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15

Great question, and one about which there has historically been a lot of speculation, and there is currently a lot of misinformation. I will first address the two answers given by other users, which are both incorrect but have been historically suggested by scientists. Then I will try to explain the current understanding (which is not simple or complete). My ...


11

Note that anyone with a Y chromosome is considered a genetic male. Also, aneuploidy is usually a result of nondisjunction (but can have other causes as well) and is usually not inherited. 47,XXX is called Triple X syndrome and occurs in approximately 1 in 1,000 female births. These individuals usually appear normal, but may have tall stature, a small head, ...


7

No, an external physical examination would be inconclusive. The reason is the TDF gene. To be more specific, if a person has XY and the gene is not active then the subject would have a female appearance. Also we cannot conclude that a person has a Y chromosome even if it has penis because that gene could be transferred on the X chromosome. Here is a ...


7

I think it is the wrong question. You assume that eukaryotes developed from a single-cell organism with circular DNA. Then, clearly, there must have been an advantage of (newly) developing a linear genome. But eukaryotes could have developed from an organism with linear DNA, too. There are still a few bacterial species with linear chromosomes, so this is not ...


6

Each chromosome is a distinct, separate DNA molecule. A chromosome of an eukariotic cell nucleus is a (long) linear molecule and so has two ends, which are called telomeres. DNA naturally forms double helix, and the molecules can further curl in what are called supercoils. In humans, the chromosomes occur in 23 pairs (totaling 46). Except for the sex ...


6

Mycoplasma genitalium was one of the first full bacterium genomes sequenced and since its a symbiotic organism that lives on the moist and warm genital skin surface it doesn't need as many genes as many bacteria. It has a 582 kbp genome sequence with only 521 genes. But that is so 1995. The 159 kbp genome of Candidatus carsonella was published in 2006. ...


6

I got $26$% as the answer. To get a recombination between C and E, there are two possible mechanisms:- C and D produce a recombinant, but D and E remain linked, therefore the final genotype will be a recombinant considering C and E(Chiasmata between C and D). Here $P_1=P_{CD}\times P'_{DE}$ where $P$ is the probability of recombination and $P'=1-P$ ...


6

If really cystic fibrose is expressed only in homozygotes, then here are some possible explanations: Mutation very improbable Autofecundation very improbable False Negative when testing the father Because of the test False Negative Rate. @kmm made a very interesting comment below. He said that according to this article 75% of the mutations coding for ...


6

During the generation of gametes (sperm, eggs), chromosomes can cross-over - this swaps paternal and maternally-derived genetic material. So none of the descendant's chromosomes would be a direct copy of Einstein's, and furthermore, each offspring receives half of its genetic complement from each parent. This means that if you pool the DNA data from small ...


5

Here is a paper you might want to take a look at: Phylogenetic Origin of Human Chromosomes 7, 16, and 19 and their Homologs in Placental Mammals From the abstract: From their origin, these chromosomes underwent the following rearrangements to give rise to current human chromosomes: centromeric fission of the two submetacentrics in ancestors of all ...


5

@nico is right. the number of chromosomes is the result of an evolutionary timeline, puncutated by sometimes spontaneous events which shape the DNA. These events occur in the course of evolution: 1) Chromosomal rearrangements. Large sections of the genome can flip around or become integrated in other chromosomes. By homologous recombination, regions of ...


4

I think the OP is asking why we have one less chromosome pair than chimpanzees (for example) [23 pairs instead of 24]. The is an abundance of evidence, as alluded to above by shigeta, that human chromosome 2 is the result of a telomere-to-telomere fusion of two ancestral chromosomes (IJdo et al., 1991). This event did not occur in our closest ancestors, ...


4

Plants have a simpler anatomical structure than mammals (is anatomical the right word, or would physiological be more appropriate?). Mammals on average don't have more genes than plants, so my understanding is that this additional complexity is the result of finer and more complex regulatory mechanisms. When you remove or duplicate an individual gene in an ...


4

Answer An comprehensive online database of the chromosome numbers of all living species most likely doesn't exist. This Wikipedia article is the best and most complete reference comprising animals that I can personally find on the internet. This source in Spanish, which I've translated with Google Translate reads: Canedo Delgado (1999) performed the ...


4

Regarding the Papers referenced above: If the authors maintain that the evidence at the site of fusion is not clear and that explanations as to how this happened are flawed that is possibly concedable. They would need to do more homework than these papers. The discussion of vestigial centromere sequences and telomere motifs being inverted is ...


4

You can package linear genomes much more efficiently than circular genomes, and bacteria simply don't require the information density to be prosperous. To be a bit more specific, it's the torque strain put on the double-helix while it's being wound that makes the difference. Linear genomes can be wound around Histones, and these Histones can be further ...


4

Yes, here is an article on it: "The role of X-chromosome inactivation in female predisposition to autoimmunity" Below see the method and results summarised. Using a DNA methylation assay, we have examined the X-chromosome inactivation patterns in peripheral blood from normal females (n = 30), female patients with a variety of autoimmune diseases (n ...


4

First of all, they are not caused by the same mechanism. They are both aneuploidies, but the mechanism is different. Turner syndrome happens when one of the gametes (most commonly father's) lacks a copy of X chromosome, or the X chromosome is heavily damaged. This means that while normal women have two X chromosomes, Turner women have only one, and nothing ...


4

"Monosomy for the X chromosome in humans creates a genetic Achilles' heel for nature to deal with." The article I quoted1 tackles this issue. It's from the turn of the century, though, so quite old in sequencing/genetic terms, but the science is still sound as far as I can tell. Here's the money quote: Hence, the X chromosome appears to have a lower ...


4

In principle, Meiosis is only the process in which the haploid egg or sperm are generated. Have a look at this figure, which shows Meiosis I and II (from the Wikipedia): During Meiosis I homologous recombination between homologous chromosomes can happen, the chromosomes are then distributed normally among the daughter cells. In Meiosis II the cells split ...


3

It turns out that this was a question that was answered by Alan Turing in the 1950s. Turing hypothesized there would be two morphogens, an inhibitor/ activator pair, each of which would produce a one of two phenotypes by activation or suppression of a trait. He hypothesized that this would produce a pattern like stripes or other patterns (e.g. black and ...


3

This question appears to start from the premise that different species of yeast are closely related, but they aren't. Saccharomyces cerevisae and Schizosaccharomyces pombe, both Ascomycetes, are thought to have diverged at least 300 million years ago (c.f. the mammalian divergence from other vertebrates was about 200 million years ago). S. cerevisiae has a ...


3

I would say this is a question that might not be resolved yet. Going back to the earliest days of DNA - a billion years ago - its hard to imagine circular DNA being the first sort to show up. Its speculated that in the transition from RNA world to DNA world, early on each gene had a separate piece of nucleic acid coding for it which would not have been ...


3

It is hard to give an answer to this question that is not just speculation. We can be reasonably confident that the most recent common ancestor of bacteria and archaea had circular genomes. However, we currently have no way to get any conclusive evidence earlier than that. 3 billion year old DNA doesn't fossilize. So it's just speculation what DNA looked ...


3

Since you didn't specify animals, I will add here for anyone coming from google that plant chromosome numbers are stored in the Index to Plant Chromosome Numbers. I would be surprised if such a database exists for animals (except maybe insects) as historically C-numbers of plants and other cytogenetics were important for taxonomy (and still is important), ...


3

So the DNA in some chromosomes must have the pieces of information about how to do the DNA replication. - I am not sure about thing. Genomes contain what is called the "origin of replication" - specific sequences in the DNA that tell DNA polymerase where to bind and to initiate replication. As for your main question, I'm a little confused as to what ...


3

An interesting condition is androgen insentivity syndrome. In AIS, the body is unable to respond to androgens, the male sex hormone, of which testosterone is the best known. The most common cause is a defective androgen receptor, and most of these diseases are X-linked. In extreme cases, complete androgen insensitivy syndrome (CAIS) can occur. Genetic males ...


3

This link seems to have good information that answers most of your questions. In my mind, there are two types of mitochondria: ones that work and ones that don't. Mitochondria do have DNA but that mDNA is there to encode proteins for their specific functions (e.g. to create ATP). So, although the mDNA may not be uniform for every mitochondrion in your body, ...


3

As @dd3 points out, average GC% indicates a need for stability and coding regions or structural regions of the genome may need to be more stable. But the largest %GC in genomes are found in thermophiles - organisms which live in high temperature water - in hotsprings and undersea geothermal vents. This review mentions how some thermophiles can be found with ...


3

Trisomy is due to non-disjunction in meiosis (the process in which eggs and sperm are created). This happens before fertilization. Trisomies are more frequently seen in children of older women. It's not fully understood why this happens, but it is likely related to the fact that the oocytes do not complete meiosis (and become eggs) until ovulation. Here's ...



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