Shrinking size of Y chromosomes

I read on the web that the size of Y chromosomes decreased in earlier period of time and the picture below tells that it is still occurring.

So basically I want to know what caused this shrinking and if it has stopped now then what caused this break ??

Y chromosome is not doomed to shrivel away to nothing, say researchers

Reports of the coming death of the male sex chromosome are greatly exaggerated, say scientists, whose work will raise a collective cheer from at least half the population. The fate of the Y chromosome, which carries the genetic switch that sends a developing embryo down the route to maleness, has been questioned since scientists first discovered that it had lost more than 90% of its genes over millions of years of evolution.

The steady withering of the Y has led some to claim that it might vanish completely over the next five million years, leaving humans to join the Okinawa spiny male rat on the list of species that make do without a sex chromosome. But that unsettling prospect is dismissed in research published on Friday by scientists at the University of California, Berkeley. Having studied the genetic makeup of 16 men, they conclude that natural selection is not about to cast the shrunken male chromosome on the evolutionary scrapheap.

The Y chromosome has shrunk over time because, unlike every other chromosome in the human body, it has no partner. This means it cannot easily be repaired when harmful mutations occur. Typically, people have 23 pairs of chromosomes including two that govern sex, which areX and Y in males, or two Xs in females. The numbers vary in some genetic disorders.Most chromosomes can repair damage that arises from mutations by swapping DNA with their opposite number, a process called recombination. But the Y is always inherited alone, so has no partner to swap with. As such, the damage builds up until the DNA is discarded, leaving the chromosome that much smaller. Today, the Y carries only 27 genes that are used to make proteins, compared with around 800 on the X chromosome. A few hundred million years ago, early versions of the X and Y were the same size.

Writing in the journal Plos Genetics,the Berkeley researchers describe the genetic diversity of Y chromosomes in eight European and eight African men. The variation was tiny, and suggests that the Y chromosome has been pared down to its bare essentials by "purifying selection"An alternative explanation for the low genetic variation of Y chromosomes is that a minority of men had a high proportion of children, passing on fewer Y chromosome variants to each successive generation. At the extreme is the Genghis Khan effect, named after the Mongol leader who fathered so many sons, his Y chromosome lives on in around 0.5% of the male global population. But the study found that if this were the driving force for low genetic variation on the Y chromosome, fewer than one in four men would have fathered children in the course of human history.

According to the researchers, all 27 genes on the Y chromosome, nearly half of which are poorly understood, are acted on by purifying selection. The fact they're still here suggests they have a valuable role to play in successful breeding.

"Natural selection is acting on the Y chromosome and has maintained the genes pretty well," said Melissa Wilson Sayres, an evolutionary biologist. "All the evidence points toward it not disappearing."

The Berkeley team now hopes to study more Y chromosomes to learn whether the genes are subjected to "positive selection", whereby beneficial mutations spread through the population.

Why a shrinking Y chromosome still doesn't mean the end of men

Is the Y chromosome really headed towards disappearing? Well, perhaps (give or take 10 million years or so.) But most likely not.

In a Q&A today with science historian Sarah Richardson on genetics and sex differences, commenter decemberfifthburner asked about reports that the Y chromosome could be headed for extinction. The Y chromosome doesn't use standard recombination for repairs, says Richardson, hence the concern over shedding genes. But new research shows that the Y-chromosome is using a recombination-like process to make repairs:

Ask an Expert Your Questions About Genetics and Sex Differences!

Sarah Richardson is a science historian at Harvard who studies how people understand the genetic…

It's certainly a fact that the evolution of the Y chromosome (from its original status as an X chromosome), because it is passed clonally from male to male and does not repair through standard recombination, has involved the sloughing off hundreds of genes. This is true in every sex chromosome system – over millions of years, the clonally inherited chromosome in the sex chromosome pair loses its function other than sex determination and diminishes in size and gene count. Now, is the human Y chromosome going extinct? In some species of voles, it already has, so it's certainly a theoretical possibility.

But why are we so concerned about this now? The Y going extinct isn't happening any time soon, if it all (estimates run in the 10 million years+ range). And, as in voles, the disappearance of the Y doesn't, in principle, mean the loss of the male sex. David Page at MIT has discovered that recombination-like processes in the Y chromosome ("gene conversion") may help repair the human Y and be keeping it in a holding pattern from further degeneration.

So, with a repair process in sight and a timeline measured in millions, just why has the idea of a disappearing Y-chromosome captured the public imagination? Richardson says it may have more to do with culture than research:

I think the popular concern about the status of the Y is continuous with concerns about the decline of men in a postfeminist age and with other biologistic discourses about males sperm count decline and males' lack of evolutionary adaptation to the supposedly "feminized" environment of the post-industrial workforce. Y chromosome degeneration theories are truly a node where a good understanding of the cultural gender context helps understand the shape of the scientific debate.

Incredible Shrinking Y

The darlings have been fretting for some years now that they may be rendered unnecessary if women get financial and biological independence, learning how to reproduce and refinance without them. What if nature played a cruel trick and demoted men, so they had to be judged merely by their appearance, pliability and talent for gazing raptly at the opposite sex, no matter how bored?

New research on the Y chromosome shows that my jittery male friends are not paranoid they are in an evolutionary shame spiral.

As Nicholas Wade wrote in The Times: 'ɺlthough most men are unaware of the peril, the Y chromosome has been shedding genes furiously over the course of evolutionary time, and it is now a fraction of the size of its partner, the X chromosome. . . . The decay of the Y stems from the fact that it is forbidden to enjoy the principal advantage of sex, which is, of course, for each member of a pair of chromosomes to swap matching pieces of DNA with its partner.''

Mr. Wade said that biologists in Cambridge, Mass., had made a remarkable discovery: '⟞nied the benefits of recombining with the X, the Y recombines with itself.''

The ultimate guys' night out. Simply put, the Y chromosome figured out a Herculean way to save itself from extinction by making an incredibly difficult hairpin turn and swapping molecular material with itself.

Self-love as a survival mechanism: the unflinching narcissism of men may send women into despair at times, but it has saved their sex for the next 5 million or 10 million years.

But, according to Olivia Judson, science's answer to the sensual British cook Nigella Lawson, men may need more than narcissism to survive.

Dr. Judson, a 33-year-old evolutionary biologist at Imperial College in London who has written a book about animals in a Dear Abby style, or Deer Abby, under the pen name Dr. Tatiana, says the worm has turned. 'ɿor a long time, it was assumed that promiscuity was good for males and bad for females in terms of the number of kids they could have,'' she explains. 'ɻut it wasn't until 1988 that it really started to become evident that females were benefiting from having sex with lots of males, with more promiscuous females having more and healthier offspring.''

In her book, Dr. Judson writes about powerful babes, noting that females in more than 80 species, like praying mantises, have been caught devouring their lovers before, during or after mating. ''I'm particularly fond,'' she told me, ''of the green spoon worm. . . . The male is 200,000 times smaller, effectively a little parasite who lives in her reproductive tract, fertilizing her eggs and regurgitating sperm through his mouth.''

And then there's the tiny female midge, who plunges her proboscis into the male midge's head during procreation. As Dr. Judson told the journalist Ken Ringle, ''Her spittle turns his innards to soup, which she slurps up, drinking until she's sucked him dry.''

The Economist recently reported on a variation of the creepy-crawly girl-eats-boy love stories. The male orb-weaving spider kills himself before the female has a chance to. Biologists now believe that the male orb-weaver dies when he turns himself into a plug to prevent other males from copulating, thus ensuring his genes are more likely to live on.

In a new book called ''Y: The Descent of Men,'' Steve Jones, a professor of genetics at University College in London, says males, always a genetic ''parasite,'' have devolved to become the ''second sex.''

The news that Dolly the sheep had been cloned without masculine aid sent a frisson through the Y populace, he writes, because men began to fear that science would cause nature to return to its original, feminine state and men would fade from view.

The Y chromosome, 'ɺ mere remnant of its once mighty structure,'' is worried about size. ''Men are wilting away,'' Dr. Jones writes. 'ɿrom sperm count to social status and from fertilization to death, as civilization advances, those who bear Y chromosomes are in relative decline.''

Perhaps that's why men are adapting, becoming more passive and turning into ''metrosexuals,'' the new term for straight men who are feminized, with a taste for facials, grooming products and home design.

The Women Are Shrinking!

The characters of the family include:

Christian Hanson - age 21, a senior in college studying physics. He's home for the summer and is working on an invention he plans to be a shrink ray. He's aware that it only affects women. He enjoys having a beer and hanging out with his buddies, but also loves his family and his girlfriend.

Carrie Hanson - age 48, a secretary at a major law firm. She's the matriarch of the family and the mother of Christian and his two sisters, and is also married to Mark. She loves her family and enjoys having her girlfriends over for drinks.

Lauren Hanson - age 24, a college graduate his is currently in law school. She is Christian's older sister who enjoys partying and getting hammered with her friends, as well teasing her younger brother for every reason she finds.

Julia Hanson - age 18, a senior in high school his is currently indecisive about her future. Julia is both very smart and very athletic, and she loves giving her older siblings a hard time.

Angela Staninski - age 22, Christian's girlfriend who is a year older than him and in nursing school. She loves her boyfriend but is relatively shy around his family. Angela doesn't have many friends and keeps to herself.

Michelle Lopez - age 22, the girlfriend of Christian's best friend Matt. Michelle is a waitress at a local bar, and Christian introduced her and Matt together. Christian and Michelle are very good friends, and Michelle is a social butterfly who will hang out with anyone.

Mark Hanson - age 47, Carrie's husband and the father of Lauren, Christian and Julia. Mark is a lawyer at the firm Carrie works at. He doesn't care for bugs and is often easily stressed out.

Matt Welch - age 22, the best friend of Christian and the boyfriend of Michelle. He's a good guy but he also is attracted to both Lauren and Angela.

Is the Y Chromosome Disappearing? Update

A book published back in 2003 titled Adam’s Curse: A Future Without Men by Bryan Sykes predicted that in the future “the human race will reach the ultimate evolutionary crisis that has been millions of years in the making: The extinction of men.”[1] His prediction is that the Y chromosome will disappear, resulting in the extinction of males as we know them. Is this what is happening really?

Duck-billed platypus, a monotreme

The Y chromosome is used in all primates, most mammals, and even in some insects and plants, to produce males.[2] Some animals, such as alligators and turtles, use a complex system that enables the temperature in which embryos develop to determine the sex.[3] Already, evolution faces a problem: a defining trait of primates—the Y chromosome—is found scattered in a wide variety of lifeforms including some plants and insects. Some strange exceptions exist. Birds are ZZ/ZW and the duck-billed platypus, a mammal, boasts ten sex chromosomes![4] Specifically, platypuses

have five male-specific chromosomes (Y chromosomes) and five chromosomes present in one copy in males and two copies in females (X chromosomes). These ten chromosomes form a multivalent chain at male meiosis, adopting an alternating pattern to segregate into XXXXX-bearing and YYYYY-bearing sperm.[5]

Some rodent groups lack the Y chromosome, such as the spinous country rat[6] of Japan which also uses XO in both sexes.[7] Another problem is most all genes are differently expressed in males and females that are not on the sex chromosomes. One example is the gene vital for female growth and sexual development that encodes for estrogen receptors, ESR1, which is located on chromosome 6.

Evolutionists postulate that both bird and mammal sex chromosomes evolved from autosomal (non-sex) chromosomes.[8] How sexual life reproduced before sex chromosomes evolved is unknown, as is the far larger question, namely how did sex itself evolve![9] The standard theory is the different sex chromosomes, the X and Y in mammals, and the Z and W in birds, evolved separately from different pairs of autosomes. No evidence exists for this theory, and an analysis of the most ancient human DNA known, from Neanderthals, revealed intact X and Y chromosomes.[10] The evolution of the sex chromosomes from the autosomal chromosomes is accepted only because evolutionists cannot figure how else the sex chromosomes could possibly have originated.

The XY chromosome in the fertilized egg produces a male, and XX, even XO (Turners Syndrome) or XXX (trisomy X), all produce females. Likewise, one Y always produces a male. Even XXY (Klinefelter syndrome) produces a male with some female traits such as minor breast enlargement, illustrating the central role of the Y chromosome in determining the male sex. I reviewed this topic in 2018 (24 Feb 2018) but new claims since then indicate another visit is in order.[11]

What is the Y Chromosome and Why is It so Important?

Humans have 46 chromosomes. The structure that carries the DNA code which makes Homo sapiens human is the set of 23 somatic chromosomes, plus the two sex chromosomes, X and Y.[12] One assumption is the X and Y chromosomes were, in the past, both equal in length and in the number of genes. The theory is the male Y chromosome eventually lost nearly all of the 640 genes it once shared with the X chromosome, and the essential genes were somehow transferred to the somatic chromosomes.[13] Thus, it now has only 27 unique genes compared to thousands on most somatic chromosomes.

This idea is not based on evidence, but on speculation on both ends of the timeline: what was assumed to be true in the past and assumptions about the future – not on data. The story has produced good reasons, however, why the Y chromosome will not disappear. Conversely, other researchers speculate that it will eventually disappear. One headline supporting the disappearance opinion proclaimed, The Y chromosome is disappearing – so what will happen to men?[14] The report added, although the Y chromosome

carries the “master switch” gene, SRY, that determines whether an embryo will develop as male (XY) or female (XX), it contains very few other genes and is the only chromosome not necessary for life. Women, after all, manage just fine without one.[15]

Another headline published at about the same time as the one just cited declared, “Study Dispels Theories Of Y Chromosome’s Demise.”[16] A key fact is, in the present day, the Y chromosome is a comparatively tiny structure, about 45 total genes compared to around 1,000 on the X, as is shown in the following illustration.

The reason for the diametrically opposed positions on the disappearance of the Y chromosome is that both positions are based on speculation and very few facts. Both sides largely agree on those facts.

A New Report Published in May of 2020

Not to be outdone, Isobel Whitcomb announced the old fear in a new article in Live Science on August 30, 2020. Once again, she raised the suggestive question, “Is the Y chromosome dying out?” She added, “The Y chromosome may be in trouble.”[17] She starts with the well-known fact that if you have two X chromosomes you normally develop ovaries, and if you have an X and Y chromosome, you normally develop testes. She then admits that research only suggests it has shrunk over time. Then she calls on evolutionary geneticists who begin their habit of storytelling:

“Our sex chromosomes weren’t always X and Y,” said Melissa Wilson, an evolutionary biologist at Arizona State University. “What determined maleness or femaleness was not specifically linked to them.” When the very first mammals evolved between 100 and 200 million years ago, they didn’t have any sex chromosomes at all. Instead, the X and Y were just like any other set of chromosomes — identical in size with corresponding structures…. animals don’t need sex chromosomes.[18]

Then, Whitcomb claims, “The only special feature of the Y chromosome is one gene, SRY, which acts as an on-off switch for the development of testes.”[19] And yet another study contradicted this claim, concluding, “A comparison of Y chromosomes in eight African and eight European men dispels the common notion that the Y’s genes are mostly unimportant and that the chromosome is destined to dwindle and disappear.” One reason for their conclusion was a finding that the worldwide genetic variation on the Y chromosome was very small compared to the comparatively large variety seen in the DNA of the non-sex chromosomes. The researchers believe that the large similarity of the Y chromosome in humans is evidence that the specific Y chromosome genes—although small in number—are very important for functions besides determining sex.[20]

The authors found that mutations are more prone to damage the 27 unique genes in the Y chromosome than in somatic chromosomes.[21] That is because the Y chromosome has no mate, thus recombination with another Y cannot occur as it does with all other chromosomes. Furthermore, as a result, “all sites on the Y are effectively linked together. Thus, selection acting on any one site will affect all sites on the Y indirectly.”[22] For detailed complex reasons I will not get into here, the study concluded, “there has to be a lot more function on the chromosome than people previously thought.”[23]

Fig 2: Recombination and crossing over during cell division (Wikimedia Commons)

Why It is Believed that the Y Chromosome is Losing Genes

Evolutionists believe the Y is shrinking because of mutations. They know that “genes develop mutations, many of which are harmful.”[24] Meiosis separates the chromosome pairs and one way of reducing the mutational load is by recombination. This occurs when paternal and maternal chromosomes randomly mix and match, again forming chromosomal pairs. For several reasons, this process makes it more likely that only functional DNA copies will be passed on to the offspring. All chromosomes including the X (for the female XX) take part in this recombination except the Y chromosome, because the Y does not have a swapping companion: thus crossing over is normally not possible (see figure 2). Y and X chromosomes, moreover, are not similar enough to recombine, and only rarely do two Y chromosomes exist in one individual.[25]

Several salient points stand out. The first is that the literature flip-flops on the question of whether the Y chromosome is shrinking. Only a small sample of articles and papers was referenced here. One says the Y chromosome is shrinking and will eventually disappear. Another argues it is not shrinking and will, as a result, not disappear. Soon another report comes out giving new reasons for the case that the male chromosome is, in fact, shrinking, which is met by yet another study that argues for the opposite, again giving plausible reasons.[26]

All of these contradictory reports depend on a few well-supported facts that are constructed on a foundation of evolutionary assumptions. Darwinism gave birth to evolutionary imagination that is used to interpret the world, and all life in it. Evolutionists use their imaginations to envision an old, slowly-evolving world, accumulating rare beneficial mutations over hundreds of millions years that are preserved by positive natural selection, while dispensing with obsolete genetic material through negative natural selection. Usually the big problem is ignored, namely, the evolution of sex itself, and its complex systems of chromosomes, meiosis, recombination and other processes designed to produce offspring that are a blend of their parents. This, F. Lagard Smith described in 2018, is “Evolution’s fatal flaw.”[27]

Fig 3: Meiosis producing a haploid state (one set of chromosomes) during the first cell division. The formation of the zygote produces the diploid condition, i.e., two sets of chromosomes. (Wikimedia Commons)

[1] Sykes, Bryan. 2003. Adam’s Curse: A Future Without Men. New York: W.W. Norton & Co.

[2] Haskett, Dorothy R. 2015. “The Y-Chromosome in Animals”. The Embryo Project Encyclopedia, May 28.

[3] Whitcomb, Isobel. 2020. “Is the Y chromosome dying out?”

[4] Grützner, Frank, et al. 2004. In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes. Nature 432:913-917.

[5] Grützner, et al., 2004, p. 913.

[6] “Sex Chromosomes: Why the Y Genes Matter”. 2015. Science 2.0, May 30.

[7] Graves, Jennifer A. Marshall. 2006. Sex Chromosome Specialization and Degeneration in Mammals. Cell 124(5):901-914, March 10, p. 912.

[9] Smith, F. LaGard. 2018. Darwin’s Secret Sex Problem: Exposing Evolution’s Fatal Flaw—The Origin of Sex. Bloomington, IN: WestBow Press.

[10] Pääbo, Svante. 2015. Neanderthal Man: In Search of Lost Genomes. New York: Basic Books, pp. 179-181, 243.

[11] Bergman, Jerry. 2018. “Is the Y Chromosome Disappearing?” Creation-Evolution Headlines, February 24.

[12] Griffin, Darren and Peter Ellis. 2018. “The Y chromosome is disappearing – so what will happen to men?” The Conversation, January 17.

[13] Hughes, Jennifer F., et al. 2015. Sex chromosome-to-autosome transposition events counter Y-chromosome gene loss in mammals. Genome Biology 16:104, May 28.

[14] Griffin, Darren. 2018. “The Y chromosome is disappearing – so what will happen to men?” PhysOrg, January 18.

[16] Sanders, Robert. 2014. “Study dispels theories of Y chromosome’s demise.” Berkeley News January 9.

[20] Sayres, Melissa A. Wilson, et al. 2014. Natural Selection Reduced Diversity on Human Y Chromosomes. PLOS Genetics, January 9.

[21] The number given varies. The National Human Genome Research Institute states that “the X chromosome is about three times larger than the Y chromosome, containing about 900 genes. While the Y chromosome has about 55 genes. “ Evidently the actual number in not known.

Illustra Media, “In the Image of God”

Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,500 college libraries in 27 countries. So far over 80,000 copies of the 40 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.

The Y chromosome is disappearing – so what will happen to men?

Chromosome Y in red, next to the much larger X chromosome. Credit: National Human Genome Research Institute

The Y chromosome may be a symbol of masculinity, but it is becoming increasingly clear that it is anything but strong and enduring. Although it carries the "master switch" gene, SRY, that determines whether an embryo will develop as male (XY) or female (XX), it contains very few other genes and is the only chromosome not necessary for life. Women, after all, manage just fine without one.

What's more, the Y chromosome has degenerated rapidly, leaving females with two perfectly normal X chromosomes, but males with an X and a shrivelled Y. If the same rate of degeneration continues, the Y chromosome has just 4.6m years left before it disappears completely. This may sound like a long time, but it isn't when you consider that life has existed on Earth for 3.5 billion years.

The Y chromosome hasn't always been like this. If we rewind the clock to 166m years ago, to the very first mammals, the story was completely different. The early "proto-Y" chromosome was originally the same size as the X chromosome and contained all the same genes. However, Y chromosomes have a fundamental flaw. Unlike all other chromosomes, which we have two copies of in each of our cells, Y chromosomes are only ever present as a single copy, passed from fathers to their sons.

This means that genes on the Y chromosome cannot undergo genetic recombination, the "shuffling" of genes that occurs in each generation which helps to eliminate damaging gene mutations. Deprived of the benefits of recombination, Y chromosomal genes degenerate over time and are eventually lost from the genome.

Despite this, recent research has shown that the Y chromosome has developed some pretty convincing mechanisms to "put the brakes on", slowing the rate of gene loss to a possible standstill.

For example, a recent Danish study, published in PLoS Genetics, sequenced portions of the Y chromosome from 62 different men and found that it is prone to large scale structural rearrangements allowing "gene amplification" – the acquisition of multiple copies of genes that promote healthy sperm function and mitigate gene loss.

The study also showed that the Y chromosome has developed unusual structures called "palindromes" (DNA sequences that read the same forwards as backwards – like the word "kayak"), which protect it from further degradation. They recorded a high rate of "gene conversion events" within the palindromic sequences on the Y chromosome – this is basically a "copy and paste" process that allows damaged genes to be repaired using an undamaged back-up copy as a template.

Looking to other species (Y chromosomes exist in mammals and some other species), a growing body of evidence indicates that Y-chromosome gene amplification is a general principle across the board. These amplified genes play critical roles in sperm production and (at least in rodents) in regulating offspring sex ratio. Writing in Molecular Biology and Evolution recently, researchers give evidence that this increase in gene copy number in mice is a result of natural selection.

Mole voles have no Y chromosomes. Credit: wikipedia

On the question of whether the Y chromosome will actually disappear, the scientific community, like the UK at the moment, is currently divided into the "leavers" and the "remainers". The latter group argues that its defence mechanisms do a great job and have rescued the Y chromosome. But the leavers say that all they are doing is allowing the Y chromosome to cling on by its fingernails, before eventually dropping off the cliff. The debate therefore continues.

A leading proponent of the leave argument, Jenny Graves from La Trobe University in Australia, claims that, if you take a long-term perspective, the Y chromosomes are inevitably doomed – even if they sometimes hold on a bit longer than expected. In a 2016 paper, she points out that Japanese spiny rats and mole voles have lost their Y chromosomes entirely – and argues that the processes of genes being lost or created on the Y chromosome inevitably lead to fertility problems. This in turn can ultimately drive the formation of entirely new species.

As we argue in a chapter in a new e-book, even if the Y chromosome in humans does disappear, it does not necessarily mean that males themselves are on their way out. Even in the species that have actually lost their Y chromosomes completely, males and females are both still necessary for reproduction.

In these cases, the SRY "master switch" gene that determines genetic maleness has moved to a different chromosome, meaning that these species produce males without needing a Y chromosome. However, the new sex-determining chromosome – the one that SRY moves on to – should then start the process of degeneration all over again due to the same lack of recombination that doomed their previous Y chromosome.

However, the interesting thing about humans is that while the Y chromosome is needed for normal human reproduction, many of the genes it carries are not necessary if you use assisted reproduction techniques. This means that genetic engineering may soon be able to replace the gene function of the Y chromosome, allowing same-sex female couples or infertile men to conceive. However, even if it became possible for everybody to conceive in this way, it seems highly unlikely that fertile humans would just stop reproducing naturally.

Although this is an interesting and hotly debated area of genetic research, there is little need to worry. We don't even know whether the Y chromosome will disappear at all. And, as we've shown, even if it does, we will most likely continue to need men so that normal reproduction can continue.

Indeed, the prospect of a "farm animal" type system where a few "lucky" males are selected to father the majority of our children is certainly not on the horizon. In any event, there will be far more pressing concerns over the next 4.6m years.

This article was originally published on The Conversation. Read the original article.

Who knows Y chromosome is shrinking

SIZE doesn't matter. At least when it comes to a man's chromosomes.

Researchers from the Australian National University have discovered that the Y chromosome is shrinking as it loses genetic material but Paul Waters said men were not at risk of becoming extinct.

''Men will always be men, irrespective of the size of the Y chromosome,'' he said.

Researchers analysed DNA samples from tammar wallabies, finding more genes on the male chromosome than expected.

Dr Waters said the genetic material retained on the marsupials' Y chromosome had been lost in placental mammals such as humans, indicating different rates of gene loss across species.

''That was completely unexpected,'' he said.

''What is going to happen to the human Y chromosome is not exactly clear. You can't predict these things.''

Dr Waters said the Y chromosomes had been lost in some species of rodents and said it was possible a similar disappearance could occur in humans.

''Theoretically it could be lost,'' he said.

''The genes that determine maleness would get replaced with a switch that determines maleness.''

The Y chromosome contains male-specific genes including the testis-determining gene. Dr Waters said this and other genes on the Y chromosome that were important for male development would be moved if the chromosome did disappear.

Neanderthal Y Chromosome Looks Modern

A new study analyzed the genome of what they termed “our closest relatives, Neanderthals and Denisovans,” which were then compared with modern humans.[1] Most previous Neanderthal DNA research has been on females because the X chromosome preserves better than the fragile Y. Fortunately, a team of geneticists from the United States, China and Europe were able to study the male Y chromosomes from three Neanderthals and two Denisovans. The team found from their limited sample that the modern human and Neanderthal Y chromosomes were more closely related to each other than the Neanderthal and the Denisovan Y chromosomes.[2] They concluded that Homo sapiens and Neanderthals were more similar genetically than were the Neanderthal and the Denisovian. The U.S. National Library of Medicine summary presented the conventional evolutionary explanation by writing the following:

Neanderthals were very early (archaic) humans who lived in Europe and Western Asia from about 400,000 years ago until they became extinct about 40,000 years ago. Denisovans are another population of early humans who lived in Asia and were distantly related to Neanderthals. (Much less is known about the Denisovans because scientists have uncovered fewer fossils of these ancient people.) The precise way that modern humans, Neanderthals, and Denisovans are related is still under study. However, research has shown that modern humans overlapped with Neanderthal and Denisovan populations for a period, and that they had children together (interbred). As a result, many people living today have a small amount of genetic material from these distant ancestors.[3]

Myth: The Y Chromosome Will Eventually Disappear, Resulting in Male Extinction

X and Y chromosomes differ dramatically in size and content

Given the dates scientists claim Neanderthals and Denisovans lived, from 400,000 to 40,000 Darwin-years ago, one would expect evidence for the shrinking Y chromosome prediction. Yet as far as we can tell, in 400,000 to 40,000 Darwin years, the Y chromosome has not changed one iota in length.

The Y chromosome is used in all primates, most mammals, and even in some insects and plants, to produce males.[4] The assumption that the Y chromosome will disappear is based on the theory that it originally evolved from a much larger autosome chromosome. The evolution of the sex chromosomes from the autosome chromosomes is postulated largely because evolutionists cannot figure how else the sex chromosomes could possibly have originated.

Thus, evolutionists assume that the X and Y chromosomes were once both equal in length and in the number of genes. The theory postulates the male Y chromosome lost nearly all of the 640 genes it once shared with the X chromosome. Furthermore, the essential genes once on the X chromosome were somehow transferred to the somatic chromosomes.[5] As a result, the theory concludes, the Y chromosome in humans now has only 27 unique genes compared to thousands on most somatic chromosomes.

The 2020 Study by Martin Petr, et al.

The 2020 study by Petr, et al. did not focus on the length of the Y chromosome, but rather on comparisons of Neanderthals and modern humans. Nonetheless, they did not find evidence that the 400,000 to 40,000 Darwin-years-old Neanderthal and Denisovan Y chromosomes were significantly longer than those of modern man, which would be expected if the Y chromosome was shrinking. They did find evidence of what are evidently minor differences between the Neanderthal and the Denisovan Y chromosomes.

The Cause of the Differences Found

Geneticists expect major differences in 40,000 to 400,000 Darwin-years-old Y chromosomes due to damage to DNA by the ubiquitous background ionizing radiation on the Earth’s surface. The radiation is from both geophysical sources (in soil minerals including potassium, uranium and thorium and their decay products) and astrophysical sources (such as cosmic radiation). These sources of ionizing radiation disintegrate DNA which is constantly repaired in living organisms. Once the creature is dead, however, DNA is no longer repaired. Consequently, DNA damage accumulates.[6] They explain away the lack of accumulated damage with over such long periods of time by affirming their deep-time belief. They say that although

DNA is a relatively weak molecule’ comparatively speaking, yet under certain conditions it persists in the fossil record.[7]

DNA showing ultraviolet damage to the base-pair bonds. (Wiki Commons)

The researchers noted that “Neanderthals have been shown to have accumulated an excess of deleterious variation compared with modern humans” which may account for some of the genetic differences found. This is the opposite of what we would expect due to the mutation accumulation trend seen today. One speculates that the “deterioration” occurred due to damage by radiation-caused mutations. This caused the difference found in the Petr, et al. study, especially considering the claim that the Y chromosome DNA tested were from 400,000 to 40,000 Darwin-years old.

The researchers wrongly predicted that Neanderthal Y chromosomes would still have the primitive DNA and “will therefore be more similar to Denisovans than to modern humans.”[8] What was found was

like maternally inherited mtDNA, modern human and Neanderthal Y chromosomes were more related to each other than to the Denisovan Y…. all but the earliest Neanderthal mtDNA samples are far more similar to those of modern humans than to those from Denisovans.[9]

[Ancient] nuclear and mtDNA sequences revealed phylogenetic discrepancies between the three groups that are hard to explain”

Hard to explain, that is, by evolution. But the data make sense given what we know about mutations.[10]

Problems with the Study

One problem with the study is the small sample size. This is expected due to the fact that virtually none of the male Neanderthal and Denisovan remains studied so far have contained well-preserved Y chromosome DNA. The problem of DNA preservation is enormous. Even in cases where the body was known to be buried only a decade ago the DNA is often close to worthless.

Another problem is the time span. If the samples were actually 400,000 to 40,000 years old—based on mutation rates documented in contemporary studies—the DNA would have long deteriorated to the point that few organized DNA strands would have remained even under good preservation conditions. Professors Meisenberg and Simmons note that the

mutational load is kept in check by natural selection. In most traditional societies, almost half of all children used to die before they had a chance to reproduce. Investigators can only guess that those who died had, on average, more “mildly detrimental” mutations than those who survived.[11]

Sanford’s book examines the impact of near-neutral mutations that are invisible to selection.

Mutations are widely recognized as a major cause of many diseases, including cancer and heart disease. An estimated 99.9% of all mutations are, in the long run, harmful. In a review of the mechanisms that drive genetic degeneration, Charlesworth and Charlesworth concluded that “most mutations with observable phenotypic effects are deleterious.”[12] Estimates vary greatly, but generally, around one new mutation occurs in “each round of cell division, even in cells with unimpaired DNA repair and in the absence of external mutagens.”[13]

As a result, for germ-line mutations, “every child is born with an estimated 100 to 200 new mutations that were not present in the parents.”[14] Cornell University Professor John Sanford puts the number of point mutations at about 200 in each generation, and for all new mutation types the number is closer to 1,000 in each generation.[15] Given this data, the fact that some intact DNA strands still exist in the Y chromosome is evidence that they are not nearly as old as claimed.

This study is one of many that is slowly filling in our knowledge about human life on the early Earth. As the evidence builds, the trend is supporting, not evolution, but a creation worldview. I am looking forward to the next research studies in this self-correcting enterprise called Science.

Packing 6 feet of DNA into a cell nucleus is a highly complex process. (Wiki Commons). See the Illustra film, 󈬂 Trillion Feet of You” to see how it is done.

[1] Sci-News News Staff. 2020. Scientists Sequence Y Chromosome DNA of Denisovans and Neanderthals. Sci-News,. September 25.

[2] Petr, Martin et al. 2020. The evolutionary history of Neanderthal and Denisovan Y chromosomes. Science 369(6511):1653-1656. doi: 10.1126/science.abb6460.

[3] U.S. National Library of Medicine. 2020. What does it mean to have Neanderthal or Denisovan DNA? Genetics Home Reference, August 17. [Not a stand-alone website as of October 1, 2020.]

[4] Haskett, Dorothy R. 2015. “The Y-Chromosome in Animals”. The Embryo Project Encyclopedia, May 28.

[5] Hughes, Jennifer F., et al. 2015. Sex chromosome-to-autosome transposition events counter Y-chromosome gene loss in mammals. Genome Biology 16,Article number 104, May 28.

[6] Mitchell, David Eske Willerslev and Anders Hansen. 2015. Damage and repair of ancient DNA. Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis 571(1-2):265-276.

[7] Poinar, Hendrik N. 2002. The Genetic Secrets Some Fossils Hold. Accounts of Chemical. Research. 35(8): 676–684, February 2.

[11]Meisenberg, Gerhard and William H. Simmons. 2006. Principles of Medical Biochemistry. 2nd edition. Linn, MO: Mosby/Elsevier, p. 153.

[12] Charlesworth, Brian and Deborah Charlesworth. 1998. Some evolutionary consequences of deleterious mutations.” Genetica, 102/103:3-19, p. 3.

[13] Meisenberg and Simmons, 2006, p. 153.

[14] Meisenberg and Simmons, 2006, p. 153.

[15] Sanford, John. 2014. Genetic Entropy. 4 th edition Lima, NY: Ivan Press.

Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,500 college libraries in 27 countries. So far over 80,000 copies of the 40 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.

Y Chromosome Is More Than a Sex Switch

The small, stumpy Y chromosome—possessed by male mammals but not females, and often shrugged off as doing little more than determining the sex of a developing fetus—may impact human biology in a big way. Two independent studies have concluded that the sex chromosome, which shrank millions of years ago, retains the handful of genes that it does not by chance, but because they are key to our survival. The findings may also explain differences in disease susceptibility between men and women.

“The old textbook description says that once maleness is determined by a few Y chromosome genes and you have gonads, all other sex differences stem from there,” says geneticist Andrew Clark of Cornell University, who was not involved in either study. “These papers open up the door to a much richer and more complex way to think about the Y chromosome.”

The sex chromosomes of mammals have evolved over millions of years, originating from two identical chromosomes. Now, males possess one X and one Y chromosome and females have two Xs. The presence or absence of the Y chromosome is what determines sex—the Y chromosome contains several genes key to testes formation. But while the X chromosome has remained large throughout evolution, with about 2000 genes, the Y chromosome lost most of its genetic material early in its evolution it now retains less than 100 of those original genes. That’s led some scientists to hypothesize that the chromosome is largely indispensable and could shrink away entirely.

To determine which Y chromosome genes are shared across species, Daniel Winston Bellott, a biologist at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, and colleagues compared the Y chromosomes of eight mammals, including humans, chimpanzees, monkeys, mice, rats, bulls, and opossums. The overlap, they found, wasn’t just in those genes known to determine the sex of an embryo. Eighteen diverse genes stood out as being highly similar between the species. The genes had broad functions including controlling the expression of genes in many other areas of the genome. The fact that all the species have retained these genes, despite massive changes to the overall Y chromosome, hints that they’re vital to mammalian survival.

“The thing that really came home to us was that these ancestral Y chromosome genes—these real survivors of millions of years of evolution—are regulators of lots of different processes,” Bellott says.

Bellott and his colleagues looked closer at the properties of the ancestral Y chromosome genes and found that the majority of them were dosage-dependent—that is, they required two copies of the gene to function. (For many genes on the sex chromosomes, only one copy is needed in females, the copy on the second X chromosome is turned off and in males, the gene is missing altogether.) But with these genes, the female has one on each X chromosome and the male has a copy on both the X and Y chromosomes. Thus, despite the disappearance of nearby genes, these genes have persisted on the Y chromosome, the team reports online today in Nature.

“The Y chromosome doesn’t just say you’re a male it doesn’t just say you’re a male and you’re fertile. It says that you’re a male, you’re fertile, and you’re going to survive,” Bellott explains. His group next plans to look in more detail at what the ancestral Y chromosome genes do, where they’re expressed in the body, and which are required for an organism’s survival.

In a second Nature paper, also published online today, another group of researchers used a different genetic sequencing approach, and a different set of mammals, to ask similar questions about the evolution of the Y chromosome. Like Bellott’s paper, the second study concluded that one reason that the Y chromosome has remained stable over recent history is the dosage dependence of the remaining genes.

“Knowing now that the Y chromosome can have effects all over the genome, I think it becomes even more important to look at its implications on diseases,” Clark says. “The chromosome is clearly much more than a single trigger that determines maleness.” Because genes on the Y chromosome often vary slightly in sequence—and even function—from the corresponding genes on the X, males could have slightly different patterns of gene expression throughout the body compared with females, due to not only their hormone levels, but also their entire Y chromosome. These gene expression variances could explain the differences in disease risks, or disease symptoms, between males and females, Clark says.

Watch the video: Shrinking Y chromosome E (January 2022).