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 meiosis. Two things happen during meiosis, each of which makes the offspring more variable. That means they are different from the parents and from each other.
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.
The consequence of assortment and crossing over makes it certain that 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 disadvantagesEdit
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 to be the same, a population reproducing sexually would produce only half as many offspring as a population reproducing asexually.
- 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.
- Fertilization: sperm penetrates the cell membrane of ovum. What now exists is a single cell called a zygote.
- 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, 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 lining of digestive system, respiratory system, liver, pancreas, and bladder.