Continuous and discontinuous variation questions

Question 1

Studies of variation in human characteristics often compare identical twins reared apart with identical twins reared together. Evaluate this design as a way of separating the genetic contribution to a characteristic from the environmental one.

Accepted Answer

B1 identical twins have the same genotype, so any difference in phenotype between two of them must have come from the conditions they grew in rather than from their alleles B1 comparing pairs reared apart with pairs reared together changes the environment while the genotype is held constant, which is as close to a controlled experiment as human inheritance allows B1 against it: identical twins separated in infancy are rare, so the samples are very small and a few unusual pairs can move the result a long way B1 against it: twins placed for adoption are often placed in homes of a similar kind, and every pair shared a uterus before they were separated, so the two environments differ less than the phrase 'reared apart' suggests B1 a judgement: the design is the strongest human evidence that a characteristic has a genetic component, but any figure it yields applies to that population and to the range of environments actually sampled, so it is an estimate rather than a fixed proportion

Question 2

Compare continuous variation with discontinuous variation, referring to the genes involved, the effect of the environment and the graph used to display each.

Accepted Answer

B1 continuous variation is controlled by many genes acting together, called polygenes, whereas discontinuous variation is controlled by one gene or very few B1 the environment changes a continuous characteristic a great deal, whereas it has little or no effect on a discontinuous one B1 a continuous characteristic spreads over an unbroken range with no gaps, whereas a discontinuous one falls into separate classes B1 continuous data are drawn as a histogram with the bars touching, whereas discontinuous data are drawn as a bar chart with the bars separated

Question 3

The table gives the length of the longest leaf on each of eight plants of one clover species growing in a single field, constructed for this question rather than measured. Calculate the mean leaf length, and calculate the standard deviation of the eight values to two significant figures.

Accepted Answer

M1 mean = sum of the values ÷ number of values = 176 ÷ 8 A1 mean = 22.0 mm M1 the deviations from the mean are −4, −2, −1, 0, 0, 1, 2 and 4, so Σ(x − mean)² = 16 + 4 + 1 + 0 + 0 + 1 + 4 + 16 = 42 mm² A1 standard deviation = √(42 ÷ 7) = √6.0 = 2.4 mm, dividing by n − 1 because this is a sample

Question 4

Body length in one species of fish shows continuous variation. The table gives the body lengths of 200 of these fish netted from a lake on one day, constructed for this question rather than measured, and the distribution has two peaks. Suggest an explanation for the two peaks, and suggest one further measurement that would test it.

Accepted Answer

B1 within each peak the lengths run over an unbroken range with no gaps, so this is not a discontinuous characteristic falling into two distinct classes B1 the sample is likely to hold two groups whose mean lengths differ, most plausibly two age groups, since fish hatched this year have had less time to grow than fish hatched the year before; accept males and females differing in mean length B1 each group has its own continuous distribution about its own mean, and the two overlap, which is why nine fish are still recorded in the 90 to 99 mm class between the peaks A1 record the age of each fish from the growth rings on its scales, or record its sex, and plot the two groups separately: each should then give a single unbroken spread with one peak

Question 5

A characteristic is controlled by several genes, each with two alleles of equal and additive effect. With one such gene there are 3 phenotype classes, and with two such genes there are 5. Calculate the number of phenotype classes with three such genes, and calculate the number with six such genes.

Accepted Answer

M1 each additional gene adds two more possible contributing alleles, so the number of classes follows the pattern (2 × number of genes) + 1 A1 for three genes: (2 × 3) + 1 = 7 classes M1 apply the same pattern to six genes: (2 × 6) + 1 A1 = 13 classes, closer together than for three genes and easier for environmental variation to blur into an apparently continuous spread

Question 6

A characteristic controlled by four genes, each with two alleles of equal and additive effect, gives nine phenotype classes. Explain why adding more such genes makes the variation appear continuous.

Accepted Answer

B1 each extra gene adds two more possible contributing alleles, so the number of phenotype classes increases B1 the difference between one class and the next therefore becomes smaller, until it is smaller than the differences that can be measured B1 environmental variation then shifts individuals across those small gaps, so no separate classes remain and the distribution is unbroken

Question 7

Cuttings taken from one hydrangea plant are grown in acid soil and in alkaline soil, and they produce flowers of different colours. Explain what this shows about variation.

Accepted Answer

B1 the cuttings are clones, so all of them have the same genotype and any difference between them cannot be genetic B1 the difference in flower colour is therefore environmental variation, caused here by the difference in soil pH and the availability of ions that follows from it B1 the phenotype is produced by the genotype developing in a particular environment, and this environmental difference is not passed to any offspring because the alleles are unchanged

Question 8

Himalayan rabbits carry an allele for a fur-pigment enzyme that works at low temperature but not at high temperature, giving black fur on the cool extremities and white fur on the warmer trunk. A patch of white fur is shaved from a rabbit's back and an ice pack is bound over the area while the fur regrows. Explain the colour of the fur that regrows under the ice pack.

Accepted Answer

B1 the allele the rabbit carries codes for an enzyme in the pigment pathway that is only active at low temperature, so pigment is made only where the skin is cool B1 binding ice to the shaved patch lowers the local skin temperature below the point at which the enzyme works, even though the genotype at that patch is unchanged B1 the fur that regrows under the ice pack will be black, because the local environmental condition, low temperature, rather than a different genotype, determines whether the enzyme is active and pigment is made

Question 9

Two wheat plants of identical genotype are grown, one in soil given plenty of nitrogen fertiliser and one in poor soil with little nitrogen, and the well-fertilised plant grows much taller. Suggest why the two plants differ in height despite being genetically identical, and suggest what would happen to the height difference if seed from each plant were grown together in the same, well-fertilised soil.

Accepted Answer

B1 the height difference is environmental variation caused by the difference in nitrogen availability, not genetic variation, because the two plants share the same genotype B1 the phenotype reached is produced by the genotype developing under a particular set of conditions, and the conditions differ here while the genotype does not B1 grown together in the same well-fertilised soil, the height difference would disappear or become much smaller, because the environmental cause of the original difference would no longer apply and offspring inherit alleles, not the environmentally produced phenotype of a parent

Question 10

A student claims that height shows continuous variation only because of differences in diet and exercise, and has no genetic basis at all. Explain why this claim is incorrect.

Accepted Answer

B1 height is controlled by many genes, called polygenes, each contributing a small additive amount to the total, so there is a genetic basis to height in the same sense that there is for a discontinuous characteristic such as blood group B1 evidence for the genetic component is that, across a range of adequate diets, taller parents tend to have taller children on average, which would not be expected if height had no genetic basis B1 the environment adds further variation on top of the genetic component, and only within the range the genotype permits, so height is controlled by both genes and environment together rather than by environment alone

Question 11

Two genetically identical (cloned) strawberry plants are grown, one in a greenhouse and one outdoors, and the outdoor plant produces smaller fruit. Separately, the height of 50 unrelated strawberry plants of different genotypes, grown together in the same greenhouse, is found to vary continuously. Identify which difference is environmental variation and which is at least partly genetic variation, justifying each choice.

Accepted Answer

B1 the fruit-size difference between the two clones is environmental variation, because the two plants are genetically identical, so the difference cannot be caused by different alleles and must be caused by the difference in growing conditions B1 the height variation among the 50 unrelated plants is at least partly genetic variation, because the plants have different genotypes but were grown in the same conditions, so the difference cannot be attributed to environment alone B1 the logic in each case is the same: holding the genotype constant while the environment varies isolates environmental variation, and holding the environment constant while the genotype varies isolates genetic variation

Question 12

Seed coat colour in a crop plant is controlled by a single gene and falls into two sharply distinct classes regardless of growing conditions, whereas seed mass varies smoothly across a wide range and is heavier in plants grown with more water. Explain, with reference to the number of genes involved, why environmental conditions affect seed mass but not seed coat colour.

Accepted Answer

B1 seed coat colour is discontinuous, controlled by a single gene, so there is no genotype lying between the possible genotypes and therefore no phenotype lying between the two colour classes for environmental conditions to shift an individual across B1 seed mass is continuous, controlled by many genes, called polygenes, each contributing a small additive amount, giving many closely spaced classes with very small gaps between them B1 environmental conditions such as water availability can shift an individual's value within the range set by its genotype, and because the classes for a polygenic characteristic are so close together this shift is enough to move individuals across the small gaps between them, an effect too small to matter for a characteristic with only a few widely separated genotype classes

Question 13

State what is meant by discontinuous variation, and give one named example of a human characteristic that shows it.

Accepted Answer

B1 variation in which the individuals of a population fall into a small number of distinct categories, with no intermediates between them B1 ABO blood group, or another correct example such as the ability to roll the tongue

Question 14

A student measures the shell height of 90 snails from one wood, and separately compares the shell height of two different snail species. Identify which set of data shows intraspecific variation and which shows interspecific variation, and justify each choice.

Accepted Answer

B1 the 90 snails from one wood show intraspecific variation, because all the individuals measured belong to the same species B1 the comparison of two species shows interspecific variation, because the measurements come from members of different species

Question 15

State how many different genotypes are possible for the ABO blood group gene in a population, and state how many phenotypes these genotypes give rise to.

Accepted Answer

B1 six genotypes are possible: I^AI^A, I^AI^O, I^BI^B, I^BI^O, I^AI^B and I^OI^O B1 these six genotypes give four phenotypes, groups A, B, AB and O, because I^A and I^B are codominant with each other and both are dominant to I^O, so two different genotypes give group A and two give group B

Plant	Length of longest leaf / mm
1	18
2	20
3	21
4	22
5	22
6	23
7	24
8	26

Body length / mm	Number of fish
50 to 59	8
60 to 69	31
70 to 79	44
80 to 89	22
90 to 99	9
100 to 109	18
110 to 119	37
120 to 129	24
130 to 139	7