Genetics 2

Question 1

define directional selection

Answer 1

a type of natural selection that occurs when an environmental change favours a new phenotype and results in a change in the population mean

Question 2

define the founder effect

Answer 2

when a small sample of an original population establishes in a new area, its gene pool is not as diverse as that of the parent population

Question 3

define genetic bottleneck

Answer 3

a sharp reduction in size of population due to environmental catastrophes such as earthquakes, floods disease or human activities such as habitat destruction, over hunting or genocide which reduces genetic diversity , as the population expands it is less genetically diverse than before

Question 4

define stabilising selection

Answer 4

natural selection leading to constancy within a population, intermediate phenotypes are favoured and extreme phenotypes selected against, alleles for extreme phenotypes may be removed from the population, stabilising selection reduces genetic variation within the population

Question 5

describe natural selection

Answer 5

• mutations and migration introduce new alleles into populations,
• some individuals within a population will be better adapted to survive and reproduce passing on the advantageous alleles
• over time allele frequencies within the population will change

Question 6

when does stabilising selection occur

Answer 6

• normally occurs when the environment remains unchanged

- draw the graph

Question 7

when does directional selection occur

Answer 7

• if the environment changes by becoming colder there may now be an advantage to being larger so a new larger mass becomes the ideal and will be selected for
• if more larger individuals survive and reproduce they will be more likely to pass genes and alleles for larger size to their offpspring
• over several generation, there is a gradual shift in the optimum value for the trait

Question 8

what are the three types of selection

Answer 8

• stabilising
• directional
• disruptive

Question 9

describe genetic drift

Answer 9

• if a population descends from a small number of parents the gene pool will lack genetic variation
• some alleles resulting from mutation and offer neither and advantage or disadvantage on the individuals so there will be no selection pressure acting upon them, however chance events may drastically alter the allele frequency
• for example if there was a catastrophic event and decreases in population size then the population recovers and increases in size it will have less genetic diversity than before and may lack particular alleles

Question 10

what happens after a genetic bottleneck

Answer 10

• after a population size has shrunk and increased again the genetic diversity will be reduced
• loss of advantageous alleles or disproportionate frequency of harmful alleles, therefore has low population shrinks to such a small size leading the species becoming endangered and then extinct
• but if the ones survive have an advantageous characteristics such as resistance than it can improve the gene pool whilst shrinking genetic diversity

Question 11

describe the founder effect

Answer 11

if a new population is established by a small number of individuals who originate from a larger parent population, the new population is likely to exhibit loss of genetic variation
- some groups of migrating humans are not fully genetically representative of the parent popualtion have set up populations in new areas and if they have remained isolated from other human populations then they will have a smaller gene pool

Question 12

what are factors affecting the allele frequencies within populations

Answer 12

• population size
• mutation rate
• migration
• natural selection
• changes to the environment
• isolation of a population from other populations of the same species
• non random mating
• genetic drift
• gene flow

Question 13

what does the hardy-weinberg principle describe

Answer 13

• it describes and predicts a balanced equilibrium in the frequencies of alleles and genotypes within a breeding population
• it can also be used to determine the frequencies of those carrying a recessive allele for a genetic disorder with a recessive inheritance pattern, if we know the incidence of aaffected babies born each year in that population

Question 14

what does the hardy-weinberg principle assume

Answer 14

• the popualtion is large enough to make sampling error negligible
• mating within the population occurs at random
• there is no selective advantage for any genotype and hence no selection
• there is no mutation, migration or genetic drift

Question 15

what is allopatric speciation

Answer 15

• formation of two different species from one original species due to geographical isolation

Question 16

what is speciation

Answer 16

the splitting of a genetically similar population in two or more populations that undergo genetic differentiation and eventually reproductive isolation leading to the evolution of two or more new species

Question 17

what is sympatric speciation

Answer 17

• formation of two different species from one original species due to reproductive isolation while the populations inhabit the same geographical location

Question 18

describe geographical isolation

Answer 18

• populations are isolated by geographical features such as lakes, oceans and mountains, act as barrier to gene flow between the populations
• the isolated populations being subject to different selection pressures in two different environments undergo independent changes to the allele frequencies and/or chromosome arrangements within gene pools
• may be as a result of genetic mutation or genetic drift
• population becomes adapted to its environment
• alllopatric speciation

Question 19

describe reproductive isolation

Answer 19

• biological and behavioural changes within a species may lead to reproductive isolation of one population from another
• if a mutation leads to some organisms in a population changing their foraging behaviour and becoming active at dawn, dusk or night rather than during the day therefore they have a new niche and will unlikley to mate with members who forage at different times
  genetic changes can also lead to reproductive isolation, a change in chromosme number may
• prevent gamete fusion
• makes the zygotes less viable so that they fail to develop
• lead to infertile hybrid offspring with an odd number of chromosomes so that chromosome pairing during meiosis cannot occur
  mating between members of the reproductively isolated populations may also be prevented by mutations leading to changes in
• courtship behaviour
• animal genitalia or plant flower structure
• this is called sympatric speciation

Question 20

what is artificial selection

Answer 20

• selective breeding of organisms, involves humans choosing the desired phenotypes and interbreeding those phenotypes individually, therefore selecting the geneotypes that contribute to the gene pool of the next generation of these organisms

Question 21

what is inbreeding depression

Answer 21

at each stage of selective breeding the individuals with the desirable characterstics and no or few undesirable characterstics are selected, therefore the genetic diversity in gene pool of the selected breed is reduced, if related individuals are crossed inbreeding depression can result and the chances of an individual inheriting two copies of a recessive harmful allele are increased

Question 22

what is hybird vigour

Answer 22

• breeders sometimes outcross individuals belonging to two different varieties to obtain individuals that are heterozygous at many gene loci this is hybird vigour

Question 23

what are gene banks

Answer 23

gene banks store genomes but in their organisms

Question 24

give example of gene banks

Answer 24

• rare breed farms
• wild popualtions of organisms
• crops in cultivation
• botanic garderns and zoos
• seed banks
• sperm banks
• cells in tissue culture
• frozen embryos

Question 25

what are the ethical considerations of artificial selection

Answer 25

• domesticated animals retain many juvenile characteristics making them friendly, docile and playful but less able to defend themselves so they are easy prey
• livestock animals such as pigs selected to have more lean meat and less fat might succumb to low environmental temperatures during winter if they were not housed
• some breeds are susceptible to disease
• some coat colours selected fail to camouflage the animals