A wildlife centre is trying to increase the number of an endangered mountain frog. Fig. 1 shows the external fertilisation procedure used in a breeding tank...
Question 1 Report
A wildlife centre is trying to increase the number of an endangered mountain frog. Fig. 1 shows the external fertilisation procedure used in a breeding tank. Eggs from one female are placed in clean water and sperm from one male is added. The resulting zygotes divide repeatedly to form embryos. The centre has also collected eggs from several unrelated females and sperm from several unrelated males. Keepers want a breeding plan that produces healthy young frogs while maintaining genetic variation in the population. They release young frogs only after they can feed independently.
(a) What is the cell labelled X called before fertilisation? [1]
(b) Describe how a zygote forms in the breeding tank. [2]
(c) Explain why the embryos become larger by mitosis before becoming tadpoles. [2]
(d) Give two differences between external fertilisation in these frogs and internal fertilisation in humans. [2]
(e) Explain why using several unrelated adult frogs is better than using one male and one female repeatedly. [3]
(f) Describe two ways the keepers could improve the reliability of their breeding results. [2]
(g) Give one reason why releasing very young tadpoles may have a low success rate. [1]
Answer Details
(a) Cell X is an egg cell, also called an ovum. It is the female gamete, present before fertilisation. [1]
(b) A zygote forms in two linked stages:
- A sperm cell enters an egg cell. [1]
- The sperm nucleus fuses with the egg nucleus. This combines genetic material from the male and female gametes, producing one zygote cell. [1]
(c) Mitosis is cell division that produces genetically identical daughter cells. Repeated mitosis produces more cells. [1] This allows the embryo to grow and develop into a tadpole while the new cells retain the same genetic information as the original zygote. [1]
(d) Two valid differences are:
- In mountain frogs, the gametes meet outside the body, in water; in humans, the gametes meet inside the female reproductive system. [1]
- Frogs release eggs and sperm into water, whereas in humans sperm is transferred into the female during intercourse. [1]
(e) Using several unrelated males and females increases genetic variation among the young frogs. [1] It reduces inbreeding, so harmful recessive alleles are less likely to become common or be expressed in offspring. [1] Greater variation also means that, if disease occurs or environmental conditions change, some offspring are more likely to have characteristics that allow them to survive and reproduce. [1]
(f) Reliability means obtaining results that are consistent when the breeding process is repeated. Two suitable improvements are:
- Repeat breeding attempts, then compare the fertilisation and survival results. Repetition reduces the influence of chance. [1]
- Keep the water temperature similar in each breeding attempt, so temperature is controlled rather than causing differences in the results. [1]
Other acceptable controls include using equal or known numbers of eggs, and recording fertilisation and survival with a clear, consistent criterion.
(g) Very young tadpoles may have a low survival rate because they may be unable to feed independently. [1] They could also be less able to avoid predators or cope with unsuitable water conditions. Releasing them only once they can feed removes one important cause of early mortality.
Examination reminder: For explanations about genetic variation, link the variation to a survival advantage, such as some individuals surviving a disease or environmental change, rather than only stating that variation is increased.
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