All the methods of reproduction you read in the previous section are asexual methods of reproduction.
In human beings and several other animals the mode of reproduction is sexual.
During sexual reproduction, DNA from two seperate individuals - the male parent and the female parent, combines to create a new human being.
Recall, that DNA is stored in chromosomes of the cell.
In human beings, each cell contains 23 pairs of chromosomes. Such a cell is known as a diploid cell.
One of the 23 pairs is called the sex chromosome and is different in females and males.
The female sex chromosome is XX, while the male sex chromosome is XY.
The way in which gamete cells divide, makes sure that the new cell will have only half the number of chromosomes as the mother cell.
Recall the process of meiosis from grade 9.
Most cells in your body divide through mitosis. However gamete cells divide through meiosis!
During mitosis division, the mother cell divides into two indentical cells. The new cells will have the same number of chromosomes as the mother cell.
On the other hand, when a gamete cell divides, it produces four new cells, not two. Each of these four new cells will have only half the number of chromosomes as the mother cell.
During the process of fertilisation when the male and the female gamete cell combine, they both have 23 chromosomes each and together they form 23 pairs of chromosomes.
If the X chromosome from the female and the X chromosome from the male combine, then the offspring will also be a female.
If on the other hand the X chromosome from the female and the Y chromosome from the male combine, then the offspring will be a male.
Let's have a quick look at the reproductive organs in males and females which produce gametes.
The male reproductive system consists of parts which produces sperm(male gamete) and deliver it to the site of fertilisation.
Sperm are formed in the testes.
The sperms formed in the testes are then carried via the vas deferens to the urethra.
The penis is the external male organ - which is specialized for delievery of sperm during copulation.
The female reproductive organ consists of the oviducts, the ovary, the uterus, the cervix and the vagina.
The ovaries produce one mature egg (female gamete) every month.
The egg is then carried from the ovary to the womb through the oviduct.
There are two oviducts which unite at the uterus.
If the male gamete and the female gamete fertilise then the development of the zygote into an infant happens in the uterus.
To support the development of a child, the uterus prepares itself every month by thickening the inner lining of the uterus.
If fertilisation does not occur then the uterine lining sheds over a period of 5 days.
The uterine lining exits the female body through the vagina as blood and mucous.
This cycle is known as menstruation.
If the male gamete and the female gamete do fertilise, then a zygote is formed.
Once a zygote is formed, it divides repeatedly by mitosis to form a pair of cells.
Once a 64 cell ball has been created, it enters the uterus and gets embedded in the wall of the uterus.
It then takes approximately 268 days or 9 months for a fully formed baby to be born.