A heartly thanks to all the teachers of science and

technology club to provide me this opportunity to give

presentation & thanks to the editors of Encarta & also

thanks to Nancy Hamilton Research team for providing

such good sources to students which are very helpful in

making projects and presentations.

CONTENTS

INTRODCTION

EMERGENCE OF GENETICS

PHYSICAL BASIS OF HEREDITY

THE TRANSMISSION OF GENES

QUANTITATIVE INHERITANCE

GENE LINKAGE AND GENE MAPPING

GENE ACTION : DNA AND THE CODE OF LIFE

CYTOPLASMIC INHERITANCE

FURDER READING

INTRODCTION

Genetics, scientific study of how physical, biochemical, and

behavioural traits are transmitted from parents to their offspring.

The word itself was coined in 1906 by the British biologist William

Bateson. Geneticists determine the mechanisms of inheritance

whereby the offspring of sexually reproducing organisms do not

exactly resemble their parents, and the differences and

similarities between parents and offspring recur from generation

to generation in repeated patterns. The investigation of these

patterns has led to some of the most exciting discoveries in

modern biology.Gregor Mendel

Known as the father of modern genetics, Gregor Mendel developed the principles of heredity while studying seven pairs of inherited characteristics in pea plants. Although the significance of his work was not recognized during his lifetime, it has become the basis

for the present-day field of genetics.

EMERGENCE OF GENETICS

The science of genetics began in 1900, when several plant breeders independently discovered the work of the Austrian monk Gregor Mendel, which, although published in 1866, had been virtually ignored. Working with garden peas, Mendel described the patterns of inheritance in terms of seven pairs of contrasting traits that appeared in different pea-plant varieties. He observed that the traits were inherited as separate units, each of which was inherited independently of the others (see Mendel's Laws). He suggested that each parent has pairs of units but contributes only one unit from each pair to its offspring. The units that Mendel described were later given the name genes.

PHYSICAL BASIS OF HEREDITY Soon after Mendel's work

was rediscovered, scientists realized that the patterns of inheritance he had described paralleled the action of chromosomes in dividing cells, and they proposed that the Mendelian units of inheritance, the genes, are carried by the chromosomes. This led to intensive studies of cell division.

Fruit Fly Chromosomes

The chromosomes of the fruit fly, Drosophila melanogaster, lend themselves well to genetic experiments. There are only 4 pairs—one of which, marked here X and Y, determines the fly’s sex—compared with the human complement of 23 pairs. In addition, the fly’s chromosomes are very large. Thomas Hunt Morgan and his associates based their theory of heredity on studies using Drosophila. They found that chromosomes were passed from parent to offspring in a way that Gregor Mendel ascribed to inherited characteristics. They proposed, correctly, that genes in fact occupy specific physical locations on chromosomes.

Every cell comes from the division of a pre-existing cell. All the cells that make up a

human being, for example, are derived from the successive divisions of a single cell,

the zygote (see Fertilization), which is formed by the union of an egg and a sperm. The

great majority of the cells produced by the division of the zygote are, in the

composition of their hereditary material, identical to one another and to the zygote

itself (assuming that no mutations occur; see below). Each cell of a higher organism

is composed of a jellylike layer of material, the cytoplasm, which contains many small

structures. This cytoplasmic material surrounds a prominent body called the nucleus.

Every nucleus contains a number of minute, threadlike chromosomes. Some relatively

simple organisms, such as cyanobacteria and bacteria, have no distinct nucleus but

do have cytoplasm, which contains one or more chromosomes.

Chromosomes vary in size and shape and usually occur in pairs. The

members of each pair, called homologues, closely resemble each other. Most

cells in the human body contain 23 pairs of chromosomes, whereas most

cells of the fruit fly Drosophila contain four pairs, and the bacterium

Escherichia coli has a single chromosome in the form of a ring. Every

chromosome in a cell is now known to contain many genes, and each gene

is located at a particular site, or locus, on the chromosome.

The process of cell division by which a new cell comes to have an identical number of chromosomes as the parent cell is called mitosis (see Reproduction). In mitotic division each chromosome divides into two equal parts, and the two parts travel to opposite ends of the cell. After the cell divides, each of the two resulting cells has the same number of chromosomes and genes as the original cell (see Cell: Division, Reproduction, and Differentiation). Every cell formed in this process thus has the same genetic material. Simple one-celled organisms and some multicellular forms reproduce by mitosis; it is also the process by which complex organisms achieve growth and replace worn-out tissue.

Higher organisms that reproduce sexually are formed from the union of two

special sex cells known as gametes. Gametes are produced by meiosis, the

process by which germ cells divide. It differs from mitosis in one important

way: in meiosis a single chromosome from each pair of chromosomes is

transmitted from the original cell to each of the new cells. Thus, each gamete

contains half the number of chromosomes that are found in the other body

cells. When two gametes unite in fertilization, the resulting cell, called the

zygote, contains the full, double set of chromosomes. Half of these

chromosomes normally come from one parent and half from the other.

THE TRANSMISSION OF GENES

The union of gametes brings together two sets of genes, one set from each parent. Each gene—that is, each specific site on a chromosome that affects a particular trait—is therefore represented by two copies, one coming from the mother and one from the father (for exceptions to this rule, see Sex and Sex Linkage, below). Each copy is located at the same position on each of the paired chromosomes of the zygote. When the two copies are identical, the individual is said to be homozygous for that particular gene. When they are different—that is, when each parent has contributed a different form, or allele, of the same gene—the individual is said to be heterozygous for that gene. Both alleles are carried in the genetic material of the individual, but if one is dominant, only that one will be manifested. In later generations, however, as was shown by Mendel, the recessive trait may show itself again (in individuals homozygous for its allele).

.

Albinism

Albinism, the lack of normal pigmentation, occurs in all groups of people. A rare condition, albinism occurs when a person inherits a recessive allele, or group of genes, for pigmentation from each parent. In this case, production of the enzyme tyrosinase is defective. Tyrosinase is necessary to the formation of melanin, the normal human skin pigment. Without melanin, the skin lacks protection from the sun and is subject to premature ageing and skin cancer. The eyes, too, colourless except for the red blood vessels of the retina that show through, cannot tolerate light. Albinos tend to squint even in normal indoor lighting and frequently have vision problems. Tinted glasses or contact lenses can help.

For example, the ability of a person to form pigment in the skin, hair, and eyes depends on the

presence of a particular allele (A), whereas the lack of this ability, known as albinism, is caused by

another allele (a) of the same gene. (For convenience, alleles are usually designated by a single

letter; the dominant allele is represented by a capital letter and the recessive allele by a small letter.)

The effects of A are dominant; of a, recessive. Therefore, heterozygous people (Aa), as well as people

homozygous (AA) for the pigment-producing allele, have normal pigmentation. People homozygous

for the allele that results in a lack of pigment (aa) are albinos. Each child of a couple who are both

heterozygous (Aa) has a probability of one in four of being homozygous AA, one in two of being

heterozygous Aa, and one in four of being homozygous aa. Only the individuals carrying aa will be

albino. Note that each child has a one-in-four chance of being affected with albinism; it is not

accurate to say that one-quarter of the children in a family will be affected. Both alleles will be carried

in the genetic material of heterozygous offspring, who will produce gametes bearing one or the other

allele. A distinction is made between the appearance, or outward characteristics, of an organism and

the genes and alleles it carries. The observable traits constitute the organism's phenotype, and the

genetic makeup is known as its genotype.

It is not always the case that one allele is dominant and the other recessive. The

four-o'clock plant, for example, may have flowers that are red, white, or pink.

Plants with red flowers have two copies of the allele R for red flower colour and

hence are homozygous RR. Plants with white flowers have two copies of the

allele r for white flower colour and are homozygous rr. Plants with one copy of

each allele, heterozygous Rr, are pink—a blend of the colours produced by the

two alleles.The action of genes is seldom a simple matter of a single gene controlling a

single trait. Often one gene may control more than one trait, and one trait may

depend on many genes. For example, the action of at least two dominant genes

is required to produce purple pigment in the purple-flowered sweet pea. Sweet

peas that are homozygous for either or both of the recessive alleles involved in

the colour traits produce white flowers. Thus, the effects of a gene can depend

on which other genes are present.

QUANTITATIVE INHERITANCE

Traits that are expressed as variations in quantity or extent, such as weight, height, or degree of pigmentation, usually depend on many genes as well as on environmental influences. Often the effects of different genes appear to be additive—that is, each gene seems to produce a small increment or decrement independent of the other genes. The height of a plant, for example, might be determined by a series of four genes: A, B, C, and D. Suppose that the plant has an average height of 25 cm (10 in) when its genotype is aabbccdd, and that each replacement by a pair of dominant alleles increases the average height by approximately 10 cm (4 in). In that case a plant that is AABBccdd will be 45 cm (18 in) tall, and one that is AABBCCDD will be 65 cm (26 in) tall. In reality, the results are rarely as regular as this. Different genes may make different contributions to the total measurement, and some genes may interact so that the contribution of one depends on the presence of another. The inheritance of quantitative characteristics that depend on several genes is called polygenic inheritance. A combination of genetic and environmental influences is known as multifactorial inheritence.

GENE LINKAGE AND GENE

MAPPING

Mendel's principle that genes controlling different

traits are inherited independently of one another

turns out to be true only when the genes occur on

different chromosomes. The American geneticist

Thomas Hunt Morgan and his co-workers, in an

extensive series of experiments using fruit flies

(which breed rapidly), showed that genes are

arranged on the chromosomes in a linear fashion;

and that when genes occur on the same

chromosome, they are inherited as a single unit for

as long as the chromosome itself remains intact.

Genes inherited in this way are said to be linked.

Perkin Elmer/Applied Biosytems DivisionGenetic Mapping

This gel scan showing the arrangement of chromosomes within a cell allows experts to take a closer look at the genetic make-up of each individual. With the completion of the human genome project in 2005, geneticists hope to compile a map identifying and locating every gene in the human body.

Morgan and his group also found, however, that such linkage is rarely complete. Combinations

of alleles characteristic of each parent can become reshuffled among some of their offspring.

During meiosis, a pair of homologous chromosomes may exchange material in a process called

recombination, or crossing-over. (The effect of crossing-over can be seen under a microscope

as an X-shaped joint between the two chromosomes.) Crossovers occur more or less at

random along the length of the chromosomes, so the frequency of recombination between two

genes depends on their distance from each other on the chromosome. If the genes are

relatively far apart, recombinant gametes will be common; if they are relatively close,

recombinant gametes will be rare. In the offspring produced by the gametes, the crossovers

show up as new combinations of visible traits. The more crossovers that occur, the greater the

percentage of offspring that show the new combinations. Consequently, by arranging suitable

breeding experiments, scientists can plot, or map, the relative positions of the genes along the

chromosome.

In recent years geneticists have used organisms such as bacteria, moulds, and viruses, which

rapidly produce extremely large numbers of offspring, to detect recombinations that occur only

rarely. Thus, they are able to make maps of genes that are quite close together. The method

introduced at Morgan's laboratory has now become so exact that differences occurring within a

single gene can be mapped. These maps have shown that not only do the genes occur in linear

fashion along the chromosome, but they themselves are linear structures. The detection of rare

recombinants can reveal the existence of structures even smaller than those observed through

the most powerful microscopes.

In recent years geneticists have used organisms such as bacteria, moulds, and viruses, which

rapidly produce extremely large numbers of offspring, to detect recombinations that occur only

rarely. Thus, they are able to make maps of genes that are quite close together. The method

introduced at Morgan's laboratory has now become so exact that differences occurring within a

single gene can be mapped. These maps have shown that not only do the genes occur in linear

fashion along the chromosome, but they themselves are linear structures. The detection of rare

recombinants can reveal the existence of structures even smaller than those observed through

the most powerful microscopes.

By March 2000, the entire genome (the complete set of genetic information) of the fruit fly had

been deciphered and mapped by another, faster method, whole-genome shotgun sequencing,

which splits the genome into tiny fragments and uses supercomputers to work out how these

fragments would reassemble and, therefore, the sequence of the fly’s genetic blueprint. This is

also one of the methods being used in the Human Genome Project (also see below).

GENE ACTION : DNA AND THE CODE OF

LIFE

For more than 50 years after the science of genetics was

established and the patterns of inheritance through genes

were clarified, the largest questions remained unanswered:

how are the chromosomes and their genes copied from cell to

cell, and how do they direct the structure and behaviour of

living things? Two American geneticists, George Wells Beadle

and Edward Lawrie Tatum, provided one of the first important

clues in the early 1940s. Working with the fungi Neurospora

and Penicillium, they found that genes direct the formation of

enzymes through the units of which they are composed. Each

unit (a polypeptide) is produced by a specific gene. This work

launched studies into the chemical nature of the gene and

helped to establish the field of molecular genetics

That chromosomes were almost entirely composed of two kinds of chemical substances, protein

and nucleic acids, had long been known. Partly because of the close relationship established

between genes and enzymes, which are proteins, protein at first seemed the fundamental

substance that determined heredity. In 1944, however, the Canadian bacteriologist Oswald

Theodore Avery proved that deoxyribonucleic acid (DNA) performed this role. He extracted DNA

from one strain of bacteria and introduced it into another strain. The second strain not only

acquired characteristics of the first but passed them on to subsequent generations. By this time

DNA was known to be made up of substances called nucleotides. Each nucleotide consists of a

phosphate, a sugar known as deoxyribose, and any one of four nitrogen-containing bases. The four

nitrogen bases are adenine (A), thymine (T), guanine (G), and cytosine (C).

In 1953, putting together the accumulated chemical knowledge, geneticists James Dewey Watson

of the United States and Francis Harry Compton Crick of Great Britain worked out the structure of

DNA. This knowledge immediately provided the means of understanding how hereditary

information is copied. Watson and Crick found that the DNA molecule is composed of two long

strands in the form of a double helix, somewhat resembling a long, spiral ladder. The strands, or

sides of the ladder, are made up of alternating phosphate and sugar molecules. The nitrogen

bases, joining in pairs, act as the rungs. Each base is attached to a sugar molecule and is linked

by a hydrogen bond to a complementary base on the opposite strand. Adenine always binds to

thymine, and guanine always binds to cytosine. To make a new, identical copy of the DNA

molecule, the two strands need only unwind and separate at the bases (which are weakly bound);

with more nucleotides available in the cell, new complementary bases can link with each

separated strand, and two double helixes result. If the sequence of bases were AGATC on one

existing strand, the new strand would contain the complementary, or “mirror image”, sequence

TCTAG. Since the “backbone” of every chromosome is a single long, double-stranded molecule of

DNA, the production of two identical double helixes will result in the production of two identical

chromosomes.

The DNA backbone is actually a great deal longer than the chromosome but is tightly coiled up

within it. This packing is now known to be based on minute particles of protein known as

nucleosomes, just visible under the most powerful electron microscope. The DNA is wound

around each nucleosome in succession to form a beaded structure. The structure is then further

folded so that the beads associate in regular coils. Thus, the DNA has a “coiled-coil”

configuration, like the filament of an electric light bulb.

After the discoveries of Watson and Crick, the question that remained was how the

DNA directs the formation of proteins, compounds central to all the processes of life.

Proteins are not only the major components of most cell structures, they also control

virtually all the chemical reactions that occur in living matter. The ability of a protein to

act as part of a structure, or as an enzyme affecting the rate of a particular chemical

reaction, depends on its molecular shape. This shape, in turn, depends on its

composition. Every protein is made up of one or more components called

polypeptides, and each polypeptide is a chain of subunits called amino acids. Twenty

different amino acids are commonly found in polypeptides. The number, type, and

order of amino acids in a chain ultimately determine the structure and function of the

protein of which the chain is a part.

CYTOPLASMIC INHERITANCE Some constituents of the cell besides the nucleus contain DNA.

They include the cytoplasmic bodies known as mitochondria (the energy producers of the cell) and the chloroplasts of plants, where photosynthesis takes place. These bodies are self-reproducing. The DNA is replicated in a manner similar to that in the nucleus, and sometimes its code is transcribed and translated into proteins. In 1981 the entire sequence of nucleotides in the DNA of a mitochondrion was determined; apparently, mitochondria use a code only slightly different from that used by the nucleus.

The traits determined by cytoplasmic DNA are more often inherited through the mother than through the father (exclusively through the mother in the case of Homo sapiens), because sperm and pollen usually contain less cytoplasmic material than do eggs. Some cases of apparent maternal inheritance are actually due to the transmission of viruses from mother to offspring through the egg cytoplasm.

FURDER READING

Burns, George W. The Science of Genetics. 6th ed., 1989. Collier Macmillan. Standard scientific textbook.

Gribbin, John. In Search of the Double Helix: Quantum Physics and Life. New York: McGraw-Hill, 1985. Examines molecular genetics, evolution, physics.

Griffiths, Anthony J. F., and McPherson, Joan. 100+ Principles of Genetics. New York; Oxford: W. H. Freeman, 1989. Clearly organized scientific text.

King, Robert C., and Stansfield, William D. A Dictionary of Genetics. 3d ed., 1985. Oxford University Press. Standard reference work.

King, Robert C., and Stansfield, William D. Encyclopedic Dictionary of Genetics. VCH, 1990. Comprehensive specialist reference work.

Koestler, Arthur. The Case of the Midwife Toad. 1973. New York: Random House, 1971. Implications of controversy surrounding Kemmerer's principle of genetic regression

Lloyd, J. R. Genes and Chromosomes. London: Macmillan, 1986. Concise and clear, for student or general reader.

Maclean, Norman. Macmillan Dictionary of Genetics & Cell Biology. London: Macmillan, 1987. Standard reference dictionary.

Singer, Sam. Human Genetics. Freeman, 1985. Fundamentals and advances.