Ploidy | Research & Encyclopedia Articles

Ploidy

Ploidy refers to the number of chromosomes found in a cell and is subdivided into two categories: euploidy and aneuploidy. Euploidy is one complete chromosome set or exact multiples of that set. Aneuploidy includes gain or loss of chromosomes that is less than a complete set. The optimum chromosome complement is fixed for each organism and can be described in terms of euploidy and aneuploidy, though the later is most often associated with variation or errors in the normal chromosome number.

Euploidy can refer to a broad range of chromosome findings. Haploidy, designated as the N number, is one complete chromosome set in which each chromosome is unique. Diploidy (2N) is two complete chromosome sets (pairs of chromosomes), triploidy (3N) is three sets, tetraploidy (4N) is four sets, and so on. Any cell with three or more chromosome sets is said to be polyploid. In humans, cells are diploid. They contain 46 chromosomes arranged in 23 pairs. One complete set (N) comes from the mother and the other N set from the father. Therefore, in diploid organisms, the haploid number equals the number of chromosomes in a gamete. In humans and most other animals, polyploidy is not compatible with life.

Some species of plants occur naturally as polyploids. The common dandelion, for example, is a triploid, as are several species of ferns. Phenotypic variation in some grains led to the finding that an increase in euploidy level within certain species may give rise to larger, more robust plants. In an attempt to improve crop yield, researchers designed crosses intended to generate plants with very high levels of polyploidy (5N, 6N, 7N and up). Larger plants with improved grain were obtained up to a level of polyploidy where excessive numbers of chromosomes began to affect the ability of the cell to divide accurately. From this point on, increasing the euploidy level actually results in smaller, less healthy plants.

Aneuploidy is most commonly the gain or loss of a single chromosome. For a diploid organism, gain of a single chromosome is known as trisomy (2N+1), i.e., all of the chromosomes are paired with the exception of one that has three copies. Monosomy (2N-1) is the loss of a single chromosome, so all chromosomes would be paired except one that would exist as a single chromosome. Occasionally, a cell may show a gain or loss of 2 (or more) separate chromosomes (double trisomy or double monosomy), but in general, the greater the gain or loss of chromosomes, the less viable the cell. If only a portion of a chromosome is lost, the cell is said to have a partial monosomy, and a duplication of only 1 chromosome region will result in a partial trisomy.

In man, autosomal monosomy is not compatible with life, but there are three liveborn autosomal trisomies: trisomy 13 or Patau syndrome, trisomy 18 or Edward syndrome, and trisomy 21 or Down syndrome. Sex chromosome aneuploidies tend to better tolerated, probably because of the process of X inactivation. The only liveborn monosomy in humans is Turner syndrome: 45,X. Sex chromosome trisomies include: Klinefelter syndrome (47,XXY), triple X syndrome (47,XXX), and XYY syndrome. There are also some individuals who have multiple sex chromosomes (48,XXXX ; 49,XXXXX ; 48,XXYY, etc.), but all have varying abnormal phenotypes.

Trisomy or monosomy are usually the result of nondisjunction cell division errors in which one chromosome fails to move to the proper pole during division. This can occur in either meiosis or mitosis. Meiotic nondisjunction errors result in a gamete with one too many or one too few chromosomes that, after fertilization, gives rise to an individual with the aneuploid chromosome number. Mitotic nondisjunction errors in somatic cells of the body may give rise to an aneuploid cell line but usually have no phenotypic effect on the individual. However, should this type of mitotic division error occur in the zygote or early in fetal development, a second cell line could be established that would generate a mosaic chromosome complement, i.e., two cell lines would be present in the individual that would differ by a single chromosome change.

Because only chromosomes 13, 18 and 21 are seen in living trisomic individuals, the question is often asked if there is something about cell division that favors these three chromosomes when errors occur. Data from cytogenetic analysis of spontaneously aborted fetuses suggests that nondisjunction is random with respect to the chromosomes since nearly every other possible trisomy has been reported in abortus tissue. In fact, the most common trisomy seen in abortions is trisomy 16. The conclusion is that the total number and combination of genes present on the chromosomes must be the determining factor in viability. Chromosomes 13, 18, and 21 are some of the smallest of the chromosome set, so on the basis of size alone would be expected to contain a comparatively low number of genes that would lower the risk of lethality. This is supported by recent evidence from the Human Genome Project that suggests these three chromosomes have the lowest density of coding regions.

Although trisomies of chromosomes 13, 18 and 21 and monosomy X can produce liveborn children, most of the time these aneuploidies are not compatible with life. Of all 45,X conceptions, approximately 95% spontaneously terminate. Similarly, spontaneous termination occurs for 90% of trisomy 13 conceptions, 80% of trisomy 18 conceptions, and 65% of trisomy 21 conceptions. This implies that the allelic constitution of the trisomy is also important in determining viability, and that certain trisomic genotypes are not compatible with life. Because of this, only a relative few of all trisomic conceptions are liveborn.

Aneuploidy is also seen in cancer cell lines. Gain or loss of a few too many chromosomes can occur. Some aneuploidies are characteristic of particular disorders, as loss of chromosomes 5 and/or 7 is associated with myelodysplastic syndrome. Hyperdiploidy is seen in acute lymphocytic leukemia and is a condition in which there is a selective gain of chromosomes such that 50-55 chromosomes exist in a single cell. In advanced disease, random gain and loss of chromosomes is known to occur.