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17

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, ...


9

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 ...


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 ...


6

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 ...


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 ...


6

This depends completely on the quality of the DNA. Since each chromosome is essentially a very long strand of DNA, breakages and missing sections are very common in extinct species due to degradation over time. If a full DNA read is absent, no determination of chromosomal number can be performed. Assuming a full read (covering all breakages) of the DNA ...


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 ...


5

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

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

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

"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

I'm not sure about the ubiquity of this but, in many animals, each each primary oocyte that undergoes oogenesis only produces one mature egg. The other products of meiosis are polar bodies, which are not fertilised. These cells often degenerate but can sometimes play supportive roles in embryogenesis. To answer your question, each mature egg is ...


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 ...


4

The reason is that X-inactivation is not complete (Carrell & Willard, 2005; Ahn & Lee, 2008), and as many as 15-25% of X-linked genes escape silencing (Carrell & Willard, 2005; Cheng et al, 2005). This means that some genes on the Barr body are expressed in XX-females, although often at lower levels compared to the active X-chromosome, and this ...


4

The question is very broad and complicated, since the situation may differ in prokaryotes and eukaryotes. Nevertheless, I'm citing a good paper that is closely related to your question: Studies in yeast show that initiation of recombination, which occurs by the formation of DNA double-strand breaks, determines the distribution of gene conversion and ...


4

In humans and mice anyway ,a lot of it boils down to the recognition of a specific sequence that marks recombination hotspots by PRDM9. http://www.sciencemag.org/content/327/5967/836 Edit - I'm expanding in response to the comment below... Meiotic recombination occurs at vastly greater frequencies in some locations in the genome than others and these are ...


4

They are not chosen, they represent an even half, a complete set of chromosomes. The entirity of a cells genes (found on the chromosomes) is called a genome. Somatic cells are the cells in our bodies that exclude sperm and egg. Sperm and egg are called germline cells. The billions of somatic cells in a homo sapien have the same 46 chromosomes. 23 ( ...


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 ...



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