CIE A Level Biology复习笔记18.3.4 Assisted Reproduction

Assisted Reproduction

  • Endangered mammals tend to have small and isolated populations
  • Small populations are prone to inbreeding and inbreeding depression
    • Inbreeding depression is the reduction in fitness of a population due to breeding between closely related individuals and the resulting increased homozygosity
  • When populations are isolated it can also be difficult for individuals to find suitable reproductive mates
  • Previously large mammals were transported between zoos in captive breeding programs
    • Advantage: Humans were able to monitor the health of the mother and foetus
    • Disadvantage: It was highly expensive and unreliable as sometimes individuals would refuse to mate
  • Science has come up with several solutions for inbreeding and the lack of reproductive mates in endangered mammals


  • In vitro fertilization involves the fertilization of an egg outside of the female body
    • For example in a test tube or petri dish
  • Method:
    • A needle is inserted into the female’s ovaries and eggs are extracted
    • The eggs are kept in a culture medium for a short amount of time
    • Male semen is mixed with the eggs so fertilization can occur
    • Several zygotes form and develop into embryos
    • The embryos are placed in a culture for several days
    • The embryos are transferred either into the mother, or another female
  • IVF allows is advantageous over natural mating as it allows humans to control and confirm fertilization of the embryo

Embryo transfer

  • Pregnancies are high risk for females; complications can arise which in some cases prove fatal
  • Since the population numbers of an endangered species are already very low each reproductive female is of very high value and importance
  • Embryo transfer can be used to avoid the risks of pregnancy for the vulnerable female so that she can provide many eggs for multiple offspring
  • Method:
    • An egg belonging to a female of the vulnerable species is fertilized by the sperm belonging to a male of the same species
    • A zygote forms which develops into an embryo
    • After fertilization, the embryo is taken from the uterus of the female and transferred to a surrogate female
    • The embryo develops to full term and the offspring is born
  • The surrogate mother can be from another non-vulnerable species
  • This technique has been used to try and conserve populations of several different species of African antelope


  • A surrogate is any female that becomes pregnant with the embryo from another female and carries the embryo to full term
  • Surrogate mothers require hormone treatment before they receive an embryo
    • The hormones ensure that her uterus is in the right condition for the embryo to embed
  • There are multiple ways in which the embryo might have been conceived:
    • Naturally
    • Artificial insemination (semen from the male is injected into the uterus of the female)
    • IVF
  • A surrogate female can be the same or different species to the biological mother of the embryo
    • If it is a different species it needs to be closely related to ensure compatibility of the embryo and uterus

Exam Tip

Sometimes eggs and sperm are frozen so that they can be used at a later date. Egg cells have a high water content so the internal membranes of eggs can be damaged by the freezing and thawing process.