12th Class

Sex chromosomes of some animals and man besides having genes for sex character also possess gene for non sexual (somatic) characters. These genes for non sexual characters being linked with sex chromosomes are carried with them from one generation to the other. Such non-sexual (somatic) characters linked with sex chromosomes are called sex linked characters or traits, genes for such characters are called sex linked genes and the inheritance of such characters is called sex linked inheritance. The concept of sex-linked inheritance was introduced by T. H. Morgan in 1910, while working on Drosophila melanogaster. Genes for sex linked characters occur in both segments of X and Y chromosomes. Many sex linked characters (About 120) are found in man. Such characters are mostly recessive. Types of sex linked inheritance (1) Diandric sex linked or X linked traits : Genes for these characters are located on non-homologous segment of X chromosome. more...

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 more...