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
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.
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.