The basic principles of genetics were laid down by Mendel and Galton towards the close of the 19th century.
But it is only during the past few years the science of genetics including human genetics has made rapid progress.
The discovery of the biological role of nucleic acids, the uncovering of the structure of genetic information and its role in regulating life processes are discoveries, the importance of which can hardly be over estimated.
Human genetics is much more than the study of mere hereditary disease. Basically it deals with the inheritance of characters in man.
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The facts of human genetics, so far known to us have, however, been mostly gathered from field study, or are based upon the genetics of other organisms, because human beings cannot be used as “experimental laboratory material” due to social and other following reasons:-
(i) Cells of human body are comparatively very small, whereas the karyotype is rather large with 23 pairs of chromosomes.
(ii) Expressivity of genes in human beings is very variable because of frequent human migrations and changes of habitat.
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(iii) Hybridization is a necessary part of genetic experimentation, but controlled breeding experiments cannot be conducted in the laboratories in case of human beings. Moreover, nothing much can be gained from reproductive activities of man because of lengthy ontogeny, childhood and life span.
A scientist can hardly observe more than two or three generations of his human subjects. Further, man has minimum fertility, producing a litter normally of a single child at each birth, so that the statistical computations are not possible.
(iv) Most people are heterozygous for most of the genetic traits. It is, therefore, difficult to find pure-breeding homozygous strains in human populations.
Much before establishment of human genetics, Sir Francis Galton recommended two methods to determine human genetic traits and their inheritance, viz., (a) pedigree analysis, and (b) studies of twins. Hardy and Weinberg (1908) recommended “population genetics” to determine human genetic traits.
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(a) Pedigree analysis:
A pedigree is a graphic method representing the generations of a family, with various symbols used for relationships or for particular clinical findings.
Analysis of the pedigree is only possible if both affected and unaffected individuals are shown. Patterns of inheritance may then be clear, and differential expression in one sex may be noted.
(b) Twin studies:
A number of human hereditary traits have been established through studies of twins. Twins are of three types:-
(i) Fraternal twins:
Such twins are formed by simultaneous fertilization of different ova by sperms. Hence, these are like normal brothers and sisters and called dizygotic, fraternal, no identical twins. Obviously, these vary genetically and may be of same or different sexes.
When these are of same sex, all variations between these at the time of birth (congenital variations) must be hereditary (blastogenic) because, in the womb, these have developed in the same environment.
Further genetic variations of these twins can be identical if these are reared (nurtured) in similar environmental conditions even after birth.
(ii) Identical twins:
Sometimes, the two blastomeres resulting from the first cleavage of zygote completely separate from each other and develop into independent embryos.
The twins, thus formed, are obviously similar in genotype (nature) and sex and, hence, called identical or monozygotic twins.
If these are nurtured in dissimilar environmental conditions, we can easily determine which of the human variations are genetic and which are somatic, i.e., induced by environmental conditions.
(iii) Siamese twins:
These are identical twins which have failed to separate completely from each other. These are, naturally, of no help in distinguishing blastogenic and somatogenic variations.
Genes:
Genes are the units of heredity. They contain the hereditary information encoded in their chemical structure for transmission from generation to generation. They affect development and function, both normal and abnormal.
As genes are not seen with an electron microscope, much is known about them by indirect methods.
Genes are composed of deoxyribonucleic acid (DNA), which codes for production of specific amino acid residues. These amino acids are then joined to form the proteins that comprise living organisms.
The human genome consists of all DNA and consequently all the genes in one set of human chromosomes. In humans, there are 23 chromosome sets, each of which consists of approximately 3 x 109 base pairs. It is said we inherit about 50,000 genes from the father and 50,000 from the mother.
The genes occupy a specific position or locus on the chromosomes. For example, the locus for the ABO blood groups is in chromosome 9 and the locus for the major histocompatibility complex is in chromosome 6.
The Y chromosome contains genes that determine the normal development of testis. It is known that the human X chromosome carries thegenes governinga blood group haemophilia, red-green colorblindness, glucose-6-phosphate dehydrogenase, muscular dystrophy, height and gonadal development.
Since genes are contained in the chromosomes, genes also occur in pairs. If the genes comprising a pair are alike (AA), the individual is described as homozygous for that gene, and if it is different (Aa) the individual is described as heterozygous.
i. A gene is said to be dominant when it manifests its effects both in the heterozygous and the homozygous state.
ii. A gene is said to be recessive when it manifests its effect only in the homozygous state.
iii. Genes whose combined action affects one particular character are known as polygenes or ‘multiple genes’
iv. Genotype refers to the total genetic constitution of an individual.
v. Phenotype refers to the outward expression of the genetic constitution.
Chromosomes:
Chromosomes are rod-like condensations of chromatin. They become visible in the nucleus only during cell division. They occur in pairs – one member of each pair comes from the father and other from the mother.
The largest chromosome measures about 7 p and is about five times the length of the shortest. Biochemically, the chromosomes are made up of deoxyribonucleic acid (DNA). Genetically, they consist of genes arranged like the beads of a necklace.
The number of chromosomes in each species is fixed. All individuals of the same species have the same number of chromosomes (with exceptions).
There are 22 pairs of autosomal (non-sex) chromosomes and a pair of sex chromosomes. Thus the total number of chromosome for man is 46 i.e. 23 pairs.
Karyotype is the chromosomal complement of an organism. It is made by the arrangement of metaphase chromosome pairs in a standard sequence. Karyotype names include information about:
i. the number of chromosomes per cell
ii. the composition of the sex chromosomes
iii. identification of any chromosomal abnormalities
For example, “46, XX” designates a normal female karyotype. 46 is the normal number of chromosomes. The two XX’s indicate the normal composition of sex chromosomes for a female. “47, XXY” indicates a male karyotype with an extra sex chromosome. “47, XX, +21” indicates a female individual with normal sex chromosomes who has an extra chromosome 21.
Karyotype does not show the –
i. Presence and location of small mutations in genes.
ii. Type of mutation present.