I Once Was Blind But My Offspring Might See
by Jacob (AP Biology)
Crazy stuff huh? You think, wait, if I am blind then how can my offspring not be [blind]? Well, a study at New York University showed that through mating different populations of the species, Astyanax mexicanus, or blind cavefish, some of their offspring could see.
These cave fish diverged many millions of years ago from regular fish and a certain mutation in the diverged fish made it lose its sight. Natural selection let those genes be passed on through several generations until today. Although natural selection made the blindness in cave fish become the prominent trait, the ability to see was still in the gene pool.
By mating two differing populations of cave fish the trait to see had a larger chance to be produced. This is because the code for the cavefish to be blind in DNA is determined by multiple genes and in the different populations of cavefish they have different sequences of those genes that code for blindness. When two different populations are mated, the genes that cause blindness can be replaced in the offspring generation by the genes that both parent fish have that do not cause blindness.
Let’s say that the presence of just one homozygous recessive gene causes blindness. If a group of genes represents two blind fish, AABBcc and AAbbCC, and these two are mated, a possible offspring could be AABbCc. Because the recessive genes are now hidden in the heterozygous condition the offspring would have the ability to see. This finding can be used to help scientists understand the human eye better and help prevent blindness.
Astyanax mexicanus or the Blind cavefish is not endangered but it is a threatened fish. They grow to around 8 cm. in length. They live around sandy, rocky backwaters of creeks and small rivers. They eat worms and insects. They are normally found in Texas through Mexico. Check out more on the Blind cave fish at this site.
March 4th, 2008 at 6:46 pm
That is a really interesting post, because I had always thought that if two blind organisms mated, that the offspring is going to be blind. I understand that mating two different populations of the same species could allow the ability to see in the fish. But why would natural selection allow blindness to be a prominent trait, because it is not an advantage to the environment its living in, is it? But if natural selection gave a species a trait that would make it harder to survive, wouldn’t natural selection just eventually let the organism go extinct. And if AABBcc and AABBcc are mated and they are both blind, how does an offspring with AABbCc have the ability to see. Because isn’t AA still the homozygous dominant trait for blindness. And if earlier before this discovery, if two blind fish with homozygous recessive traits mated, wouldn’t they have been able to produce an offspring with the ability to see. So the offspring mates with another fish that can see, doesn’t that just make the offspring from then on have the ability to see. What makes this a big discovery?
March 4th, 2008 at 7:35 pm
Wow, this has everything to do with what we are learning in class about genetics! Genetics are so interesting because they explain how two blind fish can produce a seeing offspring, or why two brown eyed parents can produce a blue eyed child. I had always wondered why some children have different characterisitcs from their parents, but now it makes sense.
Jacob, you said this finding can be used to help scientists understand the human eye and prevent blindness. How?
Here is an article from National Geographic that talks more about the blind cavefish producing sighted offspring: http://news.nationalgeographic.com/news/2008/01/080108-cave-fish.html
Great post Jacob!
March 4th, 2008 at 7:48 pm
Remember that question on the worksheet that asked how two albino individuals (a homozygous recessive condition) could produce a normal child? This explains it!
Alex, and everyone else with similar questions, read this great article about the blind cave fish. It discusses why natural selection would choose for blind cave fish over seeing cave fish. AP Biology students, this is important for your animal development unit as it talks about hedgehog and hox genes!
March 4th, 2008 at 8:18 pm
This post can relate to the 9th grade test question on how natural selection can’t get rid of diseases. Even though the mutation for blindness is not a disease it’s not a quality most people or fish would want! Anyways, Miss Baker explained after the test how natural selection can not get rid of diseases because the mutations will always stay in our genes, they just might not show up. So, this person would have to be a carrier. I think its neat how certain genes can be recessive or dominant in order for you to have that certain characteristic. Why are they different? So, in other words why can’t all traits be recessive in order to have that certain characteristic?
March 4th, 2008 at 8:57 pm
That is really interesting Miss Baker, because I had no idea about some of these things before I read this article. I had no idea that making body parts in an embryo is actually work for the embryo. So does this mean if the embryo is unable to make the body part, that you will simply not have it. Because it said in the article that the embryo has to work to give you eyes. And if you don’t need a certain body that you could lose it because it is not an advantage to the organism. In the article it says that if the fish loses its eyes, it wouldn’t matter being it is not helping it at all and it does not affect the animal. So if a human lived in an environment that didn’t require having hands, then could humans possibly lose hands over a period of time?
March 4th, 2008 at 9:03 pm
This is a great post and all, but I have always been aware that just because a parent has a trait, doesn’t always mean that the offspring will. There are no breakthroughs in science so this should already be well known. If you mate a blind fish with one that doesn’t have the gene, it all goes back to punnet squares and people can tell that it is a possibility that the offspring will see. Just as Miss Baker said we had a question about this. This goes for any animal and is common in genetics: sometimes offspring have different traits from parents. I don’t really see this study by New York University really as brand new. I thought this was commonly known. Nice post though Jacob, you did a good job explaining this.
I also found a link to a website that tells a lot about blind Cave fish. They are really interesting creatures!
March 4th, 2008 at 9:04 pm
woops the link is http://freshaquarium.about.com/cs/characins2/a/blindcavefish.htm
March 4th, 2008 at 9:19 pm
This really is quite cool, Great post. Here is a video of the blind cave fish.
http://www.youtube.com/watch?v=iuMVo2dv3_c
These fish are very remarkable and just like Amanda said it is a great example of what we are learning about natural selection. It has actually helped me understand this concept better. This website is pretty neat check it out. http://www.flmnh.ufl.edu/fish/InNews/blindcavefish2008.html. Scientists conducted an experiment to test the blindness of these fish and most of them were able to see for a while and the scientists concluded that by the aging of these fish their eye sight deteriorates.
March 4th, 2008 at 9:38 pm
Alex, go back to the article and try again. The economics hypothesis (organs that are too expensive to create will deteriorate) does not fit the evidence and in the article the author discusses two more hypotheses. It is the third hypothesis, pleiotrophy, that fits the evidence.
Grant, the blind cave fish were taken from two separate populations. That is the discovery.
March 5th, 2008 at 10:30 pm
This post is very interesting! As amanda has said above about two parents that are blind producing a seeing offspring blows my mind. Also, this really explains our albino question, like miss baker said. Its interesting how when multiple genes code for a trait the odds of the offspring having a certain trait are narrowed down. It related back to the coin flip example we had in class, where the odd remained 50/50 each time, but every time it was done the odds were multiplied together for a specific outcome. Great Post!
March 6th, 2008 at 9:39 am
So to recap, hedgehog and pax6 are genes that move simultaneously in positive and negative correlation. One can only become more prominent at the expense of the other. Thus the blind cavefish is not blind due to simple economics or chance; it is blind because it reduced its ability to use one of its senses that wasn’t helpful to it, in exchange for enhanced senses such as taste and touch.
March 6th, 2008 at 9:50 am
Cool post Jacob! I read the article from Miss Bakers comment and this is what I found:
You know how a person who is blind can often hear better than a person who can see? Well this is almost what happened with the cave fish over time. Except for the fact that it was not their hearing that got better but rather their ability to taste and feel with their lower jaw. Now some of you might be wandering how this happened. Well, it all comes down to pleiotropy. The basic idea of pleiotropy is that one gene can code for many different things on one organism. It is important to understand two specific genes that result in the cave fish being blind.
The one is Pax 6, which codes for the development of the eye. And the other is Hedgehog, which codes for the mid line of the animal and parts of the fish’s body like the jaw, teeth and taste buds. Now, cave fish live in the dark. Therefore, if a fish has stronger senses in its jaw and taste buds (which means it has expanded Hedgehog function), it will be able to feel for food and taste when it finds it, better than a fish that just has normal sensory in that area. So the fish that had these traits probably survived better than the fish who had better eye function (which was useless in the dark) so the fish with enhanced sensory functions passed the trait on to its offspring. However, when Hedgehog enhances the sensory function of the fish, it inhibits the expression of Pax 6. So, they are negatively coupled, which means that it enhances expression of one trait (like the senses in the jaw and taste buds) and it inhibits the expression of another trait (like eye development). This is pleiotropy working. So over time the fish that could feed better survived to reproduce and pass the trait on to their offspring. And so the frequency of the fish with undeveloped eyes increased in the cave fish population.
March 6th, 2008 at 3:39 pm
Pleiotropy is where a single gene can have different effects on an organism. Two genes that act in this way for the blind cave fish are pax6, a gene that regulates eye development, and hedgehog, which helps to set up the midline of an animal; but its also a teeth, jaw and tastebud gene. And while Hedgehog does not have anything to do with eye development, it does inhibit the pax6 gene, which is important for eyes. This means that they are negatively coupled, but it is not a defect. It is simply another way for positive selection to occur.
So if the organism is in an environment which causes its eyes to be not as important and the way it finds food, the process kicks in. By inhibiting the pax6 gene, hedgehog causes eye development goes down in the cave fish, while the jaw, teeth and tastebuds are becoming more developed. This is good for species who can survive without the use of vision. The increased sensitivity in the jaw allows the fish to probe its environment for food that it could not find before. This is a great example of Pleiotropy in action.
March 6th, 2008 at 6:51 pm
I think I have an understanding of what pleiotropy is. So this means that pax6 expression, which is the expanding jaws and taste buds will cause reduction of eyesight. So since hedgehog and pax6 are negatively paired to one another, only one can bee expanded at the expense of the other. This means that the only reason the blind cavefish are losing its eye is because it does them no use and this way the jaws and taste buds can expand, which will be more beneficial to the blind cavefish. Does this mean that if two necessary genes were paired together, that the gene that is more useful will be chosen and the other gene will be reduced to expand the advantages of the more useful gene? And are there only certain genes that can be negatively coupled together, or would it be completely random if this were to happen.
March 6th, 2008 at 7:17 pm
After debunking the first two hypotheses, Myers explained in his article, “PZ Myers On How The Cavefish Lost Its Eyes”, how pleiotrophy, when a gene can have several affects on an organism instead of just one, affects blind cave fish. The gene pax6 controls the development of the eye, as well as jaw, teeth, and taste buds while the hedgehog protein has a role in inhibiting and separating structures. When it is absent or defected, some features will appear deformed and may lead to disorders like cyclopia. Knowing how important the hedgehog molecule is to the expression of teeth, taste buds, and the jaw, animals that have an expanded hedgehog function would benefit those that relied on senses other than their eyes to search for food. The expanded function means that since hedgehog and pax6 are interdependent, increasing one (hedgehog) would affect the other negatively (pax6, which codes for eyes). This all means that while losing its eyesight, the fish are actually selecting for other heightened senses, such as smell and taste. The blindness is just a result of the negatively coupling of the pax6 gene and hedgehog.
March 6th, 2008 at 7:30 pm
When I read this post, I thought the exact same thing as Amanda and Codie; this is exactly like what we learned about genetics and does relate back to that question on the test that a lot of people missed. I guess they don’t have to compete with each other because they are blind, so the trait stays. According to the article, mutations at multiple gene sights resulted in the evolutionary impairment of eye development as well as loss of pigmentation and other cave related changes. Also, the article said that scientists used hybridization to recombine the right genes for sight, which reminded me of what we learned about artificial selection and how humans did the same thing with species of dogs. I didn’t even think about the possibility that it could be used with something other than fur color or size. According to the website, they are endangered, which does not surprise me considering they are blind!! That website you found was so cool Amanda! I loved how they came up with an eye test for these fish!! I also agree with Melody and found the same thing; in the dark caves, the fish don’t need to see, so why waste energy creating fully developed eyes? Instead, why not focus on bettering a more helpful sense, like taste to find your food, with hedgehog?
I found this great article that says that blind cavefish can detect light with a light sensitive organ in their brains!! It is in the pineal gland, which controls the body’s day night cycle. What’s even funnier is that this discovery was made by chance!! Check it out!!
http://www.foxnews.com/story/0,2933,326471,00.html
Great job Jacob!! I think I see the point
March 6th, 2008 at 8:43 pm
I never would have thought that the expansion of one gene could cause another to reduce! I think that it is awesome that blindness in the fish is determined by multiple genes, and since it is recessive, they have less of a chance of being blind than if it was only determined by one gene. You wouldn’t think that losing your sight would be advantageous, and I guess it isn’t, but the development of the wider jaw and keener sense of taste at the expense of the eye are advantageous. I guess it all depends on priority.
Also, scientists have been trying to explain why humans have appendixes. The article that Miss Baker put a link to helps explain why the fish still have eyes, and I think that this can be applied to why we have appendixes. The article explains that an unnecessary organ will not just disappear. So maybe, millions of years ago, we had two opposing genes, one being the appendix, and the development of the other was more beneficial than the expense of the appendix was harmful. I could be completely wrong in my ideas about this, so don’t take it for fact. I am just throwing it out as a possibility. Great post, Jacob.
March 6th, 2008 at 9:53 pm
A protein called Hedgehog controls the midline of an animal. It expresses teeth, tastebuds, and the jaw (all very important to the cavefish who lives in such a dark environment, these are the only things that will help it find food or escape preditors.) If the protein Hedgehog is mutated, effects could be fatal. It is responsible for inhibiting and separating structures. If this does not happen, an organism could suffer from cyclopia, meaning that it has both of it’s eyes, they just were never separated, so they have one big eye. This protein inhibits a gene called pax6 will be inhibited. This gene controls the development of eyes.
Therefore, if the hedgehog protein is present, then eyes will not develop while the jaw, taste buds, and teeth are developing.
March 6th, 2008 at 9:58 pm
As amanda has said above about two parents that are blind producing a seeing offspring blows my mind. Also, this really explains our albino question, like miss baker said. Its interesting how when multiple genes code for a trait the odds of the offspring having a certain trait are narrowed down. It related back to the coin flip example we had in class, where the odd remained 50/50 each time, but every time it was done the odds were multiplied together for a specific outcome. Great Post!
March 6th, 2008 at 10:29 pm
The problem that is discussed in the article is why the Mexican Blind should fish, which lives in total darkness, loose its eyes. There are two theories that were originally accepted but have been proven to be wrong. The first theory is that it is an economical adaptation. It takes a lot of energy to construct something diverse as an eye. If the fish didn’t make the eye, it could use the energy towards something else in its body. Yet for the blind cavefish, their embryos actually make the eyes but then suddenly stop.
The second theory is that loosing an eye doesn’t affect the fish, they just disappear, and there is no advantage towards then but also any cost. Yet again, this is proven wrong, because the genes in the fish are functional so there must be a different reason. If you go back to pleiotropy we learned that one gene can have different effects on other functions of the organism. The gene pax6 in the Mexican Blind fish controls the development of the eyes and regulates the expression of other genes in the embryonic head where the eyes will form. Knowing this, if the Mexican blind fish has a mutation or defect in development of the eye, we can hypothesize that there is probably something having a negative effect on pax6. Hedgehog, a protein that can cause to inhibit certain genes, actually inhibits pax6. Hedgehog inhibits the eye gene pax6, but also plays a major role in genes of the taste buds, teeth and the jaw. Therefore we can assume that if there is a large amount of hedgehog, it greatly suppresses pax6. In the Mexican blind fishes case, in strengthening the Hedgehog, you have an increase of taste and a larger jaw, but the feedback also has a negative effect on the pax6 which is key role into the development of eyes. We see here that the negative of having reduced eyes is actually a positive effect on the fish in more than one way.
March 6th, 2008 at 11:09 pm
Jacob, excellent post! This is really interesting, not only because it defies the stereotype of genetics (you think the offspring has to be blind because the parents were blind) but it also does not necessarily work along the terms of natural selection as we consider it in basic terms. The fish that survive in the dark environment and do not really have need for eyes still have them, and we stop and ask ourselves “Wait… if it’s energy expensive to make the eyes, why do the fish develop eyes at all? Why has natural selection allowed for these fish to still develop eyes if the do not need them?”
The answer is not in natural selection in terms of the average on of us thinks in. In the blind cave fish’s instance, natural selection has gone deeper than originally meets the eye. In the blind cave fish, a “master gene” if you will, called Pax6 works in eye development and codes for the eyes to be built in embryo development. Well, it just so happens that another gene called hedgehog is pleiotropic, meaning it codes for several different functions in the body (teeth, jaws, and taste buds), as well as turning off Pax6. This is why the fish do not have functioning eyes- the hedgehog gene turns off the Pax6 gene, which stops the development of the eyes in the later embryonic stage! And since the hedgehog gene is pleiotropic, it also develops the teeth, jaws, and taste buds, thus providing more sensory for areas of the body that the fish can use in its dark environment. Isn’t natural selection amazing!?
March 6th, 2008 at 11:21 pm
This is a very informative web site. This article deals shows how pleiotropy could be the cause blindness in cave fish. Pleiotropy is one gene that affects many different parts of the organism. When two genes under pleiotropy that effect one another are called a master gene. Two that are in cave fish are Hedgehog and Pax6. Hedgehog creates the midline of the cave fish and is also seen in the jaw taste buds and teeth and Pax6 helps develop the eye. How do these two genes have anything to deal with each other? Hedgehog actually inhibits Pax6. So if there is more Hedgehog expression then there will be less Pax6 expression. Or in other words the more Hedgehog there is then the more developed the taste sense is while making the eyes much weaker. So if the Hedgehog gene been selected by natural selection it will indirectly cause the cave fish’s eyes to diminish indirectly.
March 8th, 2008 at 10:32 pm
Thats a great post and it helps to back up what we learned in class. It helps to see what we learned in another real life example. Its also really interesting to see how genes affect not only genes like it but other genes like in the cavefish example. It is cool that lacking in one gene that seems so important can actually benefit an individual. the cavefish’s genes can combine to alter its chance of seeing and how that can affect other charateristics. Its like all the genes are intertwined and its a big chain of effects. Even after learning about it in class i thought that each gene just affected one trait. That post helped to clear up alot of misconceptions i had. Good post!
March 9th, 2008 at 1:09 am
Fantastic post! It always helps me to learn a subect when i see it used outside of school. I was intrigued by the thought of fish being able to survive without eyesight and found an interesting post on another science Blog.
http://www.livescience.com/animals/080128-blindfish-notblind.html
It starts off talking about the same subject with blindfish gaining their eyesight because of genetics, but later in the article it says that the larvae of the fish that dont have eyes, are still able to detect light and shadows. This allows them to find shadows and hide in them from predators. The site says that they are able to do this because of their Pineal Gland. It is a small organ in organism’s bodies that regulate their biological clock, and in some species it gives the organism the ability to detect light even without eyes. Apparently when a neurologist was studying blindfish he noticed that the larvae responded to shadow movement. So they followed up on this lead and removed the fishes eyes and pineal gland. They noticed that the fish would only not be able to see shadows when the pineal gland was removed, so they came to the conclusion that the blind cavefish are able to detect light because of this neurological gland in their brains.
March 9th, 2008 at 4:06 pm
If, like me, you arent in AP Bio, you might have gotten confused on the last two comments. I had no idea what PAX6 was. SO i looked it up and found good info on it. http://www.jyi.org/volumes/volume1/issue1/articles/friedman.html
This gives alot of great info about it. I learned that PAX6 codes for the formation of complex and simple eyes.
SOmething interesting also is that PAX mutations may cause some forms of cancer, just thought i would throw that out their.
Thank you Jacob, after learning about PAX6 i see, better, what you were talking about with the hedgehogs
March 10th, 2008 at 1:05 pm
I did a little research on the Astyanax mexicanus. this is what I found:
Astyanax embryos have the same favorable attributes for developmental studies as those of the zebrafish. The head of a 24 hr. cavefish embryo. The cavefish embryo initiates eye development by forming a lens vescicle and optic cup. Eye development subsequently arrests. Genetic crosses of cavefish and surface fish yield F1 progeny with uniformly small eyes and pigmentation. Backcrosses of the F1 progeny to the cavefish parent yield fish with eyes ranging from completely regressed to the small eyes characteristic of the F1s. This result shows that multiple genes are involved in eye regression. Pigmentation segregates independently of eye formation.
Websites:
http://life.umd.edu/labs/jeffery/genetics.html
http://life.umd.edu/labs/jeffery/development.html
March 10th, 2008 at 2:33 pm
This has to do with everything that we have been covering over the couple of weeks. First let me say this is a great post. I really liked the website and photos you linked on to the blog. As i was reading over the comments i read Jacob D’s comment and saw PAX6… i wasn’t really quite sure what that was but i kept reading on and saw ben’s comment. Well since it provided a link that gives brief yet descriptive information about what PAX6 is and what may code for it. So thank you ben for clearing up a few of confusing concepts. Even though we have been learning about genetics for a while, this definitely helped me understand and fit a real life example into what we have been talking about in class. So great post Jacob and way to add on to the conversation that allowed people such as myself to do a little of their own research. And i found out that PAX6 is a protein that is highly conserved across species. PAX6 stands for Paired Box Gene 6. Here is an example of the usage of PAX6. For instance mouse PAX6 can trigger eye development in Drosophila melanogaster, or a two-winged insect, such as a fly.
April 13th, 2008 at 7:07 pm
That is really good example of generation affect. Actually, i surprised about this research. Because human blind is from the ancestor. But, I thought that fishes were different with people. Actually, human and fish basics are same about the genetic. Actually, most agrarian animals evoluted from the sea animals. Maybe this is reason it. So, thanks Jacob! good job buddy!