Sexual reproduction

Sexual reproduction is how most animals and plants reproduce. Some protists and fungi also reproduce this way. Organisms that reproduce sexually have two different sexes: male and female.

In sexual reproduction, offspring are produced when sperms fertilise eggs from the female. Various steps are involved in this process.

Cell biology 
The cells of an animal or higher plant have two sets of chromosomes: they are diploid. When gametes (sex cells) are produced, they have only one set of chromosomes: they are haploid. They have undergone a process of cell division called "reduction division" or meiosis. Two things happen during meiosis, each of which makes the offspring more variable. That means they are different from their parents and from each other.

Assortment 
Assortment is when the double set of chromosomes becomes a single set in each gamete. Of each pair of chromosomes, which one goes into a single gamete is random. Because the gene alleles on each chromosome are not always the same, this means that there is genetic variation between gametes. This process was Mendel's 'first law', the law of segregation.

Crossing over 
Because crossing over occurs during meiosis, this increases the variety of the chromosomes. This makes recombination possible. 

Assortment and crossing over make it certain that normally no two offspring of the same mother and father are identical. Identical twins are the exception. They are identical genetically because they developed from the same fertilised egg.

Advantages and disadvantages 
There are advantages and disadvantages of sexual reproduction, compared to asexual reproduction. The main issues are:
Advantages: More variation assists with survival. It increases the chance that at least some offspring of a parent survive. To give an example, suppose a deadly infection occurs in the population. Greater variety increases the chance that some of the population will survive.
Disadvantages: Requires two parents. So, supposing the total number of eggs laid by a female is the same, a population reproducing sexually would produce only half as many offspring as a population reproducing asexually. The actual search for a partner, and mating, often takes huge chunks of time out of the lifetime of sexual animals.

Various terms 
Gonads are specialized sex organs where gametes are formed. In the male, the gonad is the testes; in the female, the gonad is the ovaries.

Gametes are specialized sex cells formed in gonads by gametogenesis. The male gamete is the sperm, and the female is ovum.
Spermatogenesis: the process which produces haploid sperm.
Oogenesis: the process which produces haploid egg cells.

Hermaphrodite: an organism with male and female sex organs. Examples: earthworms, snails, hydra.

Fertilization: sperm penetrates the cell membrane of ovum. What now exists is a single cell called a zygote. 
Internal fertilization: sperm are deposited in or near the female reproductive tract. 
External fertilization: eggs are shed by the female and are fertilised by sperm in the external environment.

Development 
Cleavage: early stage of embryo development. Cell number increases by cell division.
1. Morula: a solid ball of cells
2. Blastula: a hollow ball of cells filled with fluid 
3. Gastrulation: blastula continues to grow, and cells multiply by mitosis. Several hundred cells on one side begin to move in and form a two-layered embryo. It develops into a three-layered embryo with endoderm, mesoderm, and ectoderm.

Ectoderm develops into the nervous system, which includes the brain, spinal cord, and peripheral nerves. Also, the linings of various organs: mouth, anus, nostrils, epidermis including sweat glands, hair, nails.

Mesoderm develops into bones, muscles, reproductive system, kidneys, blood, blood vessels, inner layer of skin.

Endoderm develops into the linings of the digestive system, respiratory system, liver, pancreas, and bladder.

Growth and differentiation: embryonic cells multiply and change into specialized cells in tissues and organs.

Origin of sexual reproduction 
The origin of sexual reproduction is an advanced topic that cannot be dealt with here. A source that may help readers who are graduates in biological science may be helpful.

Related pages 
 Asexual reproduction
 The evolution of sex
 Gender
 Sex
 Sexual intercourse

References 

Methods of reproduction