Fig. 1 shows a veterinarian using embryo transfer to increase the number of calves from a dairy cow with a high milk yield. Eggs are fertilised using sperm ...
Question 1 Report
Fig. 1 shows a veterinarian using embryo transfer to increase the number of calves from a dairy cow with a high milk yield. Eggs are fertilised using sperm from a selected bull. At an early stage, embryos are placed into surrogate cows. A small sample of cells can be removed from each embryo and tested for a gene linked with a harmful inherited condition.
Table 1 gives the results for 40 embryos made in one breeding programme.
| Result | Number of embryos |
|---|---|
| Contain harmful gene | 11 |
| Do not contain harmful gene | 29 |
| Healthy calves born after transfer | 18 |
(a) What is an embryo? [2]
(b) Describe how the test result could be used before embryos are transferred. [2]
(c) Calculate the percentage of embryos that did not contain the harmful gene. Show your working. [2]
(d) Give two advantages of embryo transfer to the dairy farmer. [2]
(e) Explain why only 18 healthy calves were born even though 29 embryos did not contain the harmful gene. [2]
Answer Details
(a) An embryo is an early stage in an organism’s development. It is formed when a fertilised egg cell, or zygote, divides repeatedly by mitosis. [2]
(b) Test each embryo sample for the harmful gene, then select embryos that do not contain it for transfer to surrogate cows. Embryos containing the gene are not transferred. This reduces the chance of calves inheriting the condition. [2]
(c) Percentage without the harmful gene:
\[\frac{29}{40}\times100=72.5\%\]
Therefore, 72.5% of the embryos did not contain the harmful gene. [2]
(d) Two valid advantages are: more offspring can be produced from a cow with desirable features such as high milk yield; and embryos without the harmful gene can be selected, reducing inherited disease. It also allows several surrogate cows to carry valuable embryos, helping improve milk yield across the herd. [2]
(e) Being free of the harmful gene does not guarantee birth. Some embryos may fail to implant in the uterus, and some embryos or fetuses may die during development or the pregnancy may fail. [2]
Download The App On Google Playstore
Everything you need to excel in your exams
