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The basic principles of genetics were laid down by

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Mendel and Galton towards the close of the 19th century.

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But it is only during the past few years the science of

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genetics including human genetics has made rapid progress.

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The discovery of the biological role of nucleic acids

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the

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uncovering of the structure of genetic information and its

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role in regulating life processes are discoveries

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the

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importance of which can hardly be over estimated.

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With increasing control of communicable diseases and

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infant mortality

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inherited abnormalities are assuming a

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proportionately greater importance in medical practice.

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Over 2

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300 hereditary diseases have been identified and

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more are added to the list every year. According to many

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authors

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genetically conditioned diseases or diseases with a

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clear genetic component account for 25-40 per cent of all

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cases treated by the health services (1).

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Human genetics is much more than the study of mere

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hereditary diseases. It has emerged as a basic biological

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science for understanding the endogenous factors in health

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and disease and the complex interaction between nature

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and nurture. Owing to rapid specialization

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several branches

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in genetics have come into being

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e.g.

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biochemical genetics

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clinical genetics

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immunogenetics

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microbial genetics

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and so on. Achievements in these fields have created a basis

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for effective medical and preventive intervention in many

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diseases

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and also posibly of "genetic engineering"

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controlling the traits of an individual.

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Cytologic facts

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In 1956

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Tjio and Levan surprised the scientific world by

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reporting that they could find only 46 chromosomes in the

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normal human karyotype. This was immediately confirmed

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by other workers. There is now universal agreement that the

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normal human body cell (except the sex cells) contains

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46 chromosomes

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i.e.

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sex chromosomes

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XX in the female and XY in the male. The

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chromosomes vary in length

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the longest being about

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5 times as long as the smallest. Each pair of chromosomes is

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homologous. The autosomes are numbered according to

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their length

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the first pair being the longest and the last pair

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the shortest. The sex chromosomes are not included in the

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numbering

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but are merely termed X and Y. Barr and his

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group discovered that the normal female cell nucleus

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contains in addition a dark-staining area at the periphery of

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cell nucleus

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called a Barr body or "sex chromatin" body

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which is not present in normal males.

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The autosomes have been classified and divided on the

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basis of length and certain morphological similarities into

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7 groups as follows :

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Mitosis

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Group A

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Group B

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Group C

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Meiosis

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Group D

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Group E

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Group F

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Group G

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1 to 3

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4 and 5

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6 to 12

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Chromosomes

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13 to 15

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16 to 18

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pairs

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pairs

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pairs

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pairs

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The X-chromosome is included in Group 'C with

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chromosomes 6-12

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and the Y-chromosome is included in

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Group G with chromosomes 21 and 22.

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pairs

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19 and 20 pairs

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21 and 22 pairs

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During ordinary cell division

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each chromosome divides

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lengthwise into two sister chromosomes called chromatids.

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The chromatids are joined together for a short time at a

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point called centromere. Then the chromatids separate

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one

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goes to one daughter cell

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and one to the other daughter

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cell. In this manner

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each daughter cell inherits the same

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number and kind of sister chromosomes. This process of

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nuclear division is called mitosis.

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The reproductive cells (sperms and ova) are produced in

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a different manner. There are two nuclear divisions and only

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one chromosome division. This form of division is called

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reduction division' or meiosis. A detailed description of

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meiosis is unnecessary here. Broadly

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the main events in

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meiosis are : (1) The homologous chromosomes first come

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together. This is called "pairing". (2) The chromosomes then

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replicate each doubling into two chromatids which are

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held together at centromere. (3) At this stage there is

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"crossing over" and a redistribution of genetic material.

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(4) Then the homologous chromosomes separate - one goes

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to one pole of the nucleus and the other to the other pole.

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The chromosome number is thus reduced to half

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i.e.

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(5) The cell divides and the nucleus of each daughter

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contains 23 chromosomes. (6) The second meiotic division

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follows : the centromeres divide and the chromatids move

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apart. (7) The cell divides again so that the daughter cells

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each contain 23 chromosomes. During fertilization

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the two

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half-sets come together and restore the full complement of

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46. The full complement of 46 chromosomes is called the

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diploid number

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and the half set of 23 is called the haploid

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number.

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Chromosomes are rod-like condensations of chromatin.

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They become visible in the nucleus only during cell division.

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They occur in pairs - one member of each pair comes from