Sexual reproduction and variation

• Most organisms use sexual reproduction as a means of preventing extinction by keeping up their numbers
• Sexual reproduction usually involves two separate parent organisms, the male parent and the female parent
• Parent organisms produce gametes which are special sex cells for reproduction
• Animal gametes - male sperm cell

  • Sperm is produced in the testes
  • sperm are very small
  • they have a tail so that they can swim to the egg
• animal gametes - female egg cell

  • in animals, egg cells are produced in the ovaries
  • egg cells are very large as they contain nutrients should the be fertilised
• when the male and female gametes fuse, they become a zygote containing the full 46 chromosomes
• Fertilisation

  • Fertilistaion occurs when the nucleus of a sperm cell fuses with the nucleus of an egg cell
  • the cell formed from this is called a Zygote
• there are 2 types of fertilisation

  1. internal
  2. external
• internal fertilisation is when it happens inside the females body as in mammals like humans
• fertilisation in mammals

  • during fertilisation the nucleus of the sperm cell joins with the nucleus of the egg cell.
  • the process results in a fertilised egg which has a combination of genetic information from the male and female parent
  • the fertilised egg cell divides time and time again as it develops into and embryo inside the uterus

  the cell in the embryo divides and grows into an offspring
• external fertilisation

  • occurs outside the body
  • in fish and amphibians, the eggs and sperm are released into the surrounding water
  • there is a high risk of the egg and sperm being swept away and so large numbers of eggs and sperm are released to make fertilisation more likely
• Pollen - male gamete

  pollen grains are produced in the anthers
• ovules - female gametes

  the ovule is produced in the plants ovary
• Pollination


  The transfer of pollen grains from an anther to a stigma. Pollen can be transferred by an animal or by the wind.
• fertilisation in plants

  takes place inside the ovary when the nucleus of pollen grain fuses with the nucleus of the ovule to produce a zygote
• Sexual reproduction ensures variation between individuals in the population
• variation allows species to adapt to environmental changes
• disadvantages to sexual reproduction

  Often slow and complicated and relies on parents locating each other and mating successfully