Questions on artificial selection and selective breeding

Question 1

A wild grass related to wheat is found to carry a gene giving resistance to a fungal disease that is destroying a high-yielding commercial wheat variety. Describe how plant breeders could use this wild plant to produce a resistant variety that keeps the high yield of the commercial variety.

Accepted Answer

B1 cross the resistant wild plant with the high-yielding commercial variety, so the offspring inherit alleles from both parents B1 screen the offspring by deliberately infecting them with the pathogen and keeping only the plants that survive, selecting for the resistance allele B1 cross the resistant offspring back to the high-yielding commercial variety, a backcross, and screen again for resistance B1 repeat the backcrossing and screening over several generations, so the proportion of the genome matching the commercial variety increases each round while the resistance allele is retained B1 select and test the resulting plants over several sites and seasons before releasing a new variety essentially identical to the original commercial variety except for the added resistance

Question 2

Broiler chickens selected intensively for rapid growth now reach slaughter mass in about half the time they took fifty years ago, but with higher rates of leg and heart problems, because their skeletons and hearts have not been improved to keep pace with their growth rate. Discuss the arguments for and against continuing to select broiler chickens for faster growth.

Accepted Answer

B1 against: fast growth outpaces skeletal and cardiovascular development, causing leg deformities, lameness and heart failure in a high proportion of birds, a direct animal-welfare cost of the selection B1 against: this trade-off cannot easily be avoided, since the same genes are often linked to both the growth advantage and the physical strain, so selecting harder for growth tends to worsen the welfare problems B1 for: faster growth reduces the feed, land and time needed to produce each bird, lowering the cost of chicken as a food source and reducing the environmental resources used per bird produced B1 for: breeders can select simultaneously for growth rate and for leg strength or heart health if both are measured and included in the breeding programme, so the trade-off is not fixed by biology alone B1 a judgement: because the welfare cost falls directly and severely on the animal while the benefit is mainly economic, breeding programmes should give more weight to leg and heart health alongside growth rate, which some breeding companies have begun to do by including health traits in their selection targets

Question 3

Describe how a farmer could use selective breeding to increase the mean mass of wool produced per sheep in a flock.

Accepted Answer

B1 identify and select the sheep in the flock that produce the greatest mass of wool B1 breed only from those selected sheep, so that only their alleles are passed on B1 measure the wool produced by the offspring and select the best of them as the next generation of parents B1 repeat the selection and breeding over many generations, so the alleles for high wool yield become more frequent and the mean of the flock rises

Question 4

Maize breeders self-pollinate plants for several generations to produce inbred lines, then cross two inbred lines together. Explain why the inbred lines are low yielding and why the cross between them is not.

Accepted Answer

B1 each generation of self-pollination halves the proportion of heterozygotes, so after several generations the line is almost entirely homozygous B1 harmful recessive alleles that were masked in heterozygotes are now present in the homozygous state and are expressed, which is inbreeding depression B1 crossing two different inbred lines gives offspring that are heterozygous at every locus where the two lines differed, so those harmful recessive alleles are masked again B1 the offspring are therefore uniform and vigorous and outyield both parent lines, which is hybrid vigour

Question 5

Some dog breeds have been selected for a very short, flat face. Discuss the arguments for and against continuing to breed dogs to such a standard.

Accepted Answer

B1 against: the selected shape causes ill health, because the airway is too small for the skull it sits in, so the dogs have difficulty breathing and cooling themselves B1 against: breeding within a closed register means matings between close relatives, so inbreeding depression raises the incidence of inherited disease B1 for: people value the appearance and temperament of particular breeds, and breeding to a standard keeps a breed recognisable and predictable B1 a judgement: the welfare cost falls on the animal and the benefit on the owner, so breed standards that reward the extreme should be rewritten and outcrossing allowed, which several kennel clubs have begun to do

Question 6

A poultry breeder selects the highest-egg-laying hens as parents for six generations. Egg number rises for the first four generations and then stops increasing, even though the breeder continues to select the highest layers each generation. Suggest two reasons the response to selection slowed and then stopped.

Accepted Answer

B1 repeated selection has made the alleles favouring high egg number increasingly common in the flock, reducing the genetic variation for the characteristic B1 once the favourable alleles are close to fixed, with nearly every bird homozygous for them, very little genetic variation is left for selection to act on B1 keeping only a small number of the highest-laying hens and their close relatives as breeding stock over six generations is a form of inbreeding B1 this raises homozygosity generally and can produce inbreeding depression, reduced vigour and fertility, which could offset or mask any further genetic gain in egg number even where some variation remains

Question 7

A maize breeder starts self-pollinating a plant that is heterozygous at a particular locus. Assuming the proportion of heterozygotes halves with each generation of self-pollination, calculate the percentage of heterozygotes remaining after 5 generations of selfing, starting from 100%, and state what your answer suggests about a line selfed for six or seven generations.

Accepted Answer

M1 the proportion of heterozygotes halves each generation, so after n generations the proportion remaining is 100% × (½)ⁿ M1 substitute n = 5: 100 × (½)⁵ = 100 ÷ 32 A1 = 3.125%, which rounds to 3.1% A1 this shows why after six or seven generations of selfing an inbred line is described as almost completely homozygous: well under 5% of loci that started heterozygous remain so

Question 8

Explain why bulls used to improve the milk yield of dairy herds are chosen by progeny testing rather than by measurement of the bull itself.

Accepted Answer

B1 milk yield cannot be measured in a bull, because bulls do not lactate B1 the bull is therefore mated to many cows and the milk yields of his daughters are recorded and compared with the herds they are kept in B1 those yields estimate the alleles the bull passes on, so a bull that tests well can then be used on a very large scale through artificial insemination

Question 9

A wheat variety bred for the highest possible yield is grown across a whole region. Suggest why plant breeders keep collections of wild relatives and old local varieties of wheat.

Accepted Answer

B1 the high-yielding variety has been selected from a small number of parents, so its gene pool is narrow and many alleles are absent from it B1 wild relatives and old varieties hold alleles the commercial variety has lost, including alleles for resistance to pathogens and for tolerance of drought or cold B1 those alleles can be bred into the commercial variety by crossing and backcrossing when a new pathogen or a change in climate arrives, and selection cannot act on variation that is not there

Question 10

Compare natural selection with artificial selection, using the agent of selection, the characteristics favoured and the effect on genetic diversity.

Accepted Answer

B1 in both, some individuals contribute more offspring than others, so the frequency of their alleles rises, and neither creates new alleles B1 in natural selection the environment decides, and what is favoured raises survival and reproduction, whereas in artificial selection a breeder decides, and what is favoured may reduce an organism's chance of surviving B1 artificial selection usually reduces genetic diversity far faster, because a very small number of parents is used deliberately

Question 11

A small captive-breeding population of an endangered animal shows increasing rates of inherited disease after several generations of breeding between close relatives. Explain, in terms of alleles and genotypes, why this happens.

Accepted Answer

B1 close relatives share a higher than average proportion of their alleles, because they have recent common ancestors B1 mating between them therefore produces offspring that are more likely to be homozygous at any given locus than offspring of unrelated parents B1 harmful recessive alleles that were masked in heterozygous parents are exposed in the homozygous state in these offspring and are expressed, causing the reduced vigour, fertility and disease resistance seen as inbreeding depression

Question 12

One bull of high breeding value for milk yield can father tens of thousands of calves through artificial insemination, and embryo transfer lets one cow of high breeding value produce far more calves than she could carry naturally. Explain how these two technologies increase the rate at which a dairy herd's milk yield can be improved, compared with natural mating.

Accepted Answer

B1 with natural mating, a single bull or cow can only be the parent of a small number of calves in its lifetime, so an individual of high breeding value contributes only a small proportion of the alleles in the next generation B1 artificial insemination lets semen from one high-breeding-value bull be used on very large numbers of cows, including abroad since semen can be frozen and shipped, so his favourable alleles spread through a much larger proportion of the next generation than natural mating would allow B1 embryo transfer does the equivalent job on the female side, letting a cow of high breeding value be the genetic mother of many more calves than she could carry and give birth to herself, again spreading her favourable alleles faster through the population

Question 13

Some cattle breeds carry a dominant allele that prevents horns growing, the polled trait, removing the need to disbud (remove the horn buds of) calves. Suggest why breeding for the polled allele is an example of selective breeding that improves rather than harms animal welfare, and suggest a cross that would show whether a polled bull is heterozygous for the allele.

Accepted Answer

B1 disbudding causes the calf pain and a wound that could become infected, so an animal born polled avoids the procedure entirely, a direct welfare benefit, unlike breeding decided by appearance or productivity alone B1 because polled is dominant, breeding from polled parents raises the frequency of the polled allele in the population, so more calves are born hornless and disbudding becomes unnecessary for them B1 a test cross: mate the bull with several homozygous horned cows; if any calf is born horned, the bull must be heterozygous for the polled allele, since a homozygous polled bull could only pass on the polled allele

Question 14

Compare disease-resistance breeding in wheat with milk-yield improvement in dairy cattle, referring to how the desired individuals are identified and how their alleles are then multiplied through the population.

Accepted Answer

B1 resistant wheat plants are identified by directly testing them, deliberately infecting offspring with the pathogen and keeping the survivors, whereas milk yield cannot be measured in the animal bred from, a bull, so bulls are instead judged indirectly by progeny testing, recording the yields of their daughters B1 a resistant wheat plant is multiplied by repeated crossing, backcrossing and growing seed from selected plants over several generations and seasons, whereas a bull identified as high breeding value is multiplied almost immediately and on a vastly larger scale through artificial insemination of many cows in a single breeding season B1 both programmes increase the frequency of favourable alleles by breeding selectively from the individuals carrying them, but a seed-propagated crop and an animal bred by artificial insemination differ greatly in how many offspring one selected individual can have, which is why livestock breeding can change a population's average much faster once an individual's value is known

Question 15

State one example of a resistance allele from a wild relative being bred into a modern crop, and state where such genetic material is stored to keep it available for future breeding.

Accepted Answer

B1 wild rice supplied a resistance allele against grassy stunt virus, which was bred into commercial rice varieties, or another correctly named documented case B1 such material is stored in seed banks such as the Millennium Seed Bank or the Svalbard Global Seed Vault, which hold seed samples of wild relatives and old local varieties