Variation and Artificial Selection (Chapter 17)

Question 1

What are the 5 things that cause genetic variation?

Answer 1

1) independent assortment of chromosomes (and ∴ alleles) during meiosis
2) crossing over between chromatids of homologous chromosomes during meiosis
3) random mating between organisms within a species
4) random fertilisation of gametes
5) mutation

Question 2

How does independent assortment, crossing over, random mating and random fertilisation cause genetic variation?

Answer 2

• They reshuffle existing alleles in the population
• Offspring have combinations of alleles which differ from those of their parents and from each other
• This genetic variation produces phenotypic variation

Question 3

How does mutation cause genetic variation?

Answer 3

• Mutation does not reshuffle alleles already present
• It can produce completely new alleles e.g. by a new base sequence occurring in a gene as a result of a mistake during DNA replication

Question 4

What is a gene mutation?

Answer 4

An unpredictable change in a gene

Question 5

What is a new allele caused by mutation normally like and what does this mean?

Answer 5

The new allele is very often recessive ∴ it often does not show up in the population for some generations until by chance two descendants of organisms in which the mutation happened mate and produce offspring

Question 6

What is the effect of mutations in somatic cells?

Answer 6

They often have no effect on the organism and cannot be passed to offspring by sexual reproduction

Question 7

What is the effect of mutations in cells that divide to form gamers in ovaries or testes?

Answer 7

They may be inherited by offspring as the gametes may contain the mutated gene and if the gamete fuses to form a zygote, all the cells in the organism produced by the single cell dividing will contain the mutated gene

Question 8

What does genetic variation passed on by parents to offspring give?

Answer 8

Differences in phenotype, providing the raw material on which natural selection can act

Question 9

What does variation within a population mean?

Answer 9

That some individuals have features that give them an advantage over other members of that population

Question 10

How else is variation in phenotype caused?

Answer 10

• By the environment in which organisms live e.g. if an organisms has better food when growing, they will be larger
• Variation caused by the environment is not passed on by parents to their offspring

Question 11

What is discontinuous variation?

Answer 11

Qualitative differences which fall into clearly distinguishable categories, with no intermediates

Question 12

What is an example of discontinuous variation?

Answer 12

The four possible ABO blood groups: A, B, AB, O

Question 13

What is continuous variation?

Answer 13

Quantitative difference which may be small and difficult to distinguish

Question 14

What is an example of continuous variation?

Answer 14

Height - there are no distinguishable height classes, instead there is a range of heights between two extremes

Question 15

What are two characteristics that discontinuous and continuous variation share?

Answer 15

Both qualitative and quantitative difference in phenotype may be inherited and both may involve several different genes

Question 16

What are the characteristics of discontinuous variation that are different?

Answer 16

1) different alleles at a single gene locus have large effects on the phenotype
2) different genes have quite different effects on the phenotypes

Question 17

What are the characteristics on continuous variation that are different?

Answer 17

1) different alleles at a single gene locus have small effects on the phenotype
2) different genes have the same, often additive effect on the phenotype
3) a large numbers of genes may have a combined effect on a particular phenotypic effect (polygenes)

Question 18

Describe the example of continuous variation in height

Answer 18

• Height is controlled by two unlinked genes (i.e. genes on different chromosomes): A/a and B/b
• a/b contribute x cm in height, A/B contrite 2x cm in height
• Since the effect of such genes is additive, aabb = 4xcm and AABB = 8x cm - the other genotypes will fall between these two extremes
• ∴ a cross between two heterozygous will produce phenotypes in a normal distribution with 6x cm being the most common height

Question 19

What happens if more genes for height with an additive effect (polygenes), all on different chromosomes and possibly with more than two alleles are involved?

Answer 19

The number of discrete height classes would increase as more genes are involved and the difference between these classes gets less

Question 20

What do environmental effects smooth out?

Answer 20

Differences between different classes

Question 21

What happens if two or more of the genes are linked on the same chromosome?

Answer 21

• This could reduce the number of classes of offspring and increase the differences between them
• However, crossing over in meiosis restores variation

Question 22

What do environmental effects do?

Answer 22

• They may allow the full genetic potential height to be reached or may shunt it in some way
• They can influence which genes are expressed

Question 23

How could environmental effects on height prevent the full genetic potential being reached?

Answer 23

1) someone might have less food or less nutritious food than someone else with the same genetic contribution
2) a plant may be in a lower light intensity or in soil with fewer nutrients than another plant with the same genetic potential height

Question 24

What is the example of environmental effects on the colouring of Himalayan rabbits, Siamese and Burmese cats?

Answer 24

• The development of dark tips to ears, nose, paws and tail
• This colouring is caused by an allele which allows the formation of the dark pigment only at low temperatures
• The extremities are the coldest part of the animal, so the colour is produced there

Question 25

What is the experiment to show environmental effects?

Answer 25

1) cross two pure-bred varieties of maize (Black Mexican and Tom Thumb), which differ in cob length
2) F1 genetically different from parents but genetically the same as each other
3) variation in parents and F1 shows environmental effects
4) F2 shows much wider variation in cob length (both genetic and environmental)

Question 26

What does pure-bred mean?

Answer 26

Homozygous at many loci

Question 27

What is it important to know in selective breeding about variation and why?

Answer 27

• It is important to know how much phenotypic variation is genetic and how much is environmental in origin
• This is bc there is no point in selecting parents for a breeding programme on the basis of environmental variation

Question 28

What is artificial selection?

Answer 28

When humans purposefully apply selection pressures to populations

Question 29

What are desired features of cattle?

Answer 29

Fast growth rates, high milk yield and docility (making the animal easier to control)

Question 30

How have increases in desired features of cattle been achieved?

Answer 30

By selective breeding

Question 31

How does selective breeding of cattle work?

Answer 31

1) individuals showing one or more of the desired features to a larger degree than other individuals are chosen for breeding
2) some of the alleles conferring these features are passed on to the individuals’ offspring
3) again, the best animals from the generation are chosen for breeding
4) over many generations, alleles conferring the desired characteristics increase in frequency, while those conferring characteristics not desired by the breeder (disadvantageous alleles), decrease in frequency or are lost entirely

Question 32

What are problems for the breeder with selective breeding of dairy cattle?

Answer 32

1) the cattle are large and take time to reach maturity
2) the gestation period is long and the number of offspring produced is small
3) a bull cannot be assessed for milk production bc this is a sex-limited trait - instead, performance of a bull’s female offspring is looked at to see whether or not to use the bull in further crosses - this = progeny testing and is a measure of the bull’s value to the breeder

Question 33

What do selective breeders also have to consider as well as the genes affecting the desired traits?

Answer 33

• The whole genotype of an organism
• Within each organism’s genotype are all the alleles of genes that adapt it to its particular environment (background genes)

Question 34

Give an example of why a selective breeder would need to consider the background genes of an organism

Answer 34

• If parents come from the same environment and from varieties that have already undergone some artificial selection, it is likely that they will share many alleles of background genes ∴ the offspring will be adapted for the same environment
• But, if one parent comes from a different part of the world, the offspring will inherit appropriate alleles from only one parent - ∴ it may show the desired trait but not be well-adapted to its environment

Question 35

Why do background genes also need to be considered when a cross is being made between a cultivated plant and a related wild species?

Answer 35

• Some species can interbreed to give fertile offspring - often those that do not usually come into contact with each other, bc they live in different habitats or areas
• ∴ the wild parent will have unwanted alleles which have probably been selected out of the cultivated plants

Question 36

Describe selective breeding to produce new varieties of crop plants

Answer 36

1) farmers pick out the best plants that grew in one year and allow them to breed and produce the grain for the next year
2) over 1000s of years, this has brought about great changes in the cultivated varieties of crop plants, compared with their wild ancestors
3) this is still the main method, but gene tech can also be used to alter/add genes into a species in order to change its characteristics

Question 37

What are desired traits of wheat?

Answer 37

Grains rich in gluten and resistance in various fungal diseases e.g. head blight to fight the loss of yield resulting from such infections

Question 38

How long does successful introduction of an allele giving disease resistance take?

Answer 38

Many generations

Question 39

Give an example of successful selective breeding of wheat

Answer 39

• The Wheat Genetic Improvement Network was set up in the UK in 2003 to bring together research workers and commercial plant breeders
• The aim was to support the development of new varieties of wheat by screening seed collections for plants with traits e.g. disease or climate resistance and efficient use of nitrogen fertilisers
• Any plant with a suitable trait is grown in large numbers and passed to the commercial breeders

Question 40

Why is it better to have wheat plants with shorter stems?

Answer 40

1) they are easier to harvest
2) they have higher yields - because energy is put into making seeds rather than growing tall
3) they are less susceptible to being knocked flat by heavy rains
4) they produce less straw, which has little value and costs money to dispose of

Question 41

Describe the effect of mutant alleles on dwarf varieties on wheat

Answer 41

• Most of the dwarf varieties carry mutant alleles of two reduced height (Rht) genes which code for DELLA proteins which reduce the effect of GA on growth
• The mutant alleles cause dwarfism by producing more of, or more active forms of, these transcription inhibitors
• A mutant allele of a different gene (Tom Thumb) has its dwarfing effect because the plant cells do not have receptors for GA and ∴ cannot respond to it

Question 42

Give an example of how rice is being selectively bred

Answer 42

1) the International Rice Research Institute (which holds the rice gene bank) and the Global Rice Science Partnership coordinates research aimed at improving the ability of rice farmers to feed growing populations
2) researchers are hoping to use selective breeding to produce varieties of rice that show resistance to diseases e.g. bacterial flight and rice blast (fungal)

Question 43

What is inbreeding depression?

Answer 43

If maize plants are inbred (crossed with other plants with genotypes like their own), the plants in each generation become progressively smaller and weaker

Question 44

Why does inbreeding depression occur?

Answer 44

Because, in maize, homozygous plants are less vigorous than heterozygous ones

Question 45

What is outbreeding and what does it produce?

Answer 45

• Outbreeding is crossing plants with other, less closely related plants
• It reduces heterozygous plants that are healthier, grow taller and produce higher yields

Question 46

Why is the problem with outbreeding?

Answer 46

1) if outbreeding is done at random, the farmer would end up with a field full of maize in which there was a lot of variation between the individual plants
2) this would make things very difficult because to be able to harvest and sell the crop easily, the farmer needs the plants to be uniform - all roughly the same height and all ripen at the same time

Question 47

What is the challenge when growing maize?

Answer 47

To achieve both heterozygosity and uniformity

Question 48

How do farmers achieve heterozygosity and uniformity?

Answer 48

1) farmers buy maize seed from companies that special;ise in using inbreeding to produce homozygous maize plants and then crossing them
2) this produces F1 plants that all have the same genotype

Question 49

Describe the benefits of selective breeding of maize

Answer 49

• There are many different homozygous maize varieties and different crosses between them can produce a large number of different hybrids, suited for different purposes
• Every year, 1000s of new maize hybrids are trialled, searching for varieties with characteristics such as high yields, resistance to more pests and diseases and goof growth in nutrient-poor soils or where water is in short supply

Question 50

What is variation?

Answer 50

Differences between individual organisms of the same and different species