Selective Breeding

There are a number of wild species of bananas that contain large seeds in the fruits, as shown below:

The varieties of bananas that we consume do not have large seeds, which improves the experience of eating them.

Banana plants cannot reproduce asexually. 

Seeds play an important role in sexual reproduction in plants and therefore it is not possible for any species to naturally evolve to no longer produce seeds. 

Explain how it is possible for humans to develop varieties of bananas without seeds.

Separate: Biology Only

The bananas without seeds cannot reproduce sexually. 

Banana farmers on plantations have to produce a large number of plants. 

Describe two methods the farmer could use to grow a large number of banana plants without the use of seeds. 

There is currently an economically concerning disease called Black Sigtoka that affects banana plants. 

It is a fungal pathogen that causes black streaks to appear on the leaves of the banana plants. 

Explain why this reduces the yield of the banana plants. 

Separate: Biology Only

One of the reasons why the spread of Black Sigatoka is so concerning is because a huge number of banana plants are susceptible to the pathogen. 

Using your answer to parts (a) and (b), suggest why so many banana plants are susceptible to Black Sigatoka. 

Two possible solutions for the susceptibility of banana plants to Black Sigatoka are as follows:

Strategy One: Breed new seedless banana varieties from wild banana plants that are resistant to the pathogen. 

Strategy Two: Produce genetically engineered banana plants that contain DNA that makes them resistant to the pathogen. 

Discuss the use of these two strategies as potential solutions for the susceptibility of banana plants to Black Sigatoka.

Humans have been selectively breeding animals and plants for many thousands of years.

For example, some 'hypoallergenic' breeds of dogs have been selectively bred to reduce the risk of allergic reactions in humans.

Suggest two other reasons why dogs might be selectively bred.

Dalmatians are a popular breed of dog but due to extensive inbreeding, they are prone to genetic defects. All pure-bred Dalmatians produce higher-than-normal levels of uric acid due to a gene mutation. This can lead to the formation of kidney and bladder stones which have to be surgically removed.

Another breed of dog called Pointer dogs do not possess this faulty gene and have normal levels of uric acid.

Cross-breed Dalmatians, which are about 99% Dalmatian and 1% pointer, look and behave like Dalmatians but crucially do not have the gene mutation which leads to high levels of uric acid. 

Explain how selective breeding was used to produce cross-breed Dalmatians that do not suffer from kidney and bladder stones.

The gene mutation in the pure-breed Dalmatian changes one amino acid in a protein involved in regulating levels of uric acid.

Explain how a change in one amino acid in the protein could stop it from functioning normally.

Selective breeding has been carried out for thousands of years. An example of this is the Dorper breed of sheep; a fast-growing sheep with a high fertility rate that is well adapted to survive in the arid regions of South Africa.  The Dorper was developed from the following two breeds:

• Dorset Horn sheep; a British breed with up to two lambing seasons per year, but not very tolerant of heat or arid conditions
• Blackhead Persian sheep; a Somalian breed with a fast growth rate and high tolerance to heat

Explain how selective breeding resulted in the development of Dorper sheep.

Suggest two other characteristics that breeders may select for when carrying out an artificial selection in sheep.

Selective breeding can result in inbreeding which can have a negative impact on a species.

Describe the negative impact that inbreeding can have on a species.

The table shows some information about two types of potato plants.

Explain how selective breeding could be used to produce potato plants which are faster growing and produce many large potatoes.

A new breeding technique has been developed which allows the production of potatoes with improved yield and quality whilst also showing a natural protection against the fungal diseases, potato blight.

Explain the benefits of breeding potato plants that are resistant to fungal diseases such as potato blight.

Scientists carried out a genetic analysis on a population of potatoes of variety A and a population of variety B potato plants to establish the proportion of heterozygous genotypes in each population.

The data in the graph gives an indication of the genetic diversity of the population where the higher the heterozygosity, the higher the genetic diversity. Conversely, fewer heterozygous genotypes (and more homozygous genotypes) means there are fewer alleles in a population and therefore genetic diversity is reduced.

The graph shows the data collected for the two varieties or potato.

Describe what the graph shows.

Explain the issues which may be associated with a lower heterozygosity demonstrated by variety A.

Suggest how breeding together individuals from varieties A and B may reduce the effects described in (d).