Lack of knowledge often results in insensitivity, curiosity, or problems relating to one another. Working under the assumption that one way to desensitize a problem is to provide information and to talk about a subject, this curriculum will provide activities that will assist individuals in understanding how and why an individual develops from single cell to complex individual and how humans are both similar and yet very different from one another.
Armed with information about genetics and hereditary disorders, students with visual impairments will be provided with information that will allow them to understand themselves and to answer questions about their condition. Doing so will be at least a first step in helping these students to develop a more positive self image and allow them to discuss and confront problems that may exist.
A series of lessons and activities will provide information about genetic and visual impairments and also be a good beginning for peers to create more understanding and acceptance of individuals who are different. It will help students to value each individual despite their physical differences and help them gain a more positive attitude toward individuals with genetic disorders including conditions such as the four visual disorders to be discussed.
BACKGROUND INFORMATION ON GENETICS
Every human is made up of billions of microscopic cells. In fact, there are more than 200 different types of cells in the human body. (1) Each of these tiny cells carries a blueprint to create a unique living creature. This blueprint is called the genetic code.
Cells are composed of water, sugars, fats, and proteins. These substances can be combined using many different recipes. It is the combination of these proteins in the cell that determines the inherited characteristics of an individual such as body structure and intelligence. The nucleus of a cell is a control center which determines how these substances will be combined.
DNA is coiled up in the center of the cell in 46 threadlike structures called chromosomes. Chromosomes determine what kind of individual will develop. Living things inherit chromosomes from each of their parents and provide a genetic link from one generation to another.
Human beings have 23 pairs chromosomes in each cell. Each pair is different from any other pair. Members of a pair are essentially identical, with the exception of sex chromosomes in males. Sex is determined by X and Y chromosomes The female has two chromosomes called X chromosomes. The male has both an X and a Y chromosome.
There are 22 pairs of autosomes which are the non-sex determining chromosomes. Traits such as eye color, hair color and height are determined by two genes. Some traits, such as freckles, are determined by only one pair of genes while others, such as blood-type, are determined by several pairs of genes. Four pairs of genes determine skin color. These genes control the production of a chemical called melanin which adds coloring to skin. The more melanin that is produced the darker the color of the skin. Individuals who have albinism produce little or no melanin.
A gene is a short section of a longer DNA molecule. Each human cell may have as many as 100,000 genes. (2) Genes are important because they carry the formulas for making proteins and proteins dictate how each cell in our body will be constructed.
Genes are composed of DNA and contain the coded message that instructs the cell to produce a particular kind of protein which affects a particular characteristic of the cell.
The genes and the chemicals found in them, DNA and RNA, are the basic building blocks of an organism. One of these chemicals, DNA or deoxyribonucleic acid, directs the production and growth of every cell in the human body. In addition, hereditary information is passed on to the cells by another chemical messenger called RNA or ribonucleic acid.
DNA, is the molecule that makes up the genes. Composed of two coiled strands it is joined by rungs between them. These rungs or steps are composed of pairs of chemicals that carry instructions for guiding the formation of a particular protein.
DNA is made up of sugar, phosphate, and four chemicals called bases. The bases are cytosine (C), guanine (G), adenine (A) and thymine (T). The sugar and phosphate form the long strands or sides of the ladder while the bases form the steps. The steps are made of two pieces which also pair this way: a with t and g with c. Inside the nucleus loose bases attract other bases floating by in the above combinations. The new DNA strands formed are identical to the original DNA. This reproduction of DNA is the key to heredity.
THE DOUBLE HELIX
Two strands make up the DNA thread. The strands twist and wind around each other. This twisting ladder of DNA is called a double helix. When the first cell divides it copies it’s DNA plan and becomes two cells. These two cells have identical sets of plans. This process of dividing and copying the original plan continues until millions of cells later a unique individual is formed.
There are 4 chemicals that make-up the DNA contained in each cell.
The 4 chemicals are ADENINE, THYMINE , CYTOSINE , and GUANINE.
We refer to them by there the letters A, T, C, And G. Each single strand joins up in a very precise order: A always joins up with T
C always joins up with G
G always joins up with C
After the copying is finished and has created 2 identical DNA threads, each with 46 chromosomes, and the cells divide.
THE DIVIDING CELL
Each individual begins as a single fertilized cell. The DNA in this cell contains the directions that determines the make-up of this individual. Nearly all the information needed to make a unique human being are contained in the chromosomes found in those cells. When a sperm cell with 23 chromosomes merges with an egg cell containing 23 chromosomes they unite to form an individual that has genes donated from each parent.
When it is about to divide into two cells DNA coils up tightly. In the center of the cell are the chromosomes which contain the DNA. Before a cell divides the nucleus divides. Each time the cell divides all the information contained in the DNA is duplicated so that both of the cells has the same DNA blueprint. This process is called REPLICATION. In human cells the DNA condenses into 46 pairs of chromosomes after this process of replication.
In the first step of this process, the double helix unzips creating two single strands of DNA. Each single strand serves as a template for a new strand complementary to itself thus creating two new DNA molecules. The DNA helix unravels and each side of the DNA molecule pulls away from its mate. Each half is a model for a new molecule. The recipe for that particular protein is copied from one strand to another. The nucleotids attaches in the T to A, G to C, and G to C pattern and each new double strand retwist itself into a helix. The cell then divides and forms two new cells. Each cell houses 46 chromosomes.
The copy strand is called RNA which is like a small piece of DNA except that one chemical is different. This copy strand does not have thymine but instead uses a chemical call URACIL known as U. The usual DNA strand copies as usual in the following way except if the strand has an A it copies a U.
The copy strand then travels within the cell in search of the ribosome where it reads the instructions on the copy strand. Each copy stand has instructions for a particular combination of amino acids. Amino Acids are the chemicals that make all proteins. In the human body there are thousands of different proteins all made from 20 amino acids joined up together in numerous combinations.
The amino acids moves along the copy strand and finds the ribosome where it reads the A’s, C’s, G’s and U’s so that it knows in what order to join up the amino acids. They then join up together like beads of a necklace. When the copy strand has completed its job it leaves the chromosomes and the DNA zips up again.
THE SEX CELLS
DNA is found in pairs in the nucleus except for the germ cells— sperm and egg (ovum) cells which are not in pairs. Instead of containing 46 chromosomes as the other cells do, the sperm and ovum contain only 23. However, when a sperm fertilizes the egg, these single sets will pair up in a new cell. Each parent contributes their half of the genetic code and the full 46 chromosomes are restored. This cell will then multiply again and again and develop into a individual.
Sex chromosomes contain genes for several traits such as color blindness, a visual disorder in which people cannot distinguish between certain colors. The gene for color blindness is recessive. The X chromosome carries the recessive gene and the dominant gene for normal color vision.
THE INHERITANCE OF TRAITS
An organism receives two genes for each trait. One gene comes from the gamete or female reproductive cell while the second gene comes from the male gamete.
Certain genes produce certain traits but some of these genes have a stronger influence than others. The stronger gene is called DOMINANT GENE and the weaker is the RECESSIVE GENE.
Genes produce traits that are either dominant or recessive. A recessive gene produces a certain trait only if its effects are not overridden by those of a dominant gene.
If an individual has one dominant gene and one recessive gene the organism will have the trait of the dominant gene. The recessive gene will be hidden. Individuals with two same genes for a particular trait are called PURE and those with two different genes for a particular trait are called HYBRIDS.
The visible trait or physical appearance for an organism is called the PHENOTYPE. The unique combination of genetic information that is inherited from the parents is called the GENOTYPE.
A trait that is determined by a dominant gene will not appear in an offspring unless it also appears in one or both of the parents. If the trait depends on a dominant gene, the offspring of two individuals who do not show the trait will produce only normal descendants Dominant traits do not skip generations— that is children do not show the trait unless a parent did.
Recessive genes do not follow the rules that have been laid down for dominant traits. Recessive traits commonly do skip although they may not do so.
The recessive gene is not changed or destroyed. Its influence is masked by the dominant gene in the hybrid generation. It, however, is carried and passed to the next generation unchanged. Not only two but three, four or more generations may be skipped
For instance, children with albinisim often are born to parents who had no idea that they or their ancestors possessed the gene for albinism.
THE INHERITANCE OF EYE COLOR
To understand how inheritance of traits works we can study eye color.
The gene that determines brown eyes is dominant while the gene for blue eyes is recessive. If a child has a brown-eyed parent who carries two brown eye genes mates with a parent with blue-eyed genes the child will have brown eyes. This is because the brown-eyed parent has only dominant brown-eye genes to contribute to the genetic make-up of the child.
If the brown-eyed parent has a dominant brown eye gene and a recessive blue eye gene , the child will have a fifty-fifty chance of receiving a blue eye gene from each parent and will have blue eyes. Two blue eye parents will always produce blue eyed offspring because it involves only recessive blue eye genes.
The offspring of two brown-eyed individuals who each have a recessive blue eye gene have a one-in-four chance of receiving a blue eye gene from each parent and will result in having blue eyes. This illustrates how a recessive gene can be present but not obvious thus allowing a trait to appear unexpectedly after skipping generations.
THE EVOLVING CELL
When a cell divides into two it makes a copy of all its genes. Sometimes a mistake is copied because the gene was faulty or because the DNA was damaged. These faulty genes are called mutations. Usually mutations die because they cannot function properly and so other healthy cells replace them. This process takes place many times in our body. If the mutated cell is a sperm or egg cell, and it does survive, it may be passed on to a new generation. In this way, diseases and disorders can be passed on to the next generation.
Inherited mutations such as color blindness may not be serious problems but others such as sickle cell anemia or thalassemia may be devastating. However, sometimes the mutations are beneficial. In fact the various plant and animals found on the earth today exist because of this process of mutation.
Scientists have determined that the genes from a single cell that lived millions of years ago is the origin of all these life forms. This cell changed and developed to adapt to its surroundings again and again and so from this single cell more complex plants and animals developed. At some point in history the species Homo Sapiens was created. Humans adapted and developed over the centuries developing features and characteristics that allowed them to survive. These adaption included such features as the ability to stand upright , development of a movable thumb, and the ability to use tools. Each of these adaptations were due to the gene changes. This evolution from a single cell to complex human organism took place because the changes in the genes were beneficial to the organism and aided them in their survival.
All life forms— plants and animals (including human beings) are all related. In fact, 99.5% of the DNA of any human is in the same order as any other person. It is the combination of DNA in the that other .5% that makes each of us unique. (3)
With such knowledge we should be able to help our students understand how they are both very different and yet very much the same as their peers. This then can be the first step in helping them to understand those individuals who face problems due to their genetic make-up. It can also provide them with information that will allow them to work toward celebrating individual differences.