Human Hereditary Traits

Inheritance in man conforms to the same genetic principles, which govern inheritance in other animals and in plants. About 200 traits are transmitted from generation to generation in humans. These are called hereditary traits. These include dominant as well as recessive traits. Many human traits are transmitted in a simple Mendelian manner, while many others follow a non-Mendelian pattern shown when there is co-dominance, polygenes and sex-linked genes.

Human Hereditary Characters

	Characters	Dominant	Recessive
1.	Hair	Dark, Curly	Light, Straight
2.	Skin colour	Normal	Albinism
3.	Eyes	Brown, Large	Blue, Small
4.	Eyelashes	Long	Short
5.	Colour Vision	Normal	Colour blindness
6.	Lips	Thick	Thin
7.	Nose	Big	Small
8.	Nostrils	Wide	Narrow
9.	Hearing & Speech	Normal	Deaf-mutism
10.	Stature	Short	Tall
11.	Blood Clotting	Normal	Haemophilia
12.	Blood groups	A, B, AB	O
13.	Rh factor	Positive	Negative
14.	Ear	Long, Wide, Free	Short, Narrow, Fused
15.	Cheeks	Dimpled	Normal
16.	Tongue Rolling	Ability to roll	Inability to roll
17.	PTC* Tasting	Ability to taste	Inability to taste

* Phenyl-thio-carbamide

Modern Approach

Five basic approaches are followed in the recent days to study human genetics.

Pedigree Approach

Proper pedigree records of families are maintained. This helps in tracing the transmission of a particular trait through generations.

fig. 20.1 - Pedigree Approach

fig. 20.2 - Probable Genotypes of the Family Shown Above

Population Genetics

The problem faced by geneticists on account of small number of children in human family has been overcome by methods devised to analyse the fate of characters in populations. Population genetics, a relatively new branch in biology, is very helpful in the study of human genetics. It involves the study of distribution of traits in any given human population.

Human Cell Culture

The techniques of cultivating human cells in specific media and the techniques used in somatic cell genetics have enabled geneticists to understand the biochemical basis of the inheritance of many characters. Division of the human cells in the cell culture is arrested at the metaphase stage with the help of colchicine. Such cells, when fixed and stained, clearly show the number, form and structure of the chromosomes. Even the position of the centromere can be easily ascertained by this technique.

Human Cytology

The exact number of somatic chromosomes in man was not known till 1956. New techniques of staining chromosomes have revealed the exact number and this have greatly helped the study of human genetics. Using the karyotyping techniques the form, shape and size of all the 23 pairs of chromosomes have been identified.

Study of Twins

The monozygotic (identical twins) develop from a single zygote by separation of the two blastomeres after the first cleavage. Therefore, they have similar genetic traits, except for rare chances of mutation. Fraternal twins develop from different zygotes. Therefore, they are no more alike than other children born at different times in the same family. Since the identical twins carry the same genotype, study of monozygotic twin helps in distinguishing between hereditary and environmental characters. The traits that are similar in the identical twins are considered hereditary, and the features in which they differ are regarded environmental. Such studies have shown that intelligence, feeble-mindedness and many other conditions such, as diabetes mellitus, are hereditary traits in man.

In the recent years, a large number of human disorders, deformities and diseases identified have been found to be hereditary. Some of these are caused by gene mutations, others by gross chromosomal abnormalities. Human genetics has progressed so much that cloning the human beings and tailoring the phenotypes in a desired manner by genetic engineering seem possible in the near future.