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  Section: Genetics » Human Genetics
 
 
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Genetic analysis through pedigree charts

 
     
 
Content
Human Genetics
Human chromosomes
Determination of sex
Sex linked inheritance
Chromosomal aberrations
Dizygotic and monozygotic twins
Inborn errors in metabolism
Sickle-cell anaemia
Genetic analysis through pedigree charts
Chromosome mapping in humans (including RFLPs, etc.)
Gene transfer in mammalian cells
Chromosome mediated gene transfer
Transformation of cells with free DNA
Use of human genetics in medical science 
Genetic counseling
Amniocentesis and antenatal diagnosis
Gene therapy
Making a choice of baby's sex
DNA fingerprinting in forensic science
Although conventional genetic analysis in human beings is restricted due to small family size and the difficulty of experimental matings, pedigrees provided useful means of genetic analysis, particularly for single gene defects. Some examples of these single gene disorders studied from pedigrees are mentioned above, e.g. alcaptonuria, phenylketonuria, albinism and sickle cell anaemia. The sex linked traits like red green colour blindness and haemophilia earlier described in Sex Linked, Sex Influenced and Sex Limited Traits, are other examples. Two other important traits in humans include (i) dominant gene for ability to roll one's tongue (found in 85% people) and (ii) dominant gene for ability to taste the bitterness of phenylthiocarbamide (PTC). Distribution of the trait tasters vs. non-tasters for PTC is discussed in Genetics of Cancer : Proto-oncogenes, Oncogenes and Tumour Suppressor Genes. Identification of genes in human beings is also difficult partly due to difficulty in producing induced mutations, although such induced mutations in cell cultures can be produced. Single gene mutations can be autosomal or sex linked, and largely recessive. In all such cases, the data from pedigrees can be pooled and 3 : 1 ratio in the progeny of two heterozygotes and 1 : 1 ratio in the progeny of a heterozygote and a homozygous recessive, can be demonstrated, although the ratio may be distorted in favour of dominant phenotype due to miscarriages caused by deleterious effect in homozygous recessive condition. Some of the single gene disorders identified from pedigrees are listed in Table 24.3. Many others are known which are rare.

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Although, most of the single gene disorders are recessive as mentioned above, but rarely one may be dominant, if its deleterious effect is expressed later after reproduction. A notorious example is Huntington chorea which is characterized by progressive mental deterioration, making its appearance usually at about 40 years of age.

 
     
 
 
     




     
 
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