QUESTION: I hope you don't think this is a silly question, but my dad says I hold a tennis racquet just the way my Mum used to do. Now, as I understand it genes transfer information through proteins, so how do proteins affect the way I hold a tennis racquet? This question also applies to those cases of identical twins brought up separately - some of them seem to like the same kind of clothes. How can that be coded through proteins?
And here's a question that might be even sillier. If we share DNA with all life, then the blueprint for creating a lettuce must be coded in proteins. But where's the protein in lettuce? My Domestic Science teacher says there's no protein in lettuce at all.
Hope you're not offended by the silliness of the questions.
ANSWER: Hi Mary,
It is certainly not a silly question. All your questions are valid and show your scientific curiosity.
Well, coming to your Q one by one...
All traits that stem from a physical structure of living organism are determined by genes. "Tangible" traits like color of skin, hair,etc, shape and looks and so on are manifestation of genes..that is easy to understand. But what about your thinking and behavior which again stem from the brain which is a physical structure consisting of cells which derive their orders from the genes it inherited.. similarly intelligence.
Likewise, when it comes to holding a racket, let us see what are the makings of this action. You hold a racket in a particular style that comes naturally to you. While there is no single gene that will dictate how to hold a racket, certainly, your hand structure, muscles, shape, the biometrics and biomechanics are going to determine this action. All of these are in turn shaped by the genes in the different cells, which was partially inherited from your Mom. Your Dad's gene's would also play a role but whichever is dominant here will be the one to visibly manifest. Next, how is this shape-structure-biomechanics tied to your Mom's style? Given the biological structure, when your hand has to move and perform in a physical world, biomechanics will come to play. So, for a given body shape, strength and metrics, your arms will find one particular way of holding the racket most convenient or requiring least effort (energy expenditure). And needless to say, if you shared much of your Mom's hand and body construction, it is logical to conclude that you ended up holding the racket like her. And if you are already born in to an ambience of sports, your observations will subtly refine your actions. Putting all this together, you would subconsciously develop a posture and racket holding style that most closely matches your Mom.
But all of the role of genes does not undermine the part played by environment. Envt. shapes all of our traits as an adaptation, and we gain acquired behavior which is best supported by our genetic trait.
As for your 2nd Q, I am not sure I understand correctly. Yet..
It is true that we share genetic similarity even with the humble fungus. The fact is that we share key proteins required for life with almost all living creatures that have made it to date. So, no doubt many genes that coded for a lettuce are also found in humans. When your teacher says "no proteins in lettuce" she probably meant proteins as in nutritional content. But genetically speaking, a lettuce cannot exist without proteins which are encoded by genes. Infact, all genes universally encode for proteins ( and RNA of course). The very structure of a leaf is made of proteins. The enzymes that do photosynthesis are proteins.
So, genetic similarity and of course dis-similarity define a lettuce from humans. But besides these, there is a great deal of dis-similarity at what we call the regulatory level which is a whole different ball game. Think about this: if we all came from one fertilized egg, the zygote- a single cell, why are we not just a blob of spherical tissue? How did we manage to get hands, that are different from legs, eyes etc.. and moreover, how is it that our skin cannot see, our eyes cannot eat, our ears cannot smell and stomach not pump blood? why this dis-similarity within an organism?..something to think..
---------- FOLLOW-UP ----------
QUESTION: Dear Thane,
Thank you for two wonderfully clear answers.
They have made me think of two more questions, I'm afraid.
I assume that every egg and every sperm has a complete set of genes. but every child only has one complete set. Do we know what the mechanism is by which half the genes are discarded? For instance, I have brown eyes like my father, while my mother, in her photographs, has green eyes. What happened to my green-eye gene? was it absorbed into the cell, then split up and used for something else? Or is there an anti-me somewhere in another dimension? A girl with green eyes? Or a boy with green eyes, perhaps?
And going back to the lettuce, is the small amount of protein in a lettudce avaiable to turn into food? If I ate nothing but huge amounts of lettuce for the rest of my life would I die from lack of protein? Or would there be other missing nutrients that would kill me first?
Thanks, and thanks again for your first answers.
You are correct in that every egg and every sperm have a "complete" set of genes... but "completeness" is a relative term (just like everything else in the physical universe perceived by life..but that is more of philosophy..so getting back to your q..).
Let me first lay down the rules of genetically driven sexual propagation on this Earth as we know it....
1) Every species/organism has a fixed and pre-determined number of chromosomes- the physical carriers of genetic material
2) This fixed number MUST be maintained at all cost for the species to thrive and reproduce normally
3) In humans this numbers is 46
4)Every cell (called somatic cells), not including the germ cells (sperm and egg) must carry 46 chr.(in a "normal" person- ie., excluding disorders and pathogenetic condition)
5) In most animals including humans, default mode of propagation is sexual reproduction.
6) Sexual production - being a union of two individuals, will end up doubling this number of chr. which means, the resulting fetus would have 92 chr ..and the next generation - 96 x2…. which is unacceptable. When such things do happen due to errors, nature puts a FLAG and STOPs the process
So what is the process by which the child ends up getting a complete set while it came from two complete sets of sperm and an egg?
To avoid the disaster stated in (6), nature smartly came up with the idea of creating germ cells- sperm and egg- each with only HALF the number of the normal set of chr. designated for a species. Only these cells have 23 chr each and no other
So the sperm and egg have 23 chr.
When mating occurs in humans, the 23 x2 restores 46 chr.
In sperms, of the 23 cells, 22 are called autosomes and the last one is called sex chr and can be an X or a Y. So each sperm will carry 22 + one X chr OR 22 + one Y. Never both X and Y in a normal sperm.
In eggs, of the 23 cells, 22 are autosomes and the last one is sex chr and is always an X chr..
Mating and fertilization of the egg by the sperm will end up in a fetus have 44 autosomes + two X (44,XX) OR 44 + XY (44,XY). Any deviation will lead to an abnormal fetus which might die or be born as an individual with a serious disorder.
When the normally formed fetus grows up, all cells will divide by a process called Mitosis so that resulting generation of cells will have 46 chr. When this fetus grows up to be a fine young woman or man, all his/her body cells are carrying 46 chr. But in the ovary or testis, the cells are special and they undergo a different kind of cell division called Meiosis from puberty onwards, which will ensure that the eggs and sperms will carry 23 chr. And so life goes on…
So no genes are discarded. It is all programmed to work without wastage. As for the Q why your eye color matches one of the parent and not the other, is also tied to the 23 x2 rule. As you said, the sperm or the egg carries a complete set of genes. That is, One complete set carrying all genes required for life. But for a fetus to form and develop, we need TWO sets of every gene- one maternal and one paternal although only on elf them will be dominant for a given gene, and its effect will be visible externally. That is how nature works for many genes. Thus for eye color, If both are same color genes- say blue, then the eye color will be blue for the child. But if one is green and the other blue, then the dominant blue will expressed and the child will have blue eyes while carrying genes for both blue and green. When this child grows up to have children, then the green color gene may be passed on and if the partner has one or two green color gene, then the chance of the child having green eyes is higher.
The above is another set of process requiring a more detailed explanation but I think this should be ok for your current Q. So the gene for feature that is not visible externally is neither lost nor absorbed- it is just latent and will express when it gets a chance in some other generation.
About your lettuce question- Yes, if you ate nothing but lettuce, you are certain to suffer from diseases caused by nutritional deficiencies, and eventually death by malnutrition. A lettuce has some nutritional content but not everything you need for the cells to be healthy. For instance it does not have much carbs, not all amino acid, no fat and not all vitamins and minerals. That is why your diet should be a combination of food that has all the nutritional content required for normal functioning of the cell.