12th Class Biology Principle Of Inheritance And Variation Twins and I.Q.

Twins : Two birth occurring at the same time in human are called twins, they are of peculiar genetic interest. The hereditary basis of a number of human traits has been established by the study of twins. There are 3 kinds of twins.

(1) Identical or monozygotic twins : Identical twins are formed when one sperm fertilizes one egg to form a single zygote. They have the same genotype and phenotype and are of same sex. Differences if any, may be due to different environmental conditions.

(2) Siamese twins or conjoint twins : Like monozygotic twins, siamese twins also originate from one zygote but the daughter cells formed as a result of first cleavage  fail to separate completely and they remain joined at some point. They were first studied in the country Siam, hence called Siamese twins. Siamese twins usually do not survive after birth although a few cases of their survival are well known. They are always of the same sex, same genotype and phenotype.

(3) Fraternal twins : They are dizygotic twins formed from the two eggs fertilized by two sperms separately but at the same time. They may be both males, both females or one male and one female. They may have different genotypic constitution and different phenotype.

Intelligence quotient (IQ) : The ratio between actual (chronological) age and mental age multiplied with 100 is known as I.Q. Intelligence quotient is the mental competence in relation to chronological age in man. It can be denoted by following formula.

\[I.Q.=\frac{\text{Mental age}}{\text{Actual age}}\times 100\]

By applying this formula we can easily calculate the IQ, such as if a 10 year child has mental age 14, his IQ will be

\[I.Q.=\frac{14}{10}\times 100=140\]