EXPLANATION OF SECONDARY SEXUAL CHARACTERS
You have learnt in Chapter 9, that testes and ovaries are the reproductive organs. They produce the gametes, that is, sperms and ova. In girls, breasts begin to develop at puberty and boys begin to grow facial hair, that is, moustaches and beard. As these features help to distinguish the male from the female they are called secondary sexual characters. Boys also develop hair on their chest. In both, boys and girls, hair grows under the arms and in the region above the thighs or the pubic region.
The changes which occur at adolescence are controlled by hormones. Hormones are chemical substances. These are secretions from endocrine glands, or endocrine system. The male hormone or testosterone begins to be released by the testes at the onset of puberty. This causes changes in boys about which you have just learnt, for example, the growth of facial hair. Once puberty is reached in girls, ovaries begin to produce the female hormone or estrogen which makes the breasts develop. Milk secreting glands or mammary glands develop inside the breasts. The production of these hormones is under the control of another hormone secreted from an endocrine gland called pituitary gland.
Source: This topic is taken from NCERT TEXTBOOK