Pprime-omega.

The epsilon-P and beta-P globin genes arose via duplication of a proto beta-globin gene in the monotreme lineage. In the epsilon-P lineage, this resulted in monotremes with the epsilon-P-globin gene. In the non-epsilon-P lineage, this resulted in monotremes with the beta-P-globin lineage. A gene duplication event in the epsilon lineage of eutherians resulted in a gene with the regions 5-prime-epsilon-gamma-betaprime. This resulted in two separate eutherian species: the epsilon-globin eutherians and the gamma-globin eutherians.