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Sexual reproduction

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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. Different steps are involved in the 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 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. In 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 it possible to get recombination.

The consequence of assortment and crossing over makes it certain that no two offspring of the same mother and father are identical, except for identical twins. 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:

  1. 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.
  2. 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. In the male, the gonad is the testes, while in the female, the gonad is the ovaries.

Gametes are specialized sex cells formed in gonads by gametogenesis.

Oogenesis process which produces haploid egg cells. Ovum: female sex cell, largest cell in body, created from birth, fixed number able to mature. Examples: earthworms, snails, hydra. The structure that now exists (single cell) is called a zygote.

Cleavage: early stage of embryo development. Cell number increases by cell division.

1. Morula: Solid ball of cells

2. Blastula: hollow ball of cells filled with fluid (blastocoel)

3. Gastrulation: Blastula continues to grow, cells continue reproducing using mitosis. Several hundred cells on 1 side begin to move in and form a 2 layered embryo. Developed into 3 layered embryo which is endoderm, mesoderm, and ectoderm.

Ectoderm develops into nervous system, which includes the brain, spinal cord, and peripheral nerves. Also, the lining of mouth, anus, nostrils, epidermis, 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.

Growth and Differentiation differentiation: changing of unspecialized embryonic cells into specializd cells to tissue and organs.

Why do cells differentiate? Parts of DNA turned on/off determines the role of a cell.