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Triploidy

Polyploid (in Greek: πολλαπλόν - multiple) cells or organisms that contain more than two copies of each of their chromosomes. Polyploid types are termed triploid (3n), tetraploid (4n), pentaploid (5n), hexaploid (6n) and so on. Where an organism is normally diploid, a haploid (n) may arise as a spontaneous aberration; haploidy may also occur as a normal stage in an organism's life cycle. more...

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Polyploids are defined relative to the behavior of their chromosomes at meiosis. Autopolyploids (resulting from one species doubling its chromosome number to become tetraploid, which may self-fertilize or mate with other tetraploids) exhibit multisomic inheritance, and are often the result of intraspecific hybridization, while allopolyploids (resulting from two different species interbreeding and combining their chromosomes) exhibit disomic inheritance (much like a diploid), and are often a result of interspecific hybridization. In reality these are two ends of an extreme, and most polyploids exhibit some level of multisomic inheritance, even if formed from two distinct species.

Polyploidy occurs in animals but is especially common among flowering plants, including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many plants from the genus Brassica also show interesting inter-specific allotetraploids; the relationship is described by the Triangle of U.

Examples in animals are more common in the ‘lower’ forms such as flatworms, leeches, and brine shrimps. Reproduction is often by parthenogenesis (asexual reproduction by a female) since polyploids are often sterile. Polyploid salamanders and lizards are also quite common and parthenogenetic. Rare instances of polyploid mammals are known, but most often result in prenatal death.

Polyploidy can be induced in cell culture by some chemicals: the best known is colchicine, which can result in chromosome doubling, though its use may have other less obvious consequences as well.

Paleopolyploidy

Ancient genome duplications probably characterize all life. Duplication events that occurred long ago in the history of various evolutionary lineages can be difficult to detect because of subsequent diploidization (such that a polyploid starts to behave cytogentically as a diploid over time). In many cases, it is only through comparisons of sequenced genomes that these events can be inferred. Examples of unexpected but recently confirmed ancient genome duplications include the baker's yeast (Saccharomyces cerevisiae), mustard weed/thale cress (Arabidopsis thaliana), rice (Oryza sativa), and an early evolutionary ancestor of the vertebrates (which includes the human lineage) and another near the origin of the teleost fishes. It has also been suggested that all angiosperms (flowering plants) may have paleopolyploidy in their ancestry. Technically, all living organisms probably experienced a polyploidy event at some point in their evolutionary history, as it's unlikely that the first living organisms had more than one stretch of DNA (i.e., one chromosome).

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First-trimester screening works well in clinics
From OB/GYN News, 4/15/05 by Robert Finn

RENO, NEV. -- First-trimester aneuploidy screening is practical not just in the context of clinical trials but also in the everyday world of the clinic, according to a poster presented by Sriram C. Perni, M.D., and colleagues at the annual meeting of the Society for Maternal-Fetal Medicine.

Among 2,515 women evaluated at a single institution, trisomy 21 was detected in 91% of 22 pregnancies when the false-positive rate was set to 5% and in 77% of 22 pregnancies when the false-positive rate was set to 1%.

In that same group, trisomy 18 was detected in all eight affected pregnancies, whether the false-positive rate was set to 5% or 1%.

Aneuploidy screening in the first trimester relies on an algorithm incorporating four pieces of data: maternal age, blood levels of pregnancy-associated plasma protein A (PAPP-A), blood levels of free [beta]-human chorionic gonadotrophin (free [beta]-hCG), and ultrasound measurements of fetal nuchal translucency, Dr. Perni said.

A large, multicenter, clinical trial involving 8,514 patients found this algorithm to have a good sensitivity and an acceptable false-positive rate (N. Engl. J. Med. 2003;349:1405-13).

But it remained unclear whether the algorithm would perform as well in the real-world setting of a single institution, reported Dr. Perni and his colleagues at Weill Medical College of Cornell University, New York.

At their clinic, 4,883 pregnant women, who together had 5,167 fetuses, were offered first-trimester aneuploidy screening, and 2,515 women agreed.

Of those pregnancies, there were a total of 37 aneuploid fetuses--1 with trisomy 13, 8 with trisomy 18, 22 with trisomy 21, 4 with 45X, 1 with 47XXY, and 1 with triploidy.

Of the 22 cases of trisomy 21, 3 resulted in a live birth, and 19 were electively terminated. All eight fetuses with trisomy 18 were electively terminated.

Asked in an interview whether first-trimester aneuploidy screening remained controversial, Dr. Perni replied, "I don't think it's controversial, but right now it's not the standard of care. There needs to be more evidence that it's reproducible at a single institution like this.

"This is a very, very good test," continued Dr. Perni, who disclaimed any financial interest in the test.

"It can be done very early and has a very good detection rate for fixed false-positive rates," he said.

He said that many insurance carriers in New York City do cover the screening test. And while the test has become "almost the standard of care" in certain parts of the United States, "it just hasn't become vogue over the whole country."

He added, "I think the most exciting thing about this is that we can get information for couples and women specifically very early on to help them determine what they want to do."

BY ROBERT FINN

San Francisco Bureau

COPYRIGHT 2005 International Medical News Group
COPYRIGHT 2005 Gale Group

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