question: Why the sudden disparity in chromosome number despite difference in complexity and intelligence?
comment: Just because a book has more pages doesn’t mean it’s a better book. Chromosomes in different species do a lot of different things, and some are more or less efficient, some specialized and some not. The number of individual chromosomes (out of context of a species) is unrelated to pretty much anything
passages: Segregation distorters that are present in sexual chromosomes (as is the case with the X chromosome in several "Drosophila" species) are denominated sex-ratio distorters, as they induce a sex-ratio bias in the offspring of the carrier individual.
  
  Section::::Nuclear genes.:Segregation distortion.:Killer and target.
   BULLET::::- Diffuse centromeres – in asexual organisms where the offspring is an exact genetic copy of the parent, there are limited sources of genetic variability. One thing that increased variability, however, is having diffused instead of localized centromeres. Being diffused allows the chromatids to split apart in many different ways allowing for chromosome fragmentation and polyploidy creating more variability.
   BULLET::::- XYY syndrome. XYY boys are usually taller than their siblings. Like XXY boys and XXX girls, they are more likely to have learning difficulties.
  
  Section::::Aberrations.:Sperm aneuploidy.
  
  Exposure of males to certain lifestyle, environmental and/or occupational hazards may increase the risk of aneuploid spermatozoa. In particular, risk of aneuploidy is increased by tobacco smoking, and occupational exposure to benzene, insecticides, and perfluorinated compounds. Increased aneuploidy is often associated with increased DNA damage in spermatozoa.
  
  Section::::See also.
  
  BULLET::::- Aneuploidy
  
  BULLET::::- Chromosome segregation
  
  BULLET::::- DNA
  
  BULLET::::- Genetic deletion
  
  BULLET::::- For information about chromosomes in genetic algorithms, see chromosome (genetic algorithm)
   The arrangement of OPN1LW and OPN1MW, as well as the high similarity of the two genes, allows for frequent recombination between the two. Unequal recombination between female X chromosomes during meiosis is the main cause of the varying number of OPN1LW genes and OPN1MW genes among individuals, as well as being the cause of inherited colour vision deficiencies. Recombination events usually begin with misalignment of an OPN1LW gene with an OPN1MW gene and are followed by a certain type of crossover, which can result in many different gene abnormalities. Crossover in regions between OPN1LW and OPN1MW genes can produce chromosome products with extra OPN1LW or OPN1MW genes on one chromosome and reduced OPN1LW or OPN1MW genes on the other chromosome. If crossover occurs within the misaligned genes of OPN1LW and OPN1MW, then a new array will be produced on each chromosome consisting of only partial pieces of the two genes. This would create colour vision deficiencies if either chromosome were passed onto a male offspring.
   Screening for those X chromosome aneuploidies was possible by noting the presence or absence of "female" sex chromatin bodies (Barr bodies) in the nuclei of interphase cells in buccal smears, a technique developed a decade "before" the first reported sex chromosome aneuploidy. An analogous technique to screen for Y-chromosome aneuploidies by noting supernumerary "male" sex chromatin bodies was not developed until 1970, a decade "after" the first reported sex chromosome aneuploidy.
   1865 to 1900 saw a time of theory formulation in the field of heredity/genetics. In 1883, Wilhelm Roux argued that the linear structure of chromosomes has an impact of making sure daughter cells get equal amounts of chromosomal material. This was the beginning of the chromosome theory; Roux viewed his findings as argument that chromosomes contain units of heredity. During this time frame, Hugo de Vries put forth a theory that persistent hereditary units are passed through generations and that each “unit” deals with a specific characteristic and the units can combine in different ways in the offspring.
   Section::::Chemistry.:Chiasma.
   Comparative genomics data including chromosome painting confirmed the substantial conservation of mammalian chromosomes. Total human chromosomes or their arms can efficiently paint extended chromosome regions in many placentals down to Afrotheria and Xenarthra. Gene localization data on human chromosomes can be extrapolated to the homologous chromosome regions of other species with high reliability. Usefully, humans express conserved syntenic chromosome organization similar to the ancestral condition of all placental mammals.
  
  Section::::Post-genomic time and comparative chromosomics.
   The X-chromosome has played a crucial role in the development of sexually selected characteristics for over 300 million years. During that time it has accumulated a disproportionate number of genes concerned with mental functions. For reasons that are not yet understood, there is an excess proportion of genes on the X-chromosome that are associated with the development of intelligence, with no obvious links to other significant biological functions. . In other words, a significant proportion of genes associated with intelligence is passed on to the male offspring from the maternal side and to the female offspring from either/both maternal and paternal side. There has also been interest in the possibility that haploinsufficiency for one or more X-linked genes has a specific impact on development of the Amygdala and its connections with cortical centres involved in social–cognition processing or the ‘social brain'.
   Sex linkage was first discovered in insects, e.g., T. H. Morgan's 1910 discovery of the pattern of inheritance of the white eyes mutation in Drosophila melanogaster. Such discoveries helped to explain x-linked disorders in humans, e.g., haemophilia A and B, adrenoleukodystrophy, and red-green color blindness.
  
  Section::::Humans.:Role in diseases.:Other disorders.
   BULLET::::- The results from polyploid and diploid cells should be identified and separately recorded from one another. This is because the fitness cost (survival to next generation) of chromosomal instability is lower in polyploid cells, as the cell has a greater number of chromosomes to make up for the chromosomal instability it experiences.
  
  BULLET::::- Polyploid cells are more prone to chromosomal changes, something that should be taken into account when determining the presence and degree of chromosomal instability
  
  Section::::Classification.
   Section::::Levels of nuclear organisation.:Linear DNA and chromosome basics.
   During her time at Cornell she worked in the field of maize cytogenetics with Barbara McClintock, the pair published a very influential paper in 1931 in which they described chromosomal crossover for the first time. This paper, part of her Ph.D. research, provided key evidence that chromosomes carried and exchanged genetic information and hence that genes for physical traits are carried on chromosomes. Barbara McClintock guided her Ph.D. research.
   BULLET::::3. Different proportions of chromosome morphs were found in different areas. There is, for example, a polymorph-ratio cline in "D. robusta" along an transect near Gatlinburg, TN passing from to 4,000 feet. Also, the same areas sampled at different times of year yielded significant differences in the proportions of forms. This indicates a regular cycle of changes which adjust the population to the seasonal conditions. For these results selection is by far the most likely explanation.
   Section::::Functions.:In somatic cells.
  
  Homologous pairing in most contexts will refer to germline cells, however also takes place in somatic cells. For example, in humans, somatic cells have very tightly regulated homologous pairing (separated into chromosomal territories, and pairing at specific loci under control of developmental signalling). Other species however (notably "Drosophila") exhibit homologous pairing much more frequently. Various functions of homologous pairing in somatic cells have been elucidated through high-throughput screens in the early 21st century.
  
  Section::::Problems.
   Section::::Kin selection.:Conflict over sex ratio.
   In 1931, Barbara McClintock discovered a triploid maize plant. She made key findings regarding corn's karyotype, including the size and shape of the chromosomes. McClintock used the prophase and metaphase stages of mitosis to describe the morphology of corn's chromosomes, and later showed the first ever cytological demonstration of crossing over in meiosis. Working with student Harriet Creighton, McClintock also made significant contributions to the early understanding of codependency of linked genes.
  
  Section::::Origins.:DNA repair theory.
  
  Crossing over and DNA repair are very similar processes, which utilize many of the same protein complexes. 
   There have been rare events in which genes that ensure proper chromosome pairing have evolved shortly after polyploidization. One such gene, Ph1, exists in hexaploid wheat. These genes keep the two sets of genomes separately by either spatially separating them or giving them a unique chromatin identity to facilitate recognition from its homologous pair. This prevents the need of rapid gene loss to speed up homeologous chromosome diversification.
  
  Section::::Drive for diploidization.
  
  BULLET::::- Coordinate inter-genomic gene expression
  
  BULLET::::- Maintain intra-genomic chromosome pairing at meiosis
  
  BULLET::::- Reduce costs of maintaining large, duplicated genomes
  
  Section::::Revolutionary vs. evolutionary changes.
   Genetic variation among humans occurs on many scales, from gross alterations in the human karyotype to single nucleotide changes. Chromosome abnormalities are detected in 1 of 160 live human births. Apart from sex chromosome disorders, most cases of aneuploidy result in death of the developing fetus (miscarriage); the most common extra autosomal chromosomes among live births are 21, 18 and 13.
   Section::::Levels of nuclear organisation.:Chromosome territories.
   When genotypes grown together in a diverse population have different profiles of resource use they complement each other in the exploitation of the limiting resource and therefore are subject to smaller between-plant competition.
  
  In case of disease or environmental change some plants will take over when others fail. Yield stability over years and environments can be better than pure lines due to compensation.
   Section::::Chemistry.:MSH4 and MSH5.
   The gain or loss of DNA from chromosomes can lead to a variety of genetic disorders. Human examples include:
  
  BULLET::::- Cri du chat, which is caused by the deletion of part of the short arm of chromosome 5. "Cri du chat" means "cry of the cat" in French; the condition was so-named because affected babies make high-pitched cries that sound like those of a cat. Affected individuals have wide-set eyes, a small head and jaw, moderate to severe mental health problems, and are very short.
   In her report, “The Significance of Responses of the Genome to Challenge”, McClintock studied corn to show how corn's genome would change itself to overcome threats to its survival. She used 450 self-pollinated plants that received from each parent a chromosome with a ruptured end. She used modified patterns of gene expression on different sectors of leaves of her corn plants show that transposable elements (“controlling elements”) hide in the genome, and their mobility allows them to alter the action of genes at different loci. These elements can also restructure the genome, anywhere from a few nucleotides to whole segments of chromosome. 
   It took until 1954 before the human diploid number was confirmed as 46. Considering the techniques of Winiwarter and Painter, their results were quite remarkable. Chimpanzees, the closest living relatives to modern humans, have 48 chromosomes as do the other great apes: in humans two chromosomes fused to form chromosome 2.
  
  Section::::Aberrations.
  
presuppositions: Every chromosome does the same amount of work. Disparity in chromosome number despite difference in complexity and intelligence.
corrections: Chromosomes differ in how much they accomplish. number of chromosoes is unrelated to pretty much anything.
raw_presuppositions: Every chromosome does the same amount of work. Disparity in chromosome number despite difference in complexity and intelligence.
raw_corrections: Chromosomes differ in how much they accomplish. number of chromosoes is unrelated to pretty much anything.