Genetics Revision Notes For NEET

HUMAN GENETICS

        The basic principles of inheritance as applicable to all other living organisms are equally applicable to man. The first person to study human genetics was Sir Archibald Garrod.

        Human karyotype is the chromosomal complement of an organism made by the arrangement of metaphase chromosome pairs in a standard sequence.

        The male karyotype has 44 autosomes, a single X-chromosome and a Y-chromosome (44 + XY), whereas the female karyotype shows two X-chromosomes (44 + XX). Chromosome numbers vary considerably among the organism and may differ markedly between closely related species.

        Sex-determination in human beings Male has 2 types of gametes, i.e. X and Y, while female has one pair of gametes, i.e. XX. Therefore, if an ovum fertilises with a sperm carrying X-chromosome, the zygote develops into a female (XX) and will develop into male if ovum fertilises with a sperm carrying Y-chromosome. Hence, the type of sperm, which fertilises the ovum determines the sex of a child.

        The key to sex-determination in humans is the SRY (for sex region the Y) gene located on the short arm of the Y-chromosome. In the male, the Testis Determining Factor (TDF) is produced by SRY on the Y-chromosome. TDF induces the medulla of the embryonic gonads to develop into Testis. In the absence of TDF, the gonads develop into ovaries.

        Barr body is condensed, inactive X-chromosome found in the nuclei of somatic cells of females. The number of Barr body in human, is one less than the total number of X-chromosomes.

        Dosage compensation is a term that describes the process by which organisms equalise the expression of genes between members of different biological sexes.

        Sex-linked inheritance refers to the inheritance of genes from one generation to another through the sex-chromosomes, i.e. X and Y. it can be of three types including X-linked inheritance, Y-linked inheritance or X-Y linked inheritance.

        X-linked inheritance X-linked genes are represented twice in female and once in male. The recessive X-linked genes have characteristic criss-cross inheritance, i.e. male transmits his X-linked genes to his grandson through his daughter and vice-versa.

        A number of well-known diseases and traits are caused by X-linked recessive genes, e.g. red-green colour blindness, haemophilia, Duchenne muscular dystrophy, etc.

        Holandric or Y-linked inheritance Holandric genes are those that occur on the Y-chromosome only they are not expressed in females. ‘Hairy ears’ (hypertrichosis) in man is inherited through genes on Y-chromosome. Y-linked genes are transmitted directly from father to son.

        XY-linked inheritance The genes which occur in homologous sections of X and Y-chromosomes are called XY-linked genes and they have inheritance like the autosomal genes, e.g. xeroderma, nephritis, etc.

        Sex-limited genes are present on autosomes, but their expression is determined by the presence or absence of one or more sex hormones. Hence, their expression is limited to one sex only, e.g. beard development in males and feminine voice in females.

        Sex-influences genes in some organisms, some genes are influenced by the sex of the organism, i.e. these are expressed more frequently in one sex than in other, e.g. pattern of baldness in man.

        Blood group inheritance several dozen blood group systems have been defined on the basis of antigens located on the surfaces of RBCs. Each of the blood group system is determined by a different gene or set of genes. Blood group systems found in humans are ABO, MN and Rh systems.

        Pedigree analysis is the study of inheritance of genetic traits in several generations of human family in the form of a family tree diagram. This provides a strong tool to trace the inheritance of a specific trait, abnormality or a disease.

        Study of twins there are mainly two types of twins, identical or monozygotic and fraternal (non-identical) or dizygotic. Fraternal twins have on an average 50% of their genes are common. The identical twins possess 100% of the genes in common except for an occasional mutation. They provide an excellent opportunity to distinguish effect of environment and heredity on the expression of trait.

        Human genetic disorders the diseases which are transferred from generation to generation are called genetic disorders or genetically transmitted diseases in humans. These may be inherited or arise due to mutation.

      Genetic disorders are following types

        Mendelian disorders caused due to alternation or mutation in single gene

           It is of two types

        Sex-linked recessive disorder, e.g. colour blindness, haemophilia

        Autosomal recessive disorder, e.g. thalassemia, phenylketonuria, sickle-cell anaemia.

        Chromosomal disorders caused due to excess, the absence or abnormal arrangement of one or more chromosomes. It is of two types

        Autosomal abnormality or disorder, e.g. Down’s syndrome (trisomy at 21st chromosomes).

        Sex chromosome abnormality, e.g. Klinefelter’s syndrome (44 + XXY), Turner’s syndrome (44 + OX), etc.

        Genomics is the study of genomes through analysis, sequencing and mapping of genes and non-cistronic areas with the study of their functions. The Human Genome Project (HGP) was started in 1990 and ended in 2000.

        Eugenics is the branch of science which deals with improvement of human race genetically. It can be divided into two types, negative eugenics and positive eugenics.

        Euthenics is the improvement of human race by improving the environment conditions, i.e. by subjecting them to better nutrition, better unpolluted ecological conditions, better education and sufficient amount medical facilities.

        Euphenics is the symptomatic treatment of genetic disease of man. It deals with the control of several inherited human diseases, especially errors of metabolism with genetic engineering.

        Intelligence quotient (IQ) the ratio between actual (chronological) age and mental age multiplied with 100 is known as IQ. It is the mental competence in relation to chronological age in man.

        Genethics is a new field of genetics which combines genetics and ethics.

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