Topic 7: Genetics, Populations, Evolution & Ecosystems

Question 1

Define genotype

Answer 1

Genetic constitution of an organism

Question 2

Define phenotype

Answer 2

Expression of the genetic constitution and its interaction with the environment

Question 4

What are alleles?

Answer 4

Variations of a particular gene

Question 5

How do alleles arise?

Answer 5

By mutations (changes in DNA base sequence)

Question 6

What is a homologous pair of chromosomes?

Answer 6

Each have the same gene in the same order but there may be variation , resulting in different alleles

Question 7

How many alleles in a diploid organism?

Answer 7

2 –> as there are 2 sets of chromosomes

Question 8

What is a dominant allele?

Answer 8

One that’s always expressed

Question 9

What is a recessive allele?

Answer 9

One that’s only expressed if there’s 2 copies (homozygous recessive)

Question 10

What are co-dominant alleles?

Answer 10

Both alleles expressed

Question 11

Define homozygous

Answer 11

Alleles at a specific locus are the same

Question 12

Define heterozygous

Answer 12

Alleles at a specific locus are different

Question 13

What do monohybrid crosses show?

Answer 13

Inheritance of one phenotypic characteristic coded for by a single gene

Question 14

What do dihybrid crosses show?

Answer 14

Inheritance of 2 phenotypic characteristics coded for by 2 different genes

Question 15

What is a sex-linked gene?

Answer 15

Gene with a locus on a sex chromosome (usually X)

Question 16

Why are males more likely to express a recessive X-linked allele?

Answer 16

Females are XX and males are XY

For alleles to be expressed, female needs to be homozygous recessive (if heterozygous, then they’re carriers)

Males only have 1 allele so the recessive allele is always expressed

Question 17

How does autosomal linkage affect inheritance of alleles?

Answer 17

1) Two genes located on same autosome

2) So alleles on the same chromosome are inherited together –> stay together during independent segregation of homologous chromosomes during meiosis

3) But crossing over between homologous can create new combinations of alleles –> if genes are close together on an autosome, they’re less likely to split by crossing over

Question 18

How do you explain results of an autosomal linkage?

Answer 18

1) Genes are linked
2) Crossing over has occurred
3) Explain which gametes are produced

Question 19

What is epistasis?

Answer 19

Interaction on non-linked genes where one masks the expression of the other

Question 20

When is a Chi-squared test used?

Answer 20

1) when determining if observed results are significantly different from expected results

2) Data is categorical

Question 21

Why are observed phenotypic ratios obtained in offspring often not the same as expected ratios in genetic crosses?

Answer 21

1) Fertilisation is random

2) Autosomal linkage

3) Small sample size –> not representative of whole population

4) Some genotypes may be lethal

Question 22

How do you calculate Chi-squared?

Answer 22

χ2 = ∑(O – E)^2/E

O = Observed frequency
E = Expected frequency

Question 23

How can chi-squared value be analysed?

Answer 23

1) Number of degrees of freedom = number of categories - 1

2) Determine critical value at p=0.05 from table

3) If χ2 is greater than critical value at p=0.05 –> difference is significant so reject null hypothesis –> less that 5% probability that difference is due to chance

4) If χ2 is less than critical value at p=0.05 –> difference is not significant so accept null hypothesis –> more that 5% probability that difference is due to chance

Question 24

Define population?

Answer 24

Group of organisms of same species in one area at one time & can interbreed

Question 25

Define gene pool?

Answer 25

All alleles of all the genes in a population at any one time

Question 26

Define allele frequency

Answer 26

Proportion of an allele of a gene in a gene pool

Question 27

What does Hardey-Weinberg principle state? What are the conditions where the condition applies?

Answer 27

Allele frequency wont change from generation to generation IF: 1) Population is large 2) No immigration / emigration 3) No mutations 4) No selection for / against specific alleles 5) Mating is random

Question 28

Why can mutations cause allele frequencies to change over generations?

Answer 28

Creates new alleles

Question 29

Why does immigration / emigration change allele frequencies over generations?

Answer 29

Introduces / removes alleles

Question 30

What is the Hardey-Weinberg equation?

Answer 30

p² + 2pq + q² = 1 p = frequency of one allele (usually dominant) q = frequency of the other allele (usually recessive) 2pq = frequency of heterozygous genotype p² = frequency of homozygous genotype (usually dominant) q² = frequency of homozygous genotype (usually recessive)

Question 31

What equation can be used simultaneously with the Hardey-Weinberg equation?

Answer 31

p + q = 1

Question 33

Why might individuals within a population of a species show a wide range of variation in phenotype?

Answer 33

1) Genetic factors: - Mutations - Crossing over - Independent segregation - Random fertilisation 2) Environmental factors: - Food availability - Light intensity

Question 34

What is evolution?

Answer 34

1) Changes in allele frequency over time / many generations in a population 2) Occurring through the process of natural selection

Question 35

Factors that may drive natural selection

Answer 35

1) Predation / disease / competition for natural survival 2) Resulting in differential survival and reproduction (natural selection)

Question 36

Explain the principles of natural selection

Answer 36

1) Mutations --> Random mutations result in new allele formed 2) Advantage --> Due to selection pressure, new allele may benefit its possessor --> Organism has selective advantage 3) Reproductive success --> Possessors are more likely to survive & have increased reproductive success 4) Inheritance --> Advantageous allele is inherited by offspring 5) Allele frequency --> Over many generations, allele increases in frequency

Question 37

Effects of stabilising selection

Answer 37

1) Organism with alleles coding for average variations of a trait have selective advantage (eg. Babies with average weight) 2) So frequency of allele coding for average variations of trait increases --> those coding for extreme variation of trait decrease 3) So range is reduced

Question 38

Effect of directional selection

Answer 38

1) Organisms with alleles coding for one extreme variation of trait has selective advantage (eg. Bacteria with high antibiotic resistance) 2) So frequency of alleles coding for extreme variation of trait increase --> those coding for the other extreme variation decrease

Question 39

Effect of disruptive selection

Answer 39

1) Organisms wit alleles coding for either extreme variation of trait have selective advantage 2) Frequency of alleles coding for both extreme variations of trait increase --> those coding for average variation of trait decrease 3) Leading to speciation

Question 40

Describe speciation

Answer 40

1) Reproductive separation of 2 populations 2) Resulting in accumulation of differences in their gene pools 3) New species arise when genetic differences lead to an inability of members of population to interbreed and produce fertile offspring

Question 41

Describe allopatric speciation

Answer 41

1) Population is split due to geographical isolation 2) Leading to reproductive isolation --> separates gene pools by preventing interbreeding / gene flow 3) Random mutations causes genetic variation in each population 4) Creating different selection pressures 5) So different advantageous alleles are selected for / passed in each population 6) Allele frequencies in each gene pool change over many generations 7) Eventually, different populations can't interbreed to produce fertile offspring

Question 42

Describe sympatric speciation

Answer 42

1) Population not geographically isolated 2) Mutations lead to reproductive isolation, separating gene pools by preventing interbreeding / gene flow 3) Different selection pressures act on each population 4) So different advantageous alleles are selected for / passed on in each population 5) So allele frequency in each population change over many generations 6) Eventually different populations can't interbreed to produce fertile offspring

Question 43

Define genetic drift

Answer 43

Mechanism of evolution in which allele frequencies in a population change over generations due to CHANCE

Question 44

Explain genetic drift and its importance in small populations

Answer 44

1) There is genetic drift as some alleles are passed onto offspring more / less often by chance --> Regardless of selection pressure / whether alleles give selective advantage 2) Strongest effects in small population as gene pool is small --> so chance has greater influence 3) any reduce genetic diversity

Question 45

What is a community?

Answer 45

All populations of different species living in the same place t the same time

Question 46

What is an ecosystem?

Answer 46

A community and non-living components of its environment

Question 47

What is a niche?

Answer 47

- Specific role of a species within a habitat (eg. What it eats, where / when it feeds) - Governed by its adaptations to both biotic and abiotic conditions

Question 48

Advantage of species occupying different niches

Answer 48

1) Less competition for food / resources 2) If they tried to occupy the same niche, they'd outcompete each other

Question 49

Define carrying capacity

Answer 49

Maximum population size of a species that an ecosystem can hold

Question 50

Factors affecting carrying capacity

Answer 50

1) Abiotic factors - eg. Light intensity / temperature / soil pH / mineral content / humidity 2) Interactions between organisms - Interspecific competition --> between organisms of different species - Intraspecific competition --> between organisms of same species - Predation --> predators kill and eat other animals

Question 51

How do abiotic factors affect population size?

Answer 51

- If conditions favourable, organisms more likely to survive and reproduce --> increases carrying capacity Eg. Increasing light intensity increases rate of photosynthesis in plants - Increases carrying capacity of variety of plants - So increases variety of habitats / niches / food sources for animals - So increasing carrying capacity of animal species

Question 52

How does interspecific competition affect population size?

Answer 52

- Reduces [named resource] available to both species --> limits their chance of survival & reproduction --> reduces population size - If one species is better adapted, it will outcompete the other --> population size of less well adapted species declines

Question 53

How does intraspecific competition affect population size?

Answer 53

1) As population size increases, resources available per organism decreases --> competition increases --> Chance of survival & reproduction decrease --> population size decreases 2) as population size decreases, resources available per organism increases --> competition decreases --> Chances of survival & reproduction increases --> population increases

Question 54

Explain changes that occur in populations of predators & prey

Answer 54

1) Population of prey increases --> predators have more food --> more predators survive and reproduce 2) Population of predators increase --> more prey killed / eaten --> less prey survive and reproduce 3) Population of prey decreases --> predators have less food --> less predators survive and reproduce 4) Population of predators decrease --> less prey killed / eaten --> more prey survive and reproduce

Question 55

How to estimate size of population of slow / non-moving organisms?

Answer 55

1) Divide area into grids --> place 2 tape measures at a right angle 2) Generate pair of coordinates using random number generator 3) Place quadrat there and count number of [named] species 4) Repeat a 10+ times and calculate mean 5) Population size = (total area of habitat / quadrat area) x mean per quadrat

Question 56

Describe mark-release-recapture method

Answer 56

1) Capture sample of species 2) Mark and release (eg. Nail polish on snail shell) 3) Allow time for organisms to randomly distribute before collecting second sample 4) Population = (number in sample 1 x number in sample 2) / number marked in sample 2

Question 57

Mark release recapture equation

Answer 57

(Number marked in sample 1 / total population size) = (Number marked in sample 2 / total number in sample 2)

Question 58

Assumptions of mark-release-recapture method

Answer 58

1) Sufficient time for marked individuals to evenly distribute 2) Marking not removed & doesn't affect chance of survival / predation 3) No immigration / emigration 4) Few births / deaths / breeding in population size

Question 59

Why can mark-release-recapture produce unreliable results?

Answer 59

1) Unlikely that organisms will evenly distribute 2) Less chance of recapturing organisms