Genetics, populations, evolution and ecosystems

Question 1

What is meant by the term genotype?

Answer 1

Genetic constitution of an organism

Question 2

What is meant by the term phenotype?

Answer 2

The expression of this genetic constitution (genotype)
and its interaction with the environment

Question 3

What are alleles and how do they arise?

Answer 3

Variations of a particular gene (same locus)→ arise by mutation (changes in DNA base sequence)

Question 4

How many alleles of a gene can be found in diploid organisms?

Answer 4

● 2 as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs)
○ But there may be many (more than 2) alleles of a single gene in a population

Question 5

Dominant allele

Answer 5

Always expressed (shown in the phenotype)

Question 6

Recessive allele

Answer 6

Only expressed when 2 copies present (homozygous recessive)
/ NOT expressed when dominant allele present (heterozygous)

Question 7

Codominant alleles

Answer 7

Both alleles expressed / contribute to phenotype (if inherited together)

Question 8

Homozygous

Answer 8

Alleles at a specific locus (on each homologous chromosome) are the same

Question 9

Heterozygous

Answer 9

Alleles at a specific locus (on each homologous chromosome) are different

Question 10

Monohybrid cross

Answer 10

inheritance of one phenotypic characteristic coded for by a single gene

Question 11

Dihybrid cross

Answer 11

inheritance of two phenotypic characteristics coded for by two different genes

Question 12

Describe how the size of a population of slow-moving or non-motile
organisms can be estimated

Answer 12

1. Divide area into a grid / squares eg. place 2 tape measures at right angles
2. Generate a pair of coordinates using a random number generator (eg. on a calculator)
3. Place a quadrat here and count number / frequency of [named species]
4. Repeat a large number of times (10 or more) and calculate a mean per quadrat
5. Population size = (total area of habitat / quadrat area) x mean per quadrat

Question 13

Describe how the mark-release-recapture method can be used to estimate
the size of a population of motile organisms

Answer 13

● Capture sample of species, mark and release
● Ensure marking is not harmful / does not affect survival
● Allow time for organisms to randomly distribute before collecting second sample
● Population = (number in sample 1 x number in sample 2) / number marked in sample 2

Question 14

What assumptions does the mark-release-recapture method make?

Answer 14

Sufficient time for marked individuals to mix / distribute evenly within the population
Marking not removed and doesn’t affect chances of survival / predation
Limited / no immigration / emigration
No / few births / deaths / breeding / change in population size (or birth & death rate are equal)

Question 15

Suggest why the mark-release-recapture method can produce unreliable
results in very large areas

Answer 15

● Unlikely that organisms will distribute randomly / evenly
● Less chance of recapturing organisms (that were marked initially)

Question 16

Succession is

Answer 16

Succession = change in a community over time due to change in abiotic factors / species

Question 17

Describe and explain how primary succession occurs

Answer 17

1. Colonisation by pioneer species (first to colonise)
2. Pioneer species (and other species at each stage in succession) change abiotic conditions
   ● Eg. they die and decompose, forming soil which retains water (humus / organic matter)
3. So environment becomes less hostile / more suitable for other species with different adaptations
   AND less suitable for previous species, so better adapted species outcompete previous species
4. As succession goes on, biodiversity increases
5. Climax community reached - final stable community (no further succession)

Question 18

Describe features of a climax community

Answer 18

● Same species present / stable community over a long time
● Abiotic factors (fairly) constant over time
● Populations (fairly) stable (around carrying capacity)