GENETICS, POPULATIONS, EVOLUTION & ECOSYSTEMS

• INHERITANCE AND HARDY-WEINBERG PRINCIPLE

  In fruit flies, males have the sex chromosomes XY and the females have XX. In

  fruit flies, a gene for eye colour is carried on the X chromosome. The allele for

  red eyes, R, is dominant to the allele for white eyes, r.

  (a) Male fruit flies are more likely than female fruit flies to have white eyes.

  Explain why. (2)

  1. Males have one allele;

  Accept males only need one allele.

  2. Females need two recessive alleles

  OR

  Females must be homozygous recessive

  OR

  Females could have dominant and recessive alleles

  OR

  Females could be heterozygous/carriers;

  Ignore references to X and Y chromosomes.

  Accept r as recessive allele and R as dominant

  allele.

  If no reference to allele, accept for one mark male

  needs one recessive gene whereas females need

  two recessive genes.
• Mutation is one cause of genetic variation in organisms.

  Give two other causes of genetic variation. (2)

  1. Crossing over;

  2. Independent segregation/assortment (of homologous chromosomes);

  Accept independent assortment of alleles.

  Accept meiosis as an alternative for 1 or 2 if neither

  of these marks is awarded.

  3. Random fusion of gametes

  OR

  Random fertilisation;

  Accept random mating.
• In a species of flowering plant, the T allele for tallness is dominant to the t allele

  for dwarfness. In the same species, two alleles CR (red) and CW (white) code for

  the colour of flowers. When homozygous red-flowered plants were crossed with

  homozygous white-flowered plants, all the offspring had pink flowers.

  (b) Name the relationship between the two alleles that code for flower colour. (1)

  Codominance;
• In genetic crosses, the observed phenotypic ratios obtained in the offspring

  are often not the same as the expected ratios.

  Suggest two reasons why. (2)

  1. Small sample size;

  2. Fusion/fertilisation of gametes is random;
• What is meant by the term phenotype? (2)

  1. (Expression / appearance / characteristic due to) genetic constitution

  / genotype / allele(s);

  2. (Expression / appearance / characteristic due to) environment;

  1. Accept: named characteristic.

  1. Accept: homozygous / heterozygous / genes /

  DNA.

  1. Ignore: chromosomes.
• This fruit fly has another characteristic controlled by a pair of codominant

  alleles, WN and WV.

  What is meant by codominant alleles? (1)

  Both alleles expressed in the phenotype (if both are present);
• In fruit flies, the genes for body colour and wing length are linked. Explain

  what this means. (1)

  (Genes / loci) on same chromosome.
• Which statistical test could the scientist use to determine whether his

  observed results were significantly different from the expected results?

  Give the reason for your choice of statistical test. (2)

  1. Chi squared test;

  2. Categorical data.
• POPULATIONS

  What is a gene pool? (1)

  All the alleles in a population;
• EVOLUTION

  Guppies are small fish. Female guppies are dull in colour. Male guppies can be

  bright or dull in colour.

  Scientists investigated the effect of female brain size on choosing a mate. They

  used laboratory-bred female guppies with large brains and with small brains.

  They set up a fish tank as shown in the diagram below.

  They observed each female for 10 minutes and recorded which male they were

  attracted towards. They repeated this with 45 large-brained females and 45

  small-brained females.

  (a) Suggest three possible limitations of this investigation (3)

  1. Laboratory-raised female (guppies) might not react/behave/choose in

  the same way (as wild guppies);

  Ignore answers relating to sample size

  Accept laboratory-raised female (guppies) might not

  be representative of wild females

  2. (Transparent) barrier might not allow for normal (courtship)

  behaviour/interaction;

  Accept choice might involve chemical/ mechanical

  signals/interaction

  Accept colour might not be the only thing females

  are attracted to

  3. Do not know if (guppies) have been used in previous experiments;
• The scientists found that only female guppies with large brains were attracted to

  male guppies bright in colour.

  (b) Suggest and explain the advantage of this behaviour to the population of

  guppies (3)

  1. (Females with large brains) will mate with males bright in colour;

  Accept answers that include references to alleles

  2. Their (male) offspring would be (more likely to be) bright in colour;

  3. (Bright in colour male) offspring could attract larger brained females;

  4. The population/offspring could (evolve to) have larger brains;

  Ignore answers relating to females only

  5. The population/offspring are better at identifying/avoiding predators;

  Ignore answers relating to females only
• Describe how the behaviour of female guppies could result in sympatric

  speciation. (3)

  1. Not geographically isolated;

  Accept are in the same area

  2. (Leading to) reproductive isolation

  OR

  Gene pools kept separate;

  Accept large brained females will only mate with

  males bright in colour and small brained females

  will only mate with males dull in colour

  3. Changes in allele frequencies;

  Reject gene frequencies

  4. Cannot breed/mate to produce fertile offspring;

  Reject inbreeding
• Lactose is the main sugar in milk and is hydrolysed by the enzyme lactase.

  Lactase is essential to newborn mammals as milk is their only source of food.

  Most mammals stop producing lactase when they start feeding on other food

  sources. Humans are an exception to this because some continue to produce

  lactase as adults. The ability to continue producing lactase is known as lactase

  persistence (LP) and is controlled by a dominant allele. A number of hypotheses

  based on different selection pressures have been put forward to explain LP in

  humans.

  (a) One hypothesis for LP in humans suggests that the selective pressure was

  related to some human populations farming cattle as a source of milk.

  Describe how farming cattle as a source of milk could have led to an

  increase in LP. (4)

  1. LP due to mutation

  OR

  Allele due to mutation;

  Reject mutation caused by drinking milk.

  Reject (LP) gene

  2. Milk provides named nutrient;

  Accept any correct named nutrient e.g. glucose,

  galactose, protein

  Ignore 'sugar' 'lactose' as named nutrient

  3. Individuals with LP more likely to survive and reproduce

  OR

  Individuals with advantageous allele more likely to survive and

  reproduce;

  Reject (LP) gene

  Accept 'individuals who produce lactase' for 'LP

  individuals'

  Accept 'pass on allele/LP/characteristic' for

  reproduce.

  4. Directional selection;

  5. Frequency of allele increases (in the offspring/next generation);

  Accept description of increasing frequency of allele

  e.g. 'higher proportion', 'more common' but ignore

  increase in number of allele
• Use the information provided to explain why the number of people showing

  LP would rapidly increase once selection for this condition had been

  established. (2)

  1. Dominant allele;

  2. (Always) expressed/shown (when present in phenotype/offspring)

  OR

  Expressed when only one (dominant allele) present;
• Lord Howe Island in the Tasman Sea possesses two species of palm tree

  which have arisen via sympatric speciation. The two species diverged from

  each other after the island was formed 6.5 million years ago. The flowering

  times of the two species are different.

  Using this information, suggest how these two species of palm tree arose

  by sympatric speciation. (5)

  1. Occurs in the same habitat / environment / population;

  2. Mutation/s cause different flowering times;

  3. Reproductive separation / isolation

  OR

  No gene flow

  OR

  Gene pools remain separate;

  4. Different allele/s passed on / selected

  OR

  Change in frequency of allele/s

  5. Disruptive (natural) selection;

  6. Eventually different species cannot (inter)breed to produce

  fertile offspring;

  1. Accept: are not geographically isolated /

  separated.

  1. Accept: same place

  3. Accept: no interbreeding but must be a

  separate idea from mark point 6 which relates

  to definition of a species.

  Note: Answers relating only to allopatric speciation

  = 3 max, mark points 3, 4 and 6.
• There was a time lag between the introduction of Bt crops and the

  appearance of the first insect species that was resistant to the Bt toxin.

  Explain why there was a time lag. (3)


  1. Initially one / few insects with favourable mutation / allele;

  2. Individuals with (favourable) mutation / allele will have more

  offspring;

  3. Takes many generations for (favourable) mutation / allele to

  become the most common allele (of this gene).
• There are nine subspecies of giraffe. These subspecies evolved when

  populations of giraffe were separated for long time periods. Each subspecies has

  distinct coloured skin markings. Some biologists have suggested that up to six of

  these subspecies should be classified as different species.

  (a) Explain how different subspecies of giraffe may have evolved from a

  common ancestor.

  Use information from the passage in your answer. (5)

  1. No interbreeding / gene pools are separate / geographic(al) isolation;

  Accept: reproductive isolation as an alternative to

  no interbreeding.

  2. Mutation linked to (different) markings/colours;

  3. Selection/survival linked to (different)

  markings/colours;

  4. Adapted organisms breed / differential reproductive

  success;

  Note: 'passed on to offspring' on its own is not

  sufficient for reproduction.

  5. Change/increase in allele frequency/frequencies;
• Biologists compared the mitochondrial DNA of the different subspecies of

  giraffe. They used the results from comparing this DNA to conclude that six

  of the nine subspecies are separate species.

  Suggest how they came to this conclusion. (2)

  1. (Compare DNA) base sequence / base

  pairing / (DNA) hybridisation;

  Ignore: compare chromosomes / 'genetic make-up'.

  Accept: (compare) genes / introns / exons.

  Note: reference to only comparing alleles is 1 max.

  2. Different in six (species) /different in different

  species / similar in three (subspecies) /similar in

  same species/subspecies;

  Ignore: compare chromosomes / 'genetic make-up'.

  Reject: 'same alleles/ same DNA bases in three

  species/subspecies'.

  Note: mark point 2 can be awarded without mark

  point 1
• POPULATIONS IN ECOSYSTEMS

  Lake Malawi in East Africa has more species of fish than any other lake in

  the world (line 1).

  Suggest and explain how this speciation may have occurred. (4)

  1. Variation/differences due to mutation/s;

  2. (Reference to) allopatric (speciation);

  Ignore sympatric speciation.

  3. Smaller/different lakes have different environmental conditions

  OR

  Smaller/different lakes have different selection pressures;

  Accept different populations for different lakes.

  4. Reproductive separation/isolation

  OR

  No gene flow

  OR

  Gene pools remain separate;

  5. Different alleles passed on/selected

  OR

  Change in frequency of allele/s;

  6. Eventually different species/populations cannot breed to

  produce fertile offspring;
• Loss of nutrients into Lake Malawi has resulted in a decrease in some fish

  populations (lines 12-13).

  Explain why. (4)

  1. (Growth/increase of) algae/surface plants/algal bloom blocks light;

  2. Reduced/no photosynthesis so (submerged) plants die;

  3. Saprobiotic (microorganisms) aerobically respire

  OR

  Saprobiotic (microorganisms) use oxygen in respiration;

  Accept: Saprobiont/saprophyte/ saprotroph

  Neutral: decomposer

  4. Less oxygen for fish to respire;
• The mark-release-recapture method can be used to estimate the size of a

  fish population (lines 13-14).

  Explain how. (4)

  1. Capture/collect sample, mark and release;

  2. Ensure marking is not harmful (to fish)

  OR

  Ensure marking does not affect survival (of fish);

  Accept examples e.g., marking should not be toxic.

  3. Allow (time for) fish to (randomly) distribute before

  collecting a second sample;

  4. (Population =) number in first sample × number in second

  sample divided by number of marked fish in second

  sample/number recaptured;
• Suggest why the mark-release-recapture method can produce unreliable

  results in very large lakes (lines 14-15). (1)

  Less chance of recapturing fish
• Succession occurs in natural ecosystems. Describe and explain how

  succession occurs. (4)

  1. (Colonisation by) pioneer species;

  2. Pioneers/species/organisms change the

  environment/habitat/conditions/factors;

  Accept example of change e.g. forms soil/humus/organic

  matter/nutrients.

  Must convey idea of change being caused by

  pioneers/species/organisms

  3. (Environment becomes) less hostile for other/new species

  OR

  (Environment becomes) more suitable for other/new species

  OR

  (Environment becomes) less suitable for previous species;

  Accept previous species out-competed.

  4. Change/increase in diversity/biodiversity;
• The sundew is a small flowering plant, growing in wet habitats such as bogs and

  marshes. The soil in bogs and marshes is acidic and has very low concentrations

  of some nutrients. The sundew can trap and digest insects.

  (a) Describe how you could estimate the size of a population of sundews in a

  small marsh. (5)

  1. Use a grid

  OR

  Divide area into squares/sections;

  Accept use of tape measures/map/area with coordinates.

  Accept Belt transect.

  2. Method of obtaining random coordinates/numbers e.g.

  calculator/computer/random numbers table/generator;

  If transect method used accept quadrats at regular

  intervals or current mark point 2.

  3. Count number/frequency in a quadrat/section;

  Accept % cover in quadrat/section.

  Ignore amount/abundance.

  4. Large sample and calculate mean/average number (per

  quadrat/section);

  Accept large sample and calculate mean %.

  Accept large sample and method of calculating mean.

  Accept many/multiple for large sample but ignore several.

  If a specific number is given it must be 10 or more.

  5. Valid method of calculating total number of sundews, e.g.

  mean number of plants per quadrat/section/m2 multiplied by

  number of quadrats/sections/m2 in marsh;

  Do not allow 'scale up' without further qualification.

  Do not award if % cover determined.
• Suggest and explain how digesting insects helps the sundew to grow in soil

  with very low concentrations of some nutrients. (2)

  1. Digestion/breakdown of proteins;

  2. Provides amino acids

  OR

  (Sundew can) produce a named (organic) nitrogen-containing

  compound e.g. proteins, amino acids, DNA, ATP;

  Ignore if nitrate or ammonium ions given as products.

  3. Digestion/breakdown of named (organic) phosphate-containing

  compound e.g. DNA, RNA;

  4. Provides named (organic) phosphate-containing product e.g. nucleotides

  OR

  (Sundew can) produce a named phosphate-containing

  compound e.g. ATP, DNA;

  Accept phosphate as a named product.
• Describe how you would determine the mean percentage cover for beach

  grass on a sand dune.(3)

  1. Method of randomly determining position (of quadrats) e.g. random

  numbers table/generator;

  Ignore line/belt transect

  2. Large number/sample of quadrats;

  Accept many/multiple

  Ignore point quadrat

  If a specified number is given, it must be 20 or more

  3. Divide total percentage by number of quadrats/samples/readings;
• Q5.

  Dengue is a serious disease that is caused by a virus. The virus is carried from

  one person to another by a mosquito, Aedes aegypti. One method used to try to

  reduce transmission of this disease is the Sterile Insect Technique (SIT). This

  involves releasing large numbers of sterile (infertile) male A. aegypti into the

  habitat. These males have been made infertile by using radiation.

  (a) Explain how using the SIT could reduce transmission of dengue (2)

  1. Compete (with fertile males) to mate / for food / resources

  OR

  intraspecific competition;

  2. Do not reproduce / breed

  OR

  Reduces population (of mosquitoes);

  1. Must convey idea of competition.

  2. Accept: 'fewer mosquitoes' / 'fewer offspring'.
• The release of radiation-sterilised A. aegypti has not been very successful

  in controlling the transmission of dengue.

  Suggest one reason why. (1)

  (Radiation) affects their 'attractiveness' / courtship / survival / life

  span;

  Accept: 'die / less likely to survive due to radiation'.

  Accept: 'disease can be transmitted by other

  means' (other than mosquitoes).
• The stream eventually recovered to reach a climax community.

  Give two features of a climax community (2)

  1. Same species present (over long time) / stable community (over long

  time);

  2. Abiotic factors (more or less) constant (over time)

  3. Populations stable (around carrying capacity)
• Describe how you would investigate the effect of an invasion by a

  non-native species of plant (a biotic environmental factor) over many years

  on the abundance of a native species of plant in a community. (3)


  1. Set up grid system with coordinates;

  2. Place large number of quadrats (at coordinates) selected at random;

  3. Count number of / estimate percentage cover of native plant in

  quadrats;

  3. Repeat at same time each year (for many years);
• Q8.

  In northern India, there is a conflict of interests between farmers of livestock (eg

  cows) and people trying to conserve ibex (a type of wild goat).

  When livestock are given extra food, their populations can grow too large and

  compete with ibex.

  (a) Name the type of competition between livestock and ibex. (1)


  Interspecific (competition);