HUMAN GENETICS
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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.
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Human karyotype is
the chromosomal complement of an organism made by the arrangement of metaphase
chromosome pairs in a standard sequence.
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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.
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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.
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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.
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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.
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Dosage compensation is a term that describes the process by which organisms equalise the
expression of genes between members of different biological sexes.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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Mendelian disorders caused due to alternation or mutation in single gene
It is of two types
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Sex-linked recessive disorder, e.g. colour blindness, haemophilia
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Autosomal recessive disorder, e.g. thalassemia, phenylketonuria, sickle-cell anaemia.
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Chromosomal disorders caused due to excess, the absence or abnormal arrangement of one or more
chromosomes. It is of two types
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Autosomal abnormality or disorder, e.g. Down’s syndrome (trisomy at 21st
chromosomes).
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Sex chromosome abnormality, e.g. Klinefelter’s syndrome (44 + XXY), Turner’s syndrome
(44 + OX), etc.
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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.
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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.
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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.
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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.
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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.
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Genethics is a
new field of genetics which combines genetics and ethics.