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A Cross-Cultural Introduction to Bioethics

C1. Genetics, DNA and Mutations

Chapter objectives

There are numerous ethical issues raised by genetic technology, so a background knowledge of DNA and genetics is needed to discuss the issues.

This chapter aims to introduce:

1. Basics of genetics that will be useful for other chapters that discuss the ethical and social issues.

2. What is mutation and how it can cause genetic disease.

C1.1. Why do humans make humans, and birds make birds?

Organisms do not pass their replica to the next generation but rather genetic material containing information needed to construct a progeny (offspring). In almost all organisms DNA is the genetic material, except for some viruses where it is RNA instead.

The genetic constitution of an organism is called its genotype. Interaction of this genetic constitution with the environment results in the physical appearance and other characteristics of an organism which is called its phenotype.

DNA works as a database or store of information needed to make an organism. It exists in the form of sequence of four nucleic acids A (adenine) T (thymine) G (guanine) and C (cytosine). When two strands of DNA are together, A binds with T and G binds with C, and these are called base pairs. There are approximately 3 billion base pairs in the human DNA. Genes are coding regions of the DNA that carry necessary information needed to make proteins, which are structures present and operating in the cells and organs. Genes are passed from one generation to the next during reproduction and are called the units of heredity. Variations in the sequence of DNA make each organism different. Genes express and function differently in all species, which makes each species and even each organism unique. Although almost all organisms have DNA (and a few viruses have their genetic information encoded as RNA), the expression of genes determine what we look like in general. Several genes get switched on or switched off during development and determine our phenotype. Environmental interactions also can determine diseases and behaviour.

`````The genetic code of all living organisms is made up of DNA.

Q1. Think about the closest organisms that are similar to human beings?

Q2. What do you think if all organisms look alike?

Q3. How many genetic diseases do you know? How many mutations do you have? How many fatal recessive alleles do you carry in your genome?


C1.2. Mechanism of genetic diseases and mutations

Every person has a different genetic sequence except for identical twins. The genes are made of DNA. DNA is a long chain of units, called bases, and there are only four kinds of base (ATCG). Each position of the DNA can be one of the four bases, and the genetic sequence is the order of these bases. In the same way the sequence of this sentence determines what we understand in reading it, the sequence of DNA determines what happens in living organisms. There are only four possible characters for each position, but even a short sequence of 20 positions could have many possible combinations of sequence. DNA is a long chain of these units, which forms a spiral geometrical structure called a double helix.

Functional lengths of DNA are called genes. Each gene may be involved in defining one particular function or character at the phenotypic level. There are many new genes discovered every week. Our genes are in long linear strings, called chromosomes. Humans possess 23 different pairs of chromosomes, a total of 46. While every human has the same set of chromosomes and thus types of genes in the same order, each gene has variant types which are called alleles. Alleles differ in their exact sequence of DNA but they should generally perform the same function. We can have many different alleles, for example there are at least 46 distinct alleles of the gene phenylalanine hydroxylase (e.g., a mutated allele of this gene is responsible for the disease PKU). There are mutations found in each of these alleles, which would make total genetic screening for PKU impracticable, but a simple cheap enzyme test can be performed.

Mutations are changes in the nucleotide base sequence, and are quite common. Mutations can be caused by random chance, by chemicals or radiation, and most commonly are caused by reactive chemicals (free radicals) formed in the ordinary process of metabolism. Specific mutations are often seen as a response to ultraviolet (UV) light or smoking. The DNA repair enzymes can repair most of these, others may escape repair and can result in abnormalities, such as cancer. If the mutation occurs in the zygote, or reproductive (germ) cells, the new offspring may carry the mutation. Somatic mutations play a role in the development of most cancers, being steps in the process. Only some mutations actually cause harm, others may make no harm (see Fig. 1). This complex system is in delicate balance, and it only requires a defect in a single gene to disrupt this balance, the effect sometimes being lethal.

Figure 1: Mutations alter Amino Acid Sequences
The original and the mutated DNA sequences may give rise to the same amino acid, a different amino acid, or stop translation. A frameshift mutation completely alters the amino acid sequence resulting in a nonsense message.

DNA Sequence Protein Sequence

Original
AACTAATTGCGTA Leu-Ile-Asp-Ala-
Neutral Mutation
AACTAGTTGCGTA Leu-Ile-Asp-Ala-
Single amino acid change
AACTACTTGCGTA Leu-Met-Asp-Ala-
Deletion, frameshift
AACT/ATTGCGTA Leu-Ile-Thr-His-
Insertion, frameshift
AACTAGATTGCGTA Leu-Ile-STOP

The cause of many genetic diseases is a simple nucleotide substitution, which occurs at a low frequency during the duplication of DNA. The effect of this nucleotide alteration is summarised in Fig 1. The effect does not always depend on the size of the deletion, but more on whether the resulting sequence has shifted in the reading frame for protein translation. This is summarised in Fig. 2. For example, in patients with muscular dystrophy, part of a gene for a protein dystrophin is deleted. The severity of the disease depends on whether it is out of frame, rather than how much is missing. As long as some type of protein can be made the muscle cells may still be able to function.

Figure 2: Effect of frameshift mutation
Original
THIS LINE CAN BE READ WELL
Single letter deletion (frameshift)
THIS LINC ANB ER EADW ELL
Whole word deletion (not a frameshift)
THIS LINE BE READ WELL

There are also more major mutations, where large fragments of DNA can be translocated to a different chromosome. Abnormal chromosome numbers can also occur, so instead of two copies there may be three copies. Because this alters the number of alleles of genes for certain proteins, this can have major affects, usually resulting in death. Trisomy 21, where there are three copies of chromosome number 21 results in Down's syndrome, and is an example where death may not necessarily be the result. In most other chromosome trisomies, death occurs during fetal growth, and/or as a result of spontaneous abortion.

Often only one of each pair of alleles of each gene is needed for normal function. Some of the alleles may be so different in their sequence from normal that the protein or enzyme they produce is nonfunctional. If this is the case then the individual will use the other functional allele of the pair and this will normally allow a completely normal life or phenotype. Sometimes one of the alleles produces an abnormal but functional product; again the individual will probably live normally. But if the individual possesses two nonfunctional, or misfunctional alleles for any gene then the effect will be a genetic disease. Normally the defective allele is not used if there is a normal, functional alternative allele, and the allele would be called recessive because of this. A recessive allele/gene is therefore one which does not get used to create the phenotype. The allele which is used is called the dominant allele/gene.

People may carry a recessive disease-causing allele without it having any effect on them, but it is possible that it will be passed on to their offspring. In some cases the defective allele is dominant which means even an individual with one normal and one defective gene will suffer from the disease. Dominant and X-linked mutations often cause severe disease and interfere with reproduction so would not last many generations. Recessive mutations have the greatest chance of being maintained in the population, no mutations would be eliminated in the first generation, as each individual would only be a carrier, and if there is only one copy, then there is no effect. They would be present for generations, for example, the most common mutation in cystic fibrosis is thought to have originated about 50,000 years ago.

Genetic disease is not usually lethal and some abnormalities have little effect. About 3-4% of children suffer from some type of genetic disease at birth. Every human possesses a specific genotype, consisting of many units called genes; each gene directs the manufacture in our body of a specific component, these components are usually proteins of which the most important class for genetic studies are enzymes. Every person has new mutations, and carry alleles which could cause disease. We all carry about twenty recessive alleles for lethal characteristics, but because these occur at low frequency the incidence of a child being born with two recessive alleles is low. Some mutations are found in the reproductive cells (ova and sperm) and others in the body (somatic) cells. Both types of mutation have the potential to cause cancer.

C1.3. Genetic screening

DNA is normally found in double-stranded form (the double helix). The four bases are given the symbols, A, T, G, and C. The base A binds with T, and the base G binds with C, between these long chains, as is shown below:

---ATTCCGAAGCTGACTGA--- parent chain

---TAAGGCTTCGACTGACT--- complementary

Genetic screening involves the use of this complementary binding. A sample of DNA is taken from a cell, and then the DNA is split into single chains. The bases in this single-stranded DNA will bind to the pairing bases. To make it easier to test, this single-stranded DNA may be fixed to a plastic filter. We can test for the presence of a certain sequence in this fixed DNA by adding a solution of single-stranded probe DNA, a short sequence of synthetically made DNA with a label on it, like a fluorescent dye. After mixing the probe with the sample, the probe that is not bound to the complementary sequence is washed away. If there are copies of the sequence in the sample, we will be able to see the probe when we hold the filter under ultraviolet light, because the probe is fluorescent. If there is no complementary sequence in the sample to the probe, then we will not see any fluorescence.

In this way, many samples can be tested, with many probes, and this is known as genetic screening. We screen for the presence or absence of particular DNA sequences that represent different genes. This screening can be used to detect a mutation, for example to tell that a fetus has a mutation that will cause a genetic disease (prenatal diagnosis). It can also be used to detect which types of bacteria may be present in a food sample, or for medical diagnosis of a patient.

Information about whether an individual has a particular DNA sequence and gene can be very powerful, especially in the diagnosis of genetic disease. There are many ethical and legal issues that result from this technology, as discussed in following chapters on genetic privacy and information. For example, presymptomatic screening means testing for a late-onset genetic disease, like Huntington's disease, before the person is sick. Such predictive power may require psychological counseling. It is very important that privacy is respected, because the information in a person's genes identifies risk factors for disease that medical insurance companies and employers could use to discriminate against people. There are already cases of discrimination against individuals after genetic testing in North America.

Many genetic diseases (such as diabetes or cancer) are caused by the effects of multiple genes, and the relationship between the environment and genes. Genetic susceptibility means that a particular gene is only one determinant for the development of a complex disorder. For example to have an allele called Apo E4 (that about 10% of Caucasians and Asians have) increases the risk of developing Alzheimer's disease, and confers a very strong susceptibility at younger age if you have two alleles. Alternatively, another allele for this gene, Apo E2, seems to be protective against Alzheimer's.

Q4. Is there any advantage to having presymptomatic screening for Alzheimer's disease when you are 20 years old? What about when you are 60 years old?

Q5. If you check on the Internet for keywords like “gene array” or “gene test”, you can find many examples of genetic tests. Find some examples and write about the advantages and disadvantages of genetic screening.

C2. Ethics of Genetic Engineering

Chapter objectives[.]

Genetic engineering has been a catalyst for discussion of ethical issues related to the modification of nature, and has been politically contentious because of the economic importance of the food industry.

This chapter aims to introduce:

1. Basics of genetic engineering.

2. Examples of genetically modified organisms (GMOs) and the purposes for which they are made.

3. Ethical issues of genetic engineering.

C2.1. What are genetic engineering and GMOs?

With many years of research, scientists have now discovered to some extent which genes do what functions in building organisms. With the help of this knowledge and new developments in scientific technologies, they are able to modify the genetic constitution of organisms for various purposes through genetic engineering. Genetic engineering or genetic modification is an all-inclusive term to cover all laboratory and industrial techniques used to alter the genetic constitution of the organisms by mixing the DNA of different genes and species together.

Genetic engineering or genetic modification is the process of recombining DNA. The living organisms made with altered DNA are called Genetically Modified Organisms (GMOs). However, the process is not so simple as precisely cutting out one gene and putting it into another place in the DNA, since genes are surrounded by other sequences in the DNA that determine whether or not a gene from one organism can function in another organism. So a careful study of the GMO is needed to be sure of its safety. Genetic engineering can be used for good causes. However, it can also potentially be misused.