(a) A wheat breeder wants to develop a new variety that produces a high grain yield and is also resistant to stem rust disease. Variety M has a high grain y...

(a)(i) The crossing programme to combine high grain yield with stem rust resistance:

1. Cross Variety M (high yield, susceptible) with Variety N (low yield, resistant) to produce F1 seeds. All F1 plants will be heterozygous for the relevant genes, carrying alleles from both parents. [1]
2. Grow the F1 plants and allow them to self-pollinate (or cross F1 plants together) to produce an F2 generation. In the F2, genetic recombination will create a wide range of trait combinations. [1]
3. Screen the F2 population by exposing plants to stem rust and measuring grain yield. Select individuals that show both high grain yield and rust resistance. Self-pollinate these selected plants and continue the process of selection and self-pollination over several further generations (F3, F4, F5...) until both traits breed true and the new variety is genetically stable. [1]

(a)(ii) The breeder must grow and test many plants at each stage because:

1. There is genetic variation among the offspring. Not all F2 or later-generation plants will carry the desired combination of alleles for both high yield and rust resistance. Most will show intermediate or undesirable combinations. [1]
2. Testing many plants increases the probability of finding rare individuals with the best trait combination, and provides statistical reliability so that selections are based on genuine genetic superiority rather than chance environmental effects. [1]

(b) One advantage of using selective breeding for disease resistance compared with chemical fungicides:

A genetically resistant variety does not require repeated application of chemical fungicides, which saves the farmer recurring costs for chemicals and labour, and avoids the environmental damage and health risks associated with fungicide residues in soil, water, and food. [1]

(c) A new disease-resistant variety may lose its resistance after several years of commercial cultivation because:

1. The pathogen (stem rust fungus) can mutate, producing new races or strains with altered virulence genes that can overcome the plant's specific resistance mechanisms. [1]
2. When the resistant variety is grown widely, it creates strong selection pressure on the pathogen population. Any mutant spore that can infect the resistant variety has an enormous advantage and multiplies rapidly across the monoculture, causing what is known as a "boom and bust" cycle of resistance breakdown. [1]