There are many common characters and disorders which do not follow simple Mendelian (single gene) inheritance. Common traits like intelligence, blood pressure, height, weight, hair colour, eye color and facial appearance have more complex genetic basis. If height were to be determined by a pair of genes (as is the case of Mendelian inheritance) then this would result in only two types of persons i.e. tall and short. (If we represent tallness as ‘T’ and shortness ‘t’ then tall individual will be ‘TT‘ or ‘Tt’ and short ones will be ‘tt’). However, in each family we get individuals whose height shows quantitative variation from one extreme to other. All the aforementioned traits cannot be distinctly classified into two groups but are measured quantitatively and therefore are called as continuous or quantitative traits.
Similarly, the following disorders do not follow Mendelian (single gene) inheritance. However these disorders cannot be measured as height or blood pressure. These disorders are either present or absent. These are called threshold traits. Threshold traits are present or absent e.g. a person is diabetic or nondiabetic. Following are examples of threshold traits in man.
| Congenital malformation | Adult onset disease |
| Neural tube defects | Diabetes mellitus |
| Pyloric stenosis | Epilepsy |
| Cleft lip | Hypertension |
| Cleft palate | Ischemic heart disease |
| Heart defects | Schizophrenia |
| Glaucoma |
The above mentioned threshold traits are considered to be determined by actions of many genes which are situated at different loci on chromosomes, each of which exerts an equal additive effect. This kind of inheritance is called polygenic inheritance. Thus in polygenic inheritance the genes do not behave as dominant or recessive but have an additive or cumulative effect on the trait. It is also believed that these common physical traits, disorders or congenital malformations are not entirely determined by the action of many genes but are to be resulted from interaction of environmental and genetic factors. Many environmental factors like diet (in case of weight), sunlight (in case of skin colour), disease, chemicals and radiation, among others, may influence the action of genes. Thus polygenic inheritance is also called as multi-factorial inheritance.
Polygenic inheritance is controlled by two or more pairs of nonallelic genes. In which the dominant alleles have a cumulative effect. Each dominant gene added will add to the character and the result will be an additive effect of all dominant genes. These genes are Polygenes and such type of inheritance is Polygenic inheritance. It is also referred as a Quantitative inheritance & Multiple genes interaction.
These genes don’t follow the Mendelian pattern of inheritance. These characters are affected by environment. Continuous variation occurs. The offsprings can be divided into a range of classes.
SKIN COLOR IN MAN:
- Studied by Davenport in Jamica.
- 3 pairs of nonallelic genes are responsible for the character.
AABBCC x aabbcc
(Negroes) (Whites)
↓
AaBbCc → F1 Generation
(Mulattoes)
F1 generation -> AaBbCc x F1 AaBbCc
The offsprings in F2 generation can be divided into a range of classes according to the number of dominant genes present.
