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by
Carolyn N. Kinder
The study of inheritance depends on the differences as well as the similarities between parents and offspring over several generations.
Heredity is very complex, and a geneticist cannot possibly analyze all the traits of an organism at once. Instead, he studies only a few traits at a time. Many other traits are present. As the geneticists work out the solution to each hereditary mystery, the geneticist must not forget that all organisms live in a complex environment. The environment may affect the degree to which a hereditary trait develops. The geneticist must try to find out which of the many parts of the environment may affect his results.
The factors must be kept as constant as possible by using controlled experiments. Only then can he tell that the differences observed are due to heredity.
Heredity determines what an organism may become, not what it will become. What an organism becomes depends on both its heredity and environment.
The modern science of genetics started with the work of Gregor Mendel. He found that a certain factor in a plant cell determined the traits the plant would have. Thirty years after his discovery this determines was given the name gene. Of the traits Mendel studied, he called dominant those at showed up in the offspring and recessive those The question I will ask is: how much of the variability observed between different individuals is due to hereditary differences between them, and how much to differences in the environments under which the individuals developed?
In most organisms, including man, genetics information is transmitted from mother to daughter cells and from one generation to the next by deoxyribonucleic acid (DNA).
Knowledge of the heredity or inheritance of plants and animals is important in many phases of our life.
The question I will ask is: How much of the variability observed between different individuals is due to hereditary differences between them, and how much to differences in the environments under which the individuals developed?
The purpose of designing a unit on “Heredity And Environment” is to help students learn more about themselves. They will learn why they develop into the kind of individual they are.
The unit will discuss heredity traits and environmental conditions, chromosomes, DNA, studies of identical twins, and several diseases linked to heredity and environment.
The students will do some hands on activities by constructing a model which represents DNA. They will explore plants with the exact same heredity and plants with different heredity. They will change the conditions in the environment to see the way the plant organisms with the same heredity may develop differently in different environments and why organisms with different heredity develop in the matter in which they do. Heredity is not the only thing that effects development. The environment also has an important effect.
The unit can be taught to students in grades five through eight. The science and math teachers are encouraged to use a team teaching approach. Other features that will be included in the unit are content, lesson plans, resources, reading list and a bibliography.
DNA is made of six parts: a sugar, a mineral (phosphate), and four special chemicals called bases. These bases are represented as A;T;C; and G. Sugar and phosphate form the chains, or sides, of the staircase. The A;G;C and T bases form the steps. See figure 1. Each step is made of two pieces, which are always paired the same way. The A base always pairs with the T base. And the G base always pairs with the C base.
Figure 1. DNA Structure
Figure 2.
Figure 4—An example of polydactyly. Extra digits on either hands or feet are almost always abnormal in structure.
A common cited example of an environmental effect on phenotype is the coloring of Siamese Cats, although these cats have a genotype for dark fur, the enzymes that produce the dark coloring function best at temperatures below the normal body temperature of the cat. Siamese Cats are noted for the dark markings on their ears, nose, paws, tail, and all areas that have a low body temperature. If the hair on the cat’s belly is shaved and an ice pack is applied, the replacement hair will be dark. Likewise, a shaved tail, kept at higher than normal temperatures, would soon be covered with light colored fur. These changes are temporary, however, unless the ice pack or heat source is maintained permanently.
The most celebrated effect of an environmental agent directly affecting the unborn, is that produced by the rubella virus. This German measles virus is capable of crossing the placenta from mother to child, and the prenatal infection, if it occurs early enough, may result in deafness and other damage to the child. Similarly, maternal infection with the rare protozoan parasite Toxoplasma can cause serious congenital defects in the fetus, and the same has been suspected for Asian influenza.
Another environmental factor is anoxia. Anoxia is a natural hazard of childbirth, and in most cases the infant makes a normal adjustment to it. When infants suffer from delayed respiration or asphyxia during birth, it is widely accepted that this is responsible for later difficulties such neurologic abnormalities.
Warburton and Fraser have emphasized that the development of a fetus depends on a precise and extremely intricate system of interactions between two sets of hereditary factors and two environments, all acting at the same time on the growing baby. The mother and the fetus each have their own environment and their own genotype.
Identical twins develop when the cells arising from a single fertilized egg separate and two complete embryos form. These embryos have exactly the same genetic information. If identical twins exhibit the same expression of a trait, then it would appear that the trait is heavily influenced by the environment. The message from such studies is that both genes and environment are important.
In your case, you may often have thought, how would you with your given heredity have turned out under different conditions? Or, under the same conditions, to what extent might you have been different with a slightly different heredity?
The only way it could be answered or, at least, partly answered is if there were two of you to start with and each were exposed to different conditions; or if you started life with somebody else at the same time within the same mother, and after you were both born, developed under approximately the same conditions. Is either of these situations at all possible? Yes, for nature has most thoughtfully provided us with twins. For the first experiment we have identical twins; for the second, “fraternal” twins. The two types differ in this way. Identical twins are the product of a single fertilized egg which, shortly after it begins to grow, splits in half to form two individuals. Each has exactly the same hereditary and factors, so among other things, identical twins must always be of the same sex.
Fraternal twins, on the other hand, are the product of two entirely different eggs simultaneously matured by the mother and fertilized, approximately at the same time, by two entirely different sperms. They carry quite different genes, and need be no more alike than any other non twin siblings in the same family, as often as not, in fact, being of opposite sex.
In other words, identical twins are from the standpoint of heredity, exactly the same individual in duplicate.
Fraternal twins are two entirely different individuals who merely through chances were born together. See fig. 5.
By comparing the twins with regards to many characteristics known to be definitely inherited or influenced by heredity, they can tell whether or not the degree of resemblance or correlation is high enough to stamp them as identical among the characteristics used for comparison are blood groups, blood pressure, pulse, respiration, and brain wave patterns, eye color, and vision, palm, sole and finger patterns: skin color, hair color, hair form and various minor hereditary abnormalities where present. The correlation in those characteristics is so much greater between any two identical twins that there is virtually no possibility of confusing their relationship.
Figure 5—How Twins are produced
In as much as identical twins have exactly the same heredity, whatever differences there are between them must be due to environment. But when identical twins are reared in different environments, there being instances of such separation in infancy and nonetheless develop marked similarities of any kind, these might be ascribed to heredity.
The study of fraternal twins takes a different direction. In their case, as they have much more similar environment in prenatal life and often thereafter than singly born individuals, the question is how much more alike this will tend to make them.
If heredity were everything, then identical twins would be exactly the same in all respects, even if reared apart. But a number of studies show that they are never exactly alike, even though they do have remarkable similarities in most respects.
On the other hand, if environment were everything, then fraternal twins, reared under the same conditions, would also be alike, regardless of how different were their genes. But here we find that although they show a closer resemblance to each other than do non-twin brothers and sisters, “fraternal”, even when of the same sex are very much alike than are “identical” reared apart.
Thus, the various studies of twins have comprised an important source of evidence for geneticists. No identical twins are really identical because they cannot possibly have had identical environments, even before birth.
If all human traits behaved in the clear-cut mendelian fashion that albinism and Huntington’s chorea do, twin studies would not be necessary as an aid in unraveling the complications that the environment often superimposes upon a mendelian pattern of heredity. There seems to be a rule that the most common defects have the largest environmental component, which makes it difficult to tell whether their hereditary basis for the abnormality is a simple dominant or recessive. The environment acts to suppress the expression of the abnormal gene in some cases but fails to do so in others! It can be appreciated that this unpredictable behavior of environmental factors would upset the classic orderly mendelian ratios, especially if they are the complicated ones that result when more than one gene pair is involved.
Pairs of twins are useful in detecting the relative effectiveness of heredity and environment upon the expression of a disease or trait. If a trait is highly hereditary, both members of a pair of identical twins will be expected to show the trait. If one identical twin shows the trait and the other member of the pair does not, the disagreement must be due to environmental differences between the two twins, because the genes of one are exactly the same as the genes of the other.
We know that the heredity of blood groups behaves according to the mendelian rules and that environment seems to have no affect on the kind of blood group a person has. Identical twins both have exactly the same blood group. Tuberculosis is a good example of a disease in which an hereditary susceptibility and an environmentally favorable situation are both necessary for the appearance of the active disease. A comparison of the behavior of identical and fraternal, and for susceptibility to tuberculosis, where heredity and environment share the responsibility, is given in table I. Notice how different the picture is for the two traits.
In table I it can be seen that both members of the 125 pairs of identical twins agreed in having the same blood groups. The 91 pairs of fraternal twins showed 60 pairs in which both members had the same blood groups and 31 pairs in which the two members had different blood groups. The different in susceptibility to tuberculosis of the two members of identical twins pairs is interesting. In 68 cases both got the disease but 29 cases one of the identical twins got the disease while the other twin remained free of it. It is striking that there could have been sufficient difference in the environments of the two members of the identical twin pairs to cause one member to remain free of it. Both must have been genetically susceptible, as one of them was, but their environments differed enough so that one got the disease and the other did not.
| Identical | Twin Pairs | Fraternal | Twin Pairs | ||
| Agree | Disagree | Agree | Disagree | ||
| Type of Blood | 125 | 0 | 60 | 31 | |
| Group | |||||
| Susceptibility | 68 | 29 | 45 | 99 |
The contrast in Agreement and Disagreement between the Members of Twin Pairs, Identical and Fraternal; for the Blood Group and Susceptibility to Tuberculosis.
The chromosomes in our cells are not affected by any change that takes place within our body cells. This means that no change that we make in ourselves or that is made in us in our lifetimes can be passed on to our children through the process of biological heredity. Such changes made in a acquired characteristics.
Cancer is another type of genetic disorder. A cancer is a clone of cells growing without normal morphogenetic controls in a living host. In man, the difficulty arises because for common cancers we also always expect and find important environmental factors involved in their onset.
CHOOSE THE BEST ANSWER:
- 1. Genetics is the study of
- ____a. digestion b. growth c. heredity
- 2. All the traits a living thing inherits are called its
- ____a. chromosomes b. heredity c. genes
- 3. Genes are located on
- ____a. chromosomes b. RNA c. cell walls
- 4. Replication means
- ____a. reproduction b. mutation c. DNA splitting in two
- 5. Identical Twins develop from
- ________a. two different fertilized eggs b. the same fertilized egg c. meiosis
- 6. DNA is found in the
- ________a. nucleus b. mitochondria c. ribosomes
- 7. Mendel gave us the laws of
- ________a. circulation b. heredity c. gravity
- 8. An organism is the product of its
- ________a. heredity b. environment c. heredity an environment
- 9. Human sex chromosomes normally have
- ________a. 23 single chromosomes b. 23 pairs of chromosomes c. 46 pairs of chromosomes
- 10. The father of genetics is
- ________a. Mendel b. Fraser c. Wurburton
Objective To get students to see that they are alike and different in many ways.
You and your classmates are alike in many ways. You are also different in size, shape, talents, and interests. How many of these similarities and differences are entirely genetic? How many can you control in some way?
For each trait given below, record a measurement taken now or a description of yourself. Try to decide how much of the trait is determined by genetics and how much by interactions with your environment.
| TRAIT | MEASUREMENT/OR | GENETIC/OR | EVIDENCE THAT |
| DESCRIPTION | ENVIRONMENT | TRAIT CAN/ | |
| CANNOTCHANGE |
LT.-handedness
PULSE RATE
NUMBER OF FINGERS
ON RIGHT HAND
NUMBER OF WORDS
READ PER MINUTE
SHOE SIZE
TALKATIVENESS
ABILITY TO PLAY
THE PIANO
Objective The student will find out about some traits that are inherited. If the student shows a dominant trait, there will be at least one dominant gene. If it is a recessive trait, wo recessive genes.
Directions Fill in the chart with the traits that you have. You may need a mirror to help you.
Objective The student will learn how to construct a model of DNA question: Why is DNA replication an essential step in cell division?
Strategy The student will use several hands-on approach to making a model of DNA. (Refer to Genetics Concepts Experiment Kit from Lab Aids, YNHTI)
Resource “DNA Made Easy”
Objective The student will visit a seed and plant store to observe plants that are similar in genotype that look alike. They will see plants of the same genotype that look different. They will observe that environment has an impact on genes expression.
Strategy We will create a bulletin board displaying the variety of plants due to environmental effects, as well as, the hereditary effect. The students will do a series of experiments with plants grown in the classroom. Student observations will be recorded in his/her lab journal.
Objective The students will write a geneotype for each person.
Cobb, Vicki. Cells. New York: Franklin Watts, 1970. This book gives the student a chance to look at many different types of cells)
Dunbar, Robert E. Heredity. New York: Franklin Watts, 1978. (Basic book on inheritance and DNA)
Facklam, Margery and Howard. From Cell to Clone. New York: Harcourt Brace Jovanovich, 1979. (Explains why the discovery of DNA is important to modern day science)
Lesser, Milton S. The Meaning of Life. New York: Amsco School Pub., 1975. (Elementary descriptions of cell structures)
Morrison, Velma Ford. There’s Only One You; The Story of Heredity New York: Julian Messner, 1978. (Good book which describes family trees)
Pfeiffer, John. The Cell. New York: Time-Life Books, 1964. (Excellent color illustrations of many different types of cells)
Silverstein, Alvin and Virginia. The Code of Life. New York: Atheneum, 1972. (Excellent book, extremely useful, very readable for all ages and interests, detailed information about DNA and RNA)
Smith, Herbert A., et al. Exploring Living Things.Illinois: Laidlaw Brothers Pub., 1980. (Excellent student text with many illustrations on cell reproduction)
Webster, Vera., et al. Life Science. New Jersey: Prentice Hall, 1980. (Excellent student text with detailed DNA structure diagrams, very readable)
Gabriel, Mordecai L., (ed), Great Experiments in Biology, Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1955. (A presentation of scientific writings in the original including those of the great geneticist, Hermann J. Muller.)
McKusick, Victor A., Human Genetics, Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1964. (One of a few authoritative books available to the layman, written in a readable style and incorporating the latest findings.)
Milunsky, Audrey, Know Your Genes, Avon Books, New York, 1979. (A clear, accurate, and readable book on medical genetics; hereditary disorders, genetic counseling, prenatal diagnosis, sex selection, twins, and more. Ethical, moral, and legal issues are given thoughtful consideration.)
Peters, James A., Classic Papers in Genetics, Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1959. (Several original papers from the outstanding men of genetics including Mendel up to those of the present day.)
Scheinfeld, Amram, Your Heredity and Environment, J. B. Lippincott Co., Philadelphia, 1964. (Explains how heredity works to produce a unique individual and the effect of the environment on the person.)
Sullivan, Navin, The Message of the Genes, Basic Books, Inc., Publishers, New York, 1967. (A lucid account of the mechanisms of heredity showing that today’s molecular biologists have arrived at a clear understanding of life itself.
Swanson, Carl R., The Cell, Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1964. (A discussion of the structure of cells, and the tools involved in the investigations of cells.)
Chedd, Graham, “Genetic Gibberish in the Code of Life”, Science, November, 1981, pp. 50-55.
Cohen, Stanley N., “The Manipulation of Genes”, Scientific American, July, 1975. (A summary of recombinant DNA techniques by one of the main innovators.)
McKusick, Victor A., “The Royal Hemophilia”, Scientific American, August, 1965. (An account of the gene for hemophilia and how it is found in the royal families of Europe.)
Patrusky, Ben, “How Do Cells Know What To Become?”, Science, May, 1981.
35-mm Transparencies
Elementary Genetics Set, Carolina Biological Supply Company, Burlington, North Carolina. (Uses inheritance of kernel color and texture in maize to explain dominance, recessiveness, and the simple Mendelian ratios resulting from monohybrid and digybrid crosses, 55 slides with narrative cassette.)
Filmstrip
Elementary Genetics Set, Carolina Biological Supply Company, Burlington, North Carolina. (Uses inheritance of kernel color and texture in maize to explain dominance, recessiveness, and the simple Mendelian ratios resulting from monohybrid and dihybrid crosses, 55 slides with narrative cassette.)
Video and 16-mm Programs
Genetics, Carolina Biological Supply Company, Burlington, North Carolina. (Heredity is used to show how anatomical and physiological traits are produced. One example used is how we become right or left-handed, a trait inherited as a simple autosomal dominant gene. Polydactylism (many fingers or toes) and blood groups (A80) in humans are used to further illustrate inheritance.
Computer Software
Genetics, Apple II (32k) Diskette. (Covers results of various crosses of peas, fruit flies, and sex-linked diseases. Question/information format.)
Linkover, Apple II (48k) Diskette. (Allows students to plan and execute a program of experiments to draw a genetic map of a single chromosome. Interaction format.)
Other Resources For Information And Materials
National Genetics Foundation, Incorporated
9 West 57th Street
New York, New York 10019
Telephone: 212-759-4432
The March of Dimes Birth Defects Foundation National Headquarters
Box 2000
White Plains, New York 10602
or local March of Dimes chapter
National Clearinghouse for Human Genetic Diseases for the Department of Health, Education, and Welfare, Bureau of Community Health Services, Genetic Services Program
Lab Materials
Resource: Genetic Concepts, Kit, Cat. # 70 Description: Gameto discs
Source: Lab Aids, Inc., Ward’s Catalog Location: Yale New Haven Teachers Institute
53 Wall Street
New Haven, CT.
Telephone: 432-1080
Resource: Gene Kit, M89101/7
Description: Pop-it Beads and Linkers
Source: Phillip Harris, Biologicals, Ltd. (Ward’s Catalog) Location: Yale New Haven Teachers Institute
53 Wall Street
New Haven, CT.
Telephone: 432-1080
Resource: DNA Made Easy
Description: Plastic DNA and RNA Building Blocks;
Transcription, Translation
Source: National Teaching Aids, Inc. (Ward’s Catalog) Location: Yale New Haven Teacher Institute
Wall Street
New Haven, CT.
Telephone: 432-1080
Speakers
Genetic professionals to speak on selected topics on Medical Genetics.
Contact: Yale Genetics
Dr. Greta Seashore
Mystic, CT. 06355
The Nature Center for Environmental Activities, Inc.
10 Woodside Lane
Westport, CT. 06880
The Stamford Museum and Nature Center
39 Scofieldtown Road
Stamford, CT 06903
West Rock Nature Recreation Center
Box 2969
New Haven, CT 06515
Norma Terris Human Education and Nature Center Salem Road
East Haddam, CT. 06423
Roaring Brook Nature Center
70 Gracy Road
Canton, CT 06010
Asimov, Isaac, The Genetic Code, New York: Signet Classics, New Jersey: New American Library, 1962.
Balzer, Levon, Life Science, Scott Foresman and Co., Glenview, Ill., 1983, pp 372-388.
Biological Science Curriculum Study, Biological Science, Houghton Mifflin Co., Boston, 1963, pp 374-404.
Borch, Ernest, The Code of Life, New York: Columbia Press, 1965.
Dawkins, Richard, The Selfish Gene, New York and Oxford: Oxford University Press, 1979.
Milunsky, A., Know Your Genes, Avon, N.Y., 1979.
Motulsky, Vogel, Human Genetics, Problems and Approaches: Springer-Verlag Berlin Heidelberg, 1986, printed in Germany.
Rothwell, Norman, Understanding Genetics, New York: Oxford University Press, 1979.
Scheinfeld, Amram, Your Heredity and Environment, J.B. Lippincott Co., Philadelphia, 1964.
Sussman, Maurice, Developmental Biology, Prentice Hall, N.J., 1973.
Winchester, A.M., Heredity, New York, Barnes and Noble, 1977.
Woese, Carl R., The Genetic Code, New York: Harper and Rowe, 1967.
Contents of 1990 Volume VI | Directory of Volumes | Index | Yale-New Haven Teachers Institute
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