Pleiotropy

Pleiotropy

Ordinarily, we know that each gene controls a single phenotypic trait, but it is not always true. It has now come to be known that a single gene may control two or more characters at a time. This phenomenon is called pleiotropy or pleiotropism and such genes are called pleiotropic genes.

Pleiotropy is also defined as the ability of a gene to have many effects. Pleiotropic genes may not have equal influence on all the traits they control. A pleiotropic gene may cause a very evident expression of its specific traits (major effect) and a less evident expression of its other traits (secondary effect).

In garden pea, the same gene controls flower and seed coat colour, and red spots in axils of leaves. In cotton, a gene for lint also affects plant size, boll size and seed number.

In Drosophila, the gene for wing size also affects the nature of balancers, eye colour, fertility, longevity, etc.

HbS

Another example of pleiotropy is a human disease called sickle-cell anaemia. It is caused by a recessive gene, HbS.

The gene, which causes this disease alters the type of haemoglobin and also changes the form of red corpuscles. The gene (HbS) causes the production of abnormal haemoglobin due to which the shape of RBC becomes distorted and sickle shaped.

• Homozygous individuals normally die early in life due to severe anaemia caused by premature destruction of sickled red cells but heterozygous individuals having both normal and abnormal hemoglobin and also having mild anaemia, are naturally protected against malaria as the parasites cannot live in these distorted cells.
• Heterozygotes may, therefore, survive better in regions where malaria is endemic. Such populations have both normal individuals (HbA HbA) and individuals heterozygous to the gene (HbA HbS). The latter acts as carrier.

QUANTITATIVE (POLYGENIC) INHERITANCE

Sometimes a single character is controlled by more than one genes, each having two allelic forms. Such traits are called polygenic traits. The effect of these genes on a particular trait is cumulative or quantitative. Thus, one gene will produce some effect, two genes will produce greater effect and the combined effect of all concerned genes for that trait will be greatest.

The genes involved in quantitative inheritance are called polygenes. Quantitative inheritance is, therefore, also called polygenic inheritance or multiple factor inheritance. Quantitative or polygenic inheritance was first studied by J. Kolreuter (1760) in case of tobacco, F. Galton (1883) in case of human beings and E. M. East (1916) in case of corolla length in Nicotiana longiflora. Nilsson-Ehle (1908) obtained the first experimental proof of polygenic inheritance in case of kernel colour in wheat.

Polygenic inheritance can be known from the frequency distribution of phenotypes. In monogenic or qualitative inheritance the phenotypes are two (3 : 1) in case of single gene pair and 4 (9 : 3 : 3 : 1) in case of two pairs of genes. In polygenic or quantitative inheritance the number of phenotypes is 3 (1 : 2 : 1) in case of one polygene pair, 5 (1: 4 : 6 : 4 : 1) in case of two polygene pairs and 7(1: 6 : 15 : 20 : 15 : 6: 1) when three polygene pairs are involved. Thus, we see that the number of intermediate types increases with the increase in the number of polygenes but the number of parental types remains the same (2 in the above cases).

The possible origin of polygenes is

• Duplication of chromosome part
• Polyploidy or increase in chromosome number
• Mutations producing genes having similar effect

The most interesting example of polygenic inheritance is the skin colour in man (studied by C.B. Davenport, 1913). Human skin colour is controlled by three separate genes, A, B and C. Each gene contributes to a unit of darkness. The black or very dark colour (Zet black/Negroes) is due to AABBCC and caucassian/very light/white is due to aabbcc.

A marriage between the two yields intermediate or mulatto (AaBbCc) offspring. When such heterozygous individuals intermarry among themselves, 8 possible gametic combinations and 64 zygotic combinations (27 genotypes) will be expected (7 phenotypic combination). These individuals are expected to show all the shades of skin colour depending upon the fact whether they have only A, AB or ABC.

PEDIGREE ANALYSIS

Pedigree is a chart showing the record of inheritance of certain genetic traits for two or more ancestral generations of individual or domesticated animals.

Females are shown by circles and males by squares. Symbols of parents are joined by a small horizontal line called marriage line. Symbols of offsprings are shown below in another horizontal line, called sibling line and is connected to marriage line by a vertical line. Marriages of the offsprings are shown like that of the parents.

Trait under study is shown by solid symbol and the recessive alleles are depicted by various types of shades. The offspring have numbered with Arabic numerals (1, 2, 3), and a generation is numbered with Roman numerals (I, II, III).

Pedigree analysis is a system to analyse the distribution and movement of traits in the family tree. It helps genetic counsellors to advice couples worried of the possibility of having genetically defective offspring when it is known that such a defect runsin their family.

A member of a family having an exceptional phenotype (e.g. colourblind or dwarf or deaf) which first attracts the attention of geneticist, is called propositus.

The history of the exceptional character in the propositus is traced back in the family and a family tree is prepared using standard symbol.

Fig. : A pedigree showing the inheritance of polydactyly in family