17.1 Variation, 17.2 Chromosomes and DNA, 17.3 Monohybrid Inheritance, 17.4 Selection, 17.5 Genetic Engineering SYLLABUS CHECKLIST

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1 Topic 17 INHERITANCE 17.1 Variation, 17.2 Chromosomes and DNA, 17.3 Monohybrid Inheritance, 17.4 Selection, 17.5 Genetic Engineering SUFEATIN SURHAN BIOLOGY MSPSBS 2010 SYLLABUS CHECKLIST Candidates should be able to: (a) describe the difference between continuous and discontinuous variation and give examples of each; (b) state that a chromosome includes a long molecule of DNA; (c) state that DNA is divided up into sections called genes; (d) explain that genes may be copied and passed on to the next generation; (e) define a gene as a unit of inheritance and distinguish clearly between the terms gene and allele; (f) describe complete dominance using the terms dominant, recessive, phenotype and genotype; (g) describe mutation as a change in the structure of a gene (sickle cell anaemia) or in the chromosome number (47 in Down's syndrome instead of 46); (h) name radiation and chemicals as factors that may increase the rate of mutation; (i) predict the results of simple crosses with expected ratios of 3:1 and 1:1, using the terms homozygous, heterozygous, F1 generation and F2 generation. (j) explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny; (k) explain codominance by reference to the inheritance of the ABO blood group phenotypes (A, B, AB, O, gene alleles I A I B and I O ); (l) describe the determination of sex in humans (XX and XY chromosomes); (m) describe variation and state that competition leads to differential survival of organisms, and reproduction by those organisms best fitted to the environment; (n) assess the importance of natural selection as a possible mechanism for evolution; (o) describe the role of artificial selection in the production of economically important plants and animals; (p) explain that DNA controls the production of proteins; (q) state that each gene controls the production of one protein; (r) explain that genes may be transferred between cells (reference should be made to transfer between organisms of the same or different species); (s) explain that the gene that controls the production of human insulin can be inserted into bacterial DNA; (t) understand that such genetically engineered bacteria can be used to produce human insulin on a commercial scale; (u) discuss potential advantages and dangers of genetic engineering.

2 VARIATION Definition: The differences that are observed among individuals within a species. There are two types of variation: 1) Continuous variation 2) Discontinuous variation Continuous Variation Many features vary gradually from one extreme form to the other. For example, we show a graduation in height from short to tall. This type of variation in which a given feature shows a gradual transition between two extreme forms (in the case of height very short and very tall) is called continuous variation. The shape of graph for continuous variation is a normal distribution: may grow taller than expected, if he is wellnourished in his childhood. More examples of continuous variation include height, weight, size of feet and intelligence. Discontinuous Variation The differences in variation may be discreet and clear cut. For example, the tongue-rolling ability is a characteristic which some people have and others do not. You are either a tongue-roller or a non-roller; there are no in-betweens. Discontinuous variation is a result of inheritance as well as mutations and cannot be altered by environmental influences. Other examples of discontinuous variation are: o Nature of hair (curly or straight) o Eye colour (brown or blue) o Tasting phenylthiocarbamine or PTC (ability to taste it as bitter or cannot taste it). The shape of graph for discontinuous variation is a discreet distribution: Continuous variation is a result of both inheritance and environmental influences. Inheritance is the passing on of characteristics from parents to offspring. Environmental influences include diet, living conditions, the amount of exercise and the frequency of practice. A person may inherit the potential to grow tall from his parents. But if he is undernourished in his childhood, he may not grow as tall as he is expected to. On the other hand, a person born of short parents 2 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

3 CHROMOSOME AND DNA A characteristic or trait is a distinct structural or functional feature of an organism. For example, tongue-rolling is a characteristic. Genetics is the study of inheritance or heredity the way in which information that determines characteristics are transmitted from parent to offspring. This genetic information is found on chromosomes. Chromosomes are inheritable materials in the nucleus of a cell. They carry the information for making a new organism. Every one of our body cells (except the gametes) contains 46 chromosomes, present as 23 pairs. The two members of a chromosome pair are exactly alike in shape and size (except for the sex chromosomes). The two chromosomes in a pair are called homologous chromosomes. Located on each chromosome are numerous genes. Each gene is a small segment of deoxyribonucleic acid (DNA) where a piece of genetic information is stored. The position on the chromosome where a gene is found is called the gene locus. Since chromosomes occur in pairs, genes are also present in pairs. The gene controlling a particular characteristic can exist in two forms known as alleles. Therefore in a pair of homologous chromosomes, they may have the genes controlling the same characteristics but these genes may not be of the same type i.e. different alleles. Alleles can exist in two forms: dominant and recessive. A dominant allele is a gene that produces its effect in the presence of the other (recessive) allele. A recessive allele is a gene whose effect is hidden in the presence of the other (dominant) allele. The dominant allele is expressed in capital letter and the recessive form is represented by the corresponding small letter. 3 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

4 For example, the gene for height consists of two alleles; allele for tall and allele for short. So allele for tall being dominant to allele for short, is represented by a capital letter T whereas the allele for short is usually represented by the small letter t) The genotype of an organism is its genetic make-up which is inherited from the individual parents. The two alleles for a particular characteristic can exist in three different combinations: 1. Homozygous dominant; TT 2. Homozygous recessive; tt 3. Heterozygous; Tt A homozygous individual has identical alleles for the particular trait. A heterozygous individual has different alleles for a particular trait. The phenotype of an organism is its observable characteristics. The phenotype of an organism is influenced both by its genotype and by the environment. When dominant and recessive alleles are present together, only the dominant allele expresses itself in the phenotype. A recessive allele will only express itself in a homozygous recessive genotype (i.e. in the absence of a dominant allele). For example a person having genotype TT or Tt will be tall whereas a person having genotype tt will be short. A dominant allele expresses itself in the phenotype in both the homozygous and heterozygous conditions. A recessive allele expresses itself in the phenotype only in the homozygous condition. 4 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

5 Monohybrid Inheritance Gametes only contain one allele for a given characteristic. This is because homologous chromosomes separate during meiosis. Half of the gametes produced contain one member of a pair; the other half contains the other member of a pair. As a result, the alleles borne on the homologous chromosomes also separate. When gametes fuse to form zygote during fertilization, the alleles recombine. But when they recombine a lot of variation results. The study of monohybrid inheritance will demonstrate this. Monohybrid inheritance refers to the inheritance of one characteristic that is expressed in contrasting forms, e.g. tall/dwarf for plant height, tongue-rolling/non-roller. 5 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

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7 An experiment using a pure stock of tall pea plants being crossed with a pure stock of dwarf pea plants and then crossing the off-springs: Test Cross This is the crossing of an organism of unknown genotype for a given trait with one which is homozygous recessive. It is done to determine the genotype of an organism expressing a dominant trait. For example, a tall pea plant may be homozygous (TT) or heterozygous (Tt). If it is (TT): Cross with recessive (tt) Offspring will be all tall (Tt) If it is (Tt): Cross with recessive (tt) Offspring will be half tall (Tt) + half short (tt). Incomplete Dominance In Incomplete dominance, neither allele in the heterozygous individual (hybrid) is completely dominant over the other. The hybrid thus shows an intermediate phenotype. 7 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

8 For example, when pure stocks of red-flowered and white-flowered snapdragon plants are crossed, the offspring all have pink flowers. Hence, there are three distinct phenotypes red, white and pink flowers. Co-dominance In this inheritance, both the alleles in the heterozygous individual (hybrid) are fully expressed, i.e. the effect of each allele is not modified by the presence of the other. This also results in three distinct phenotypes. For example, when a homozygous red bull is crossed with a homozygous white cow, all the roan offspring have coats consisting of both red and white hairs. Sex determination In humans, sex determined by the sex chromosomes. Females have two copies of the X-chromosome, while males have one X-chromosome and the one Y-chromosome (much shorther than the X- chromosome). As a result; Females produce ova that have only X-chromosomes, Males produce two kinds of sperms one kind with X-chromosome and the other kind with Y- chromosome. 8 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

9 The sex of an offspring depends on whether the ovum is fertilized by an X-bearing sperm or a Y- bearing sperm. There is usually an equal chance of the offspring being a male or a female. Sex-linked Inheritance Certain characteristics are linked with the sex chromosomes. Some examples of sex-linked characteristics are red-green colour blindness, muscular dystrophy and haemophilia. In humans, the X-chromosome may contain a recessive allele for colour blindness, muscular dystrophy and haemophilia. These inherited sex-linked characteristics frequently affect man but occur very rarely in women. They are expressed in males because of the lack of dominant counterpart on the Y-chromosome. Only females who are homozygous recessive for the trait are affected. Females with heterozygous genotype are carriers who do not show the condition but carry a recessive allele which they may pass to their offspring. In red-green colour blindness, the dominant allele for normal vision is X C and the recessive allele for colour blindness is X c. The possible genotypes and phenotypes in females and males are: Female Male Genotype X C X C X C X c X c X c X C Y X c Y Phenotype Normal colour vision Normal colour vision carrier Red-green colour blindness Normal colour vision Red-green colour blindness 9 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

10 Haemophilia is a disease in which a person s blood will not clot (lack clotting factors). The dominant allele X H produces blood clotting factors for clotting of blood. The recessive allele X h causes haemophilia. The possible genotypes and phenotypes in the females and males are: Female Male Genotype X H X H X H X h X h X h X H Y X h Y Phenotype Normal Normal carrier Haemophiliac Normal Haemophiliac Multiple alleles Certain characteristics are controlled by more than two alleles. The gene that controls the ABO blood group in humans has three different alleles: I A, I B and I O. However there can only be two alleles in any one genotype. I A and I B are co-dominant and I O is recessive to both I A and I B alleles. The following table summarises the distribution of the three types of alleles in blood groups: Genotype I A I A or I A I O I B I B or I B I O I A I B I O I O Phenotype (Blood group) Group A Group B Group AB Group O Inheritance pattern when a male (AB blood group) mates with a female (A blood group heterozygous): 10 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

11 Mutations Mutation is the sudden spontaneous change in the structure of a gene or in the chromosome number, and may be inheritable. If mutations occur in gametes of cells giving rise to gametes, they are inheritable. Most mutations are harmful and recessive. Occasionally, beneficial mutations arise in nature. Mutations produce variations in populations and provide the raw material for evolution. Sickle-cell anaemia is an example of a gene mutation. The gene involved is the gene controlling haemoglobin (a protein pigment) production. The mutated gene produces haemoglobin S (HbS) which is exactly the same as the normal haemoglobin except in one amino acid. This causes a change in structure that makes HbS less soluble. The HbS molecules tend to clump causing the red blood cells to become sickle-shaped. This interferes with the oxygen-carrying property of the red blood cells and also makes them fragile, resulting in severe anaemia. People with sickle cell anaemia usually die young. The mutated gene is recessive so it expresses itself only in the homozygous condition. The heterozygous condition gives rises to the sickle cell trait, which is a mild condition. Down s syndrome is an example of a chromosomal mutation. It is caused by a change in chromosome number from 46 to 47 (extra chromosome 21). This is caused by an abnormal meiotic division during ova formation. It usually happens in older women. The extra chromosome interferes with the normal brain and body development of the child. 11 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

12 The rate of spontaneous mutation is usually very low. However, the rate of mutation is greatly increased by mutagens in the external environment. Examples of mutagens are: Radiations like ultraviolet light, and alpha, beta and gamma radiation. Chemicals like mustard gas, formaldehyde and cyclamates (artificial sweetening agents). SELECTION Natural Selection Natural selection is the process by which the better adapted varieties of species in populations are selected by environmental pressures. Better adapted varieties are the organisms that are structurally, physiologically and behaviourally well fitted for their environment. Environmental pressures refer to competition and environmental factors such as climatic conditions. Natural selection is a never-ending process in nature since environmental factors change all the time. Only variations with survival value and are due to inherited characteristics are important in natural selection. Evolution Evolution is broadly the sum total of adaptive changes that have taken place over a very long period of time. It leads to the development of new species from existing ones- new species that lead their lives differently form the species from which they evolved. Many species also become extinct in this never-ending process. Natural Selection and Evolution Darwin proposed that natural selection is the mechanism by new species arises from existing ones. Modern view of evolution makes use of our knowledge of genes and chromosomes (which Darwin did not know anything about) to explain the source of genetic variation upon which natural selection acts. Natural selection is still regarded as the only force that regularly produces adaptive evolutionary changes. Other force that play a part in evolution are mutation and gene flow. Artificial Selection This is practiced by humans to produce economically important plants and animals with desirable qualities. To do this we: First isolate natural populations, and then Selectively breed individuals which show desirable characteristics (such as individuals ofen arise as a result of mutation). 12 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

13 GENETIC ENGINEERING Structure of chromosome: Each chromosome (chromatin) is made up of a molecule of deoxyribonucleic acid (DNA) wrapped around protein bundles. Structure of DNA: A DNA molecule is a double helix molecule where two linear strands run parallel and twisted around each other. The two strands are linked together through bases. There are four kinds of base in DNA. There are Adenine (A), Thymine (T), Cytosine (C) and Guanine (G). The bases pair with each other strictly; 1. Adenine (A) with thymine (T), and 2. Cytosine (C) with guanine (G). 13 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

14 Genes and proteins: Each DNA molecule is made up of thousands of genes. The 23 pairs of chromosomes in the human body cell contain approximately over genes. A protein is made up of one or more chains (polypeptides) of amino acids. The type and order of amino acids in these polypeptides determine the structure and function of a given protein. It is now known that the sequence of bases in a DNA molecule determines: i. The type of amino acids, and ii. The sequence in which they are arranged in a polypeptide chain. This sequence of bases is also known as the genetic code. Thus, different genes have different sequences of bases. Genetic Engineering: the technique used to artificially transfer gene(s) from one organism to another. Such transfer can be made between organisms of the same or different species. The organism that acquires a foreign gene is called a transgenic or genetically engineered (modified) organism. Advantages of genetic engineering The numerous applications of genetic engineering in manufacturing industries, agriculture, waste management, pollution control and medicine indicate its obvious advantages and benefits to mankind. Applications of genetic engineering include: 1. Production of pharmaceuticals e.g. drugs, enzymes, hormones, vaccines, antitoxins, etc. 2. Production of genetically modified foods (GM foods): Food from plants and animals that has been genetically engineered to give it the desired trait. 3. Gene therapy e.g. Human somatic cell gene therapy targets just the diseased/affected tissue. 4. Improvements in selective breeding e.g. (a) Genes from any organisms can be introduced into a plant or an animal. (b) Ability to selection of genes so as not to introduce defective genes into an organism. (c) Reduction in the time taken and space to selectively breed and organism. (d) More economical (better productivity) and better efficiency. Risks/dangers of genetic engineering While the benefits of genetic engineering are obvious, there are potential hazards. A few are listed: 1. There may be unknown effects of moving genes from one organism to another. 2. New and dangerous diseases may result. 3. Some people may be allergic to genetically modified food. 4. Misuse of genetic engineering techniques by some people may result in disastrous chemical or biological warfare. 5. It would be against nature. Some people feel that one should not play around with nature. 14 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S

15 Production of human insulin using genetic engineering Insulin is a hormone made by the pancreas and is involved in homeostasis of blood glucose level. Diabetes mellitus, a potentially fatal disorder, occurs when the pancreas is unable to synthesise insulin either totally or inadequate amounts. Diabetics need to be injected with insulin made from cattle and pigs to survive. However, many problems are associated with the use of foreign insulin. The production of human insulin using genetic engineering overcame these problems. 15 S u f e a t i n S u r h a n / B i o l o g y / M S P S B S