GENETICS, POPULATIONS, EVOLUTION & ECOSYSTEMS

Cards (31)

  • 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);