THE THREAD OF LIFE

[Music] lies hidden the secret which we may well call the thread of life [Applause] bye we are about to unfold for you an adventure in the world of science a science that deals with the nature of living things our story is about you and me and why we are alike in some ways and yet why each person is different from every other there are billions of people over the face of the earth of all sizes shapes dispositions let's look at a few nature arranges endless combinations [Music] so [Music] how does nature endow us with so many different traits or characteristics well that's what our show is about through the magic of electronics we're inviting some of the audience to come along with us and join in dr baxter oh yes hello can i ask a question surely you see i'm left-handed yes sir and my dad is left-handed too so i must have inherited that from him huh very likely does your dad have blue eyes too well sure but well maybe you're a chip off the old block then because being left-handed and having blue eyes are both traits you could have inherited dr baxter oh yes hello you're not for luck and very fashionable but i've heard a lot of odd stories about what causes such things yes but most of them are only old wives tales the truth is it's just another inherited trait like this gentleman's high fart and heavy brows well here's what's been puzzling us our baby has red hair and both of us have brown hair no mystery parents can hand down a trait that's hidden in themselves in fact a trait like red hair can be hidden from many generations the story of how heredity is passed on from generation to generation is the same for all living things human beings animals plants even the lowliest creatures let's go to the microscope and look at one of nature's simpler animals an amoeba an organism that has only one cell and like every living thing this little cell is a sort of chemical machine carrying on the many intricate functions we call life one of the functions is to reproduce watch we can see it divide [Music] now there are two daughter cells the two new cells are just like the old one they inherited all the traits of the cell they came from and this same sort of process goes on in all living creatures i see now you've got two cells okay but what i'd like to know is where'd the first cell come from in a way you're asking where life itself began we don't know that but the first cell you saw here came from some other cell all cells come from other cells that's how the chain of life has come down through the ages you started life as a single cell that cell was a combination of an egg like this from your mother and a sperm cell from your father [Music] you are seeing actual motion pictures of a human egg in the process of fertilization many sperm are trying to enter the egg only one can fertilize it when egg and sperm cells like these joined you came into existence when the fertilized egg is only hours old it begins to divide it divides over and over again [Music] the new cells begin to work in different ways some become muscles others make nerves here bone making cells skin cells blood cells trillions of cells an embryo takes shape then the greatest miracle of all a baby and not just any baby but a baby of a particular kind a baby with almond-shaped eyes or round eyes with brown skin or white skin all this begins with one tiny egg no bigger than a tiniest dot you can make with a sharp pencil but can you predict what an egg is going to grow into can you tell by looking at it no you can't many egg cells look very much alike look at this egg or this one or this one could you guess that one would become a baby one a rabbit one alone analyze the cells of various living things and find that their chemical machine is about the same we begin to wonder what shapes the development of a living being all right a lot of cells look the same their chemical machinery as you call it is about the same but still there must be some sort of a gimmick inside the cell that makes it work you're right scientists have been trying for a long time to find that gimmick but the first clue to the workings of heredity was uncovered in a way that might surprise you how is that by mathematics in one of the greatest achievements of the human mind the story begins over a hundred years ago in a monastery in a corner of the old austrian empire there lived a monk named gregor mendel a teacher a man of many talents driven by scientific curiosity he spent long busy hours working over a patch of peas in the monastery garden he noted that some of the traits of the pea plants were clear and distinct some of the strains were always tall some were always short some had wrinkled peas some smooth peas some purple flowers some white [Music] and he set out to determine how these traits were passed on from one generation to the next he cross-pollinated large numbers of the plants and he kept careful records after eight years of painstaking work he reported his findings to the scientific society in the town of bruin and so gentlemen i crossed a pure strain of tall plants with pure short plants by putting some pollen from one strain onto the flower of the other you might suppose that their offspring would be medium-sized plants but not so the next generation were all tall plants and now when i crossed these plants among themselves three-fourths of the grandchildren came out tall and one-fourth came out short on an average tall plants outnumber the short by just about three to one [Music] how does this happen from tall and short parents all tall children and from tall children a mixture of grandchildren that are both short and tall this is my theory our original parents were pure strains so each child inherited a unit for tallness and a unit for shortness but they all grew tall that's because tallness is a strong or dominating trait i call it dominant while shortness is weaker or as i call it recessive now these child plants are not pure strains even though they are tall because each contains a unit for shortness which is hidden or blocked by the dominant unit for tallness but it is still there waiting so when these child plants are crossed with each other a grandchild plant has four equal chances it may get a tall unit from each parent and be a pure tall plant like the tall grandparent a short unit from each parent becoming a short plant like the short grandparent or a tall and a short and become tall or a short and a tall and become tall narration and in every generation they are shuffled and reshuffled according to the laws of chance [Music] that i believe is what causes the puzzling mixture of traits in nature gregor mendel died january 6 1884. his work was unnoticed by the world but his careful study of nature had begun to explain one of her great mysteries he proved the existence of something that could not be seen the invisible units of heredity the same units that govern your hair color or the shape of your face or make your eyes blue what were these units how did they work these were problems other scientists had to work out it was in 1866 intel's report to the scientific society was published but years passed before the scientific world was ready to see its value now these units of heredity where i thought we'd already found out in the cell that's right well could they be some kind of particles exactly my question that's also the question biologists began to ask themselves and for the answer they looked deep into the unit of life with their microscopes inside the cell and probed into its inner core of the nucleus can you see inside a nucleus with a microscope yes it's not easy because ordinarily all you see of the nucleus is a fuzzy indistinct blob but when scientists stain the cells with a strong dye some parts begin to show up and we see strange little fibers called chromosomes there are chromosomes in the cells of every living organism they come in matched pairs in every cell of corn for example there are 10 pairs or 20 chromosomes the grasshopper 24 chromosomes and man 46 23 pairs well man has the most is that because man is the highest form of life no because a potato has 48 chromosomes some goldfish have 94 one kind of crayfish has about 200. the number doesn't seem to be significant what the chromosomes do is very significant suppose we look through a microscope at chromosomes in a live cell you're about to see some extraordinary motion picture scenes they were photographed by dr ange cycle we are watching takes about half a day in nature but we are seeing it and speed up types photography now let's look at a single cell as it divides watch the chromosomes closely each one will split into two identical new chromosomes the new chromosomes now separate into two clusters each becoming part of a newly formed nucleus [Music] a dividing wall will form and we will have two new cells each new cell has the same chromosomes as the old cell well that's very clear well sure and they pass them on from one cell to the next you might say and does that mean that heredity is carried by the chromosomes but you said those units were invisible we can see chromosomes right and what you are all asking are precisely some of the questions that occurred to scientists around the turn of the century about this time mendel's forgotten report was discovered and brought to the attention of the world meanwhile scientists looking in microscope based on the invisible particles now called genes which are passed on from generation to generation first discovered by a patient dedicated scientist working alone in a monastery garden man without even a microscope that's another reminder that the most powerful instrument in the cause of science is the human mind all right let's back up a minute genes are on chromosomes yes but you can't see them the fact is scientists are still not quite sure what to look for but they're getting closer they're beginning to find more and more of how the genes work if scientists couldn't see the genes how could they be so sure they were on the chromosomes well let's ask the scientist we have several ways of knowing dr baxter one way is by means of experiments on drosophila a harmless little insect commonly known as the fruit fly human chromosomes are too small and too numerous to be examined in detail but the chromosomes found of all places in the salivary gland of a fruit fly are huge as if taylor made for scientific study by experiment we can prove that the units or the genes are actually lined up in single file along the chromosomes and we're able to locate the exact position of hundreds of genes now this black band for example is the location of a gene that's responsible for eye pigment how do we know this for sure because in fruit flies where this segment is missing the eyes have no figment and a fear of white down here we know is a gene necessary for normal wings here for body color and so on on this particular chromosome we know the location of more than a hundred genes the lineup of genes on the chromosomes of the fruit fly is as clear to the geneticist as towns on a railroad line in fact he calls his diagrams of them chromosome maps maps like these were constructed on a theoretical basis as long ago as 1916 by the brilliant insight of the geneticist thomas hunt morgan and his students we don't have a map of the human chromosomes yet but we know the genes are there we know that they determine what our cells will do and what we will be like and do i have the same genes in every cell in practically every cell in your body but what's the use of having a blue eye gene in my big toe nobody knows and wouldn't scientists like to think up an experiment that would answer that question well then where do i get my genes in the first place from your father and mother of course the idea is this your heredity comes to you in two bundles the sperm cell from your father with its 23 chromosomes and the egg from your mother with another 23. but i thought you just told us that a human cell has 46 chromosomes that's right with the one exception of egg and sperm cells you see nature's arithmetic has to come out right so you get 23 chromosomes from your mother and 23 from your father each parent contributes only half and in this way you get the normal number of 46 chromosomes in the fertilized egg from which you grow does that mean i have just part of my father's genes that's right only half and half of your mothers well what decides which ones i get the rule for human beings is just the same as mendel found with peas chance pure chance you see the egg and the sperm are formed in a different way from all the other cells of your body and the way they are made is one of the most beautiful and ingenious of all nature's processes [Music] they are formed by special germ cells each with a normal number of 46 chromosomes let's look at just one half of the picture the father's side this cell is going to divide in order to form sperm cells it has the normal number of 46 chromosomes 23 from the man's mother 23 from his father but it doesn't divide the way other cells do this cell follows a different process called meiosis first the chromosomes match up in pairs pairs move around in the cell then each chromosome splits in two each side carrying only one half the genes and as though the signal all the chromosomes draw apart into two equal groups on either side of the cell then the whole cell divides and subdivides into four sperm cells each carrying with it 23 chromosomes and one half of the original genes which combination of chromosomes goes into which sperm cell is a matter of chance the similar process goes on in the forming of the egg cells and the coming together of these two chance combinations starts that unique and wonderful creation we call a human being then you really can't predict how things will come out no more than you can foretell the toss of a coin both are governed by that complexity of unknown causes which we call chance and just as a little demonstration let me show you the mathematical chances involved in your heredity some of you may not realize the number of combinations that are possible with 46 chips now here are 23 representing mother's chromosomes 23 for fathers and each chip has two sides of course because the chromosomes of both parents come in pairs now we put them together [Applause] and throw them out do you know what the chances are that this very same combination of 46 sides will turn up again well the odds against it are 70 million million to one that means the chances of your being exactly like your brother or sister are less than one in seventy million million this is many times the total number of people who ever lived so the chances of two persons happen just alike are slim indeed but i know two sisters who were as alike as two peas in a pod oh of course even if brothers and sisters heredities are not exactly alike they're bound to have many identical genes the more they happen to share the more clearly we see a family resemblance and in certain cases children have exactly the same heredity gene for gene these are identical twins identical to begin life as one fertilized egg this egg divides it first in the normal way but early in its growth the embryo splits into two and of course in every cell of both embryos are the same chromosomes and the same genes that's why such twins are always the same sex and so astonishingly alike getting back to the average case though a lot of genes come shuffling down through my family until they come together and bingo that's me but i don't see how they do it how can some little gizmos in the chromosomes stuck away in the nucleus of a cell how can they curl your hair or make your ears stick out it's only in recent years that we've begun to find out how genes do their work and as research and experiments go on scientists are opening a new and fascinating chapter in the science of the genes [Music] genetics [Music] you