- Sometimes, a single fertilized egg may get split and separated into two parts during the early stage of cell division0. Each part then develops into a separate individual. As these two individuals are produced from a single zygote, they look similar and are called identical twins.(either both boys or both girls)
- Sometimes two eggs are released from ovaries simultaneously and get fertilized to produced from different zygotes, they are dissimilar. (either both boys or both girls or one boy and one girl)
- Why do some animals reproduce many young ones and some only one?
All animals want to make sure that at least some of their young reach adulthood and reproduce themselves.
Some animals do this by looking after a few young ones until they can fend for themselves.
Other animals produce hundreds of eggs which they leave to develop by themselves. Most of them will die, but a few will survive.
*The population of the world reached:
- 1billion in 1804
- 2 billion in 1927
- 3 billion in 1960
- 4 billion in 1974
- 5 billion in 1987
- 6 billion in 1999
- 7 billion in 2011
|1||China||1,339,290,000||19 August 2010||Chinese official population clock|
|2||India||1,184,890,000||19 August 2010||Indian official population clock|
With the present trend of population growth, India is likely to overtake China and be the World's top most in population by the year 2025.
July 11 is observed as World Population Day.
12.3.3 Needs for and methods of family planning:
There is undoubtedly a strong relation between high national fertility rate and measures of poverty. As the population density increases, decrease in per capita income and natural resources takes place. General health also goes down. It creates an economical burden on the nation.
Large families affected both the individual as well as the community life. Economic pressure, mother's poor health, children neglected at home, poor housing, malnutrition, insufficient medical care, lack of better education are some of the disadvantages of large family size.
Limited family is advantageous to the individuals as well as to the nation and ultimately to the entire human race. For better future of mankind, the rate of growth of human population must be reduced.
Another problem related to human reproduction is the spread of sexually transmitted diseases (STDs). STDs spread from an infected person to a healthy person by unprotected sexual contact. Syphilis, gonorrhea, AIDS are some common sexually transmitted diseases which affect the sexual health of human being.
To protect individuals from these STDs, sex education and preventive measures are necessary. Use of condom not only protects the individual from STDs but also helps in population control.
Contraception (to avoid pregnancy) and to keep sexual health can be achieved by the use of condoms, oral pills, copper-T and some other methods.
The WHO has prepared guidelines and considered reproductive health as a fundamental human right.
World Population Estimates*
|Population (in billions#)||1||2||3||4||5||6||7||8||9|
|Years elapsed between milestones||––||123||33||14||13||12||13||15||19|
|* Estimated by United States Census Board||# 1 Billion = 100 Crore|