(a) State what is meant by the term line breeding. [2] (b) A pig breeder uses line breeding to improve the litter size of a herd of sows. (i) Describe how t...

(a) Line breeding is a form of selective breeding in which animals that are related, but not as closely as parent-offspring or full siblings, are mated together (e.g. half-siblings, cousins, uncle-niece). [1] The aim is to concentrate the genes of a particular outstanding ancestor in the offspring, increasing the likelihood that the ancestor's desirable traits are inherited homozygously. [1]

(b)(i) How the breeder would carry out line breeding over three generations to increase litter size:

1. Generation 1: Select the sows in the herd that produce the largest litters and mate them with a boar that is a relative of a high-performing sow (e.g. a half-brother or maternal grandson of an outstanding dam), but not a direct parent or full sibling. [1]
2. Generation 2: From the offspring, select the gilts (young females) that produce the largest litters in their first farrowing. Mate them with a boar from the same family line (e.g. a cousin or half-brother from the same outstanding ancestor). [1]
3. Generation 3: Again select the females with the largest litters from this generation and mate them with a male from the same bloodline, continuing to concentrate the genes associated with prolificacy. [1]
4. Throughout all three generations, always select strictly for large litter size and keep the breeding stock within the family line of the original outstanding ancestor, while avoiding the closest matings (parent x offspring or full sibling x sibling). [1]

(b)(ii) One risk of continuing line breeding for too many generations is inbreeding depression. As relatedness accumulates, harmful recessive alleles become homozygous, leading to reduced fertility, smaller litters, lower disease resistance, and poorer overall performance. [1]

(c) Boar D has the best feed conversion ratio at 2.8. [1] A lower feed conversion ratio means the animal converts feed into body mass more efficiently - it needs fewer kilograms of feed to gain one kilogram of body weight. Therefore Boar D is the most feed-efficient, reducing feed costs per kilogram of pork produced. [1]

(d) A farmer might choose to buy hybrid breeding stock from a commercial breeder rather than running an on-farm programme because:

1. Hybrid stock may display hybrid vigour (heterosis), giving faster growth, better feed conversion, and improved reproductive performance compared to purebred animals. [1]
2. Purchasing ready-made hybrids saves the farmer years of time and the specialist expertise required to maintain pedigree records, conduct progeny tests, and manage a multi-generation selection programme. [1]

(e) Two other characteristics a pig farmer might select for:

1. Backfat thickness / lean meat percentage - consumers and processors prefer leaner carcasses, so selecting for low backfat increases the market value of each pig. [1]
2. Growth rate / daily weight gain - faster-growing pigs reach slaughter weight sooner, reducing the total feed cost per animal and increasing throughput. [1]

(f) One way selective breeding differs from natural selection:

In selective breeding, humans deliberately choose which animals are allowed to reproduce, based on traits the farmer considers economically desirable (such as litter size or growth rate). [1] In natural selection, the environment determines which animals survive and reproduce. Traits that aid survival and reproduction in that environment are favoured, regardless of whether they are useful to humans. Selective breeding is therefore directed towards human goals, while natural selection is driven by environmental fitness. [1]