Selective Breeding, Genetic Engineering & Cloning

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Selective Breeding (Artificial Selection)

The process by which humans breed plants and animals for particular genetic characteristics (e.g., disease resistance in food crops, gentle nature in domestic dogs, cows that produce high milk yields, or large unusual flowers) by selecting and breeding desirable parents over many generations.

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Inbreeding & Reduced Gene Pool

Selective breeding leads to inbreeding, which reduces the number of different alleles in a population (smaller gene pool). Consequences: higher probability of inheriting harmful recessive genetic disorders (e.g., breathing issues in pugs, hip dysplasia in Labradors) and vulnerability of an entire crop/herd to a single new pathogen.

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Genetic Engineering Process (Higher Tier)

Modifying an organism's genome by introducing a gene from another species: 1. Isolate and cut out the desired gene using restriction enzymes. 2. Cut open a vector (bacterial plasmid or virus) using the same restriction enzyme. 3. Insert the gene into the vector using ligase enzymes. 4. Transfer the vector into target cells at an early stage of development (meristem/embryo/bacteria) so they develop with the desired characteristic.

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Genetically Modified (GM) Crops & Bacteria

GM bacteria have been engineered with the human insulin gene to produce pure human insulin at scale for Type 1 diabetics. GM crops (e.g., Bt corn or Golden Rice) are modified to be resistant to insect pests or herbicides, or to have higher nutritional value, typically increasing crop yields.

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Plant Cloning: Cuttings & Tissue Culture (Triple)

Cuttings: An older, simple method used by gardeners where a branch is cut from a parent plant, dipped in rooting powder (auxin), and planted in damp compost. Tissue culture: Small groups of cells (explants) from a plant meristem are grown on sterile nutrient agar with hormones to produce thousands of identical cloned plants rapidly, preserving rare species cleanly.

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Animal Cloning: Embryo Transplants & Adult Cell Cloning (Triple)

Embryo transplants: Cells from a developing cattle embryo are split apart before specialisation and transplanted into host surrogate mothers. Adult cell cloning: 1. Remove nucleus from an unfertilised egg cell. 2. Insert the diploid nucleus from an adult body cell (e.g., skin cell) into the empty egg cell. 3. Stimulate the cell to divide with an electric shock. 4. Implant the resulting embryo into the uterus of a surrogate mother.

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1. Q1

   A dairy farmer wants to breed a herd of cows that produce a very high volume of milk. Describe how the farmer would use selective breeding to achieve this, and explain one disadvantage of this method. [5 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. From the existing herd, select a female cow with the highest milk yield and a bull whose mother/female relatives had a very high milk yield [1 mark]. 2. Breed this cow and bull together [1 mark]. 3. Measure the milk yield of the female offspring when mature, and select the best-producing female offspring to breed with a high-yield bull [1 mark]. 4. Repeat this selection and breeding process over many generations until all cows in the herd produce a high milk yield [1 mark]. 5. Disadvantage: It causes inbreeding, which reduces the gene pool (genetic variation), increasing the risk of cows inheriting harmful recessive disorders or the entire herd being wiped out by a new disease [1 mark].

2. Q2

   Describe how human insulin can be produced on a large scale by genetically engineering bacteria. [5 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Use a restriction enzyme to isolate and cut out the human insulin gene from a human chromosome [1 mark]. 2. Remove a circular plasmid DNA vector from a bacterium and cut it open using the same restriction enzyme [1 mark]. 3. Insert the human insulin gene into the bacterial plasmid and join them using DNA ligase enzyme (forming a recombinant plasmid) [1 mark]. 4. Insert the recombinant plasmid vector back into host bacteria [1 mark]. 5. Grow the transgenic bacteria in an industrial fermenter with nutrients at optimum temperature and pH; as they divide rapidly by binary fission, they express the gene and synthesise human insulin, which is purified [1 mark].

3. Q3

   Describe how scientists cloned 'Dolly the sheep' using adult cell cloning. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. An unfertilised egg cell was taken from a female sheep (Sheep A) and its haploid nucleus was removed and discarded (enucleated) [1 mark]. 2. A diploid nucleus was removed from an adult body cell (an udder cell) of the sheep to be cloned (Sheep B) and inserted into the empty egg cell [1 mark]. 3. A mild electric shock was applied to fuse the nucleus with the egg cell and stimulate it to divide by mitosis into a ball of cells (an embryo) [1 mark]. 4. Once developed, the embryo was implanted into the uterus of a surrogate mother sheep (Sheep C), who gave birth to Dolly (a genetic clone of Sheep B) [1 mark].

4. Q4

   Compare the methods and outcomes of cloning cattle using embryo transplants versus adult cell cloning. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. In embryo transplants, a prize cow is artificially inseminated with sperm from a prize bull, and the resulting embryo is split into separate cells before they specialise and implanted into surrogates [1 mark]. 2. Because embryo transplants start with sexual fertilisation, the cloned calves are genetically identical to each other, but you cannot know their exact characteristics in advance because they have a new random mix of parental alleles [1 mark]. 3. In adult cell cloning, the diploid nucleus of an adult body cell is placed into an enucleated egg cell [1 mark]. 4. Therefore, adult cell cloning produces an offspring that is 100% genetically identical to a known adult animal whose desirable phenotype has already been proven [1 mark].

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