Biology 9700/52 — May/June 2012

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Planning · Analysis, Conclusions and Evaluation

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

Hybridisation is used by plant breeders to develop new varieties of crop and garden plants.
A student investigated the ability of one species of grass, species A, to hybridise with two other species of grass, B and C by studying the chromosomes of the hybrids.

The diploid number of chromosomes for each species was:

• species A 12
• species B 20
• species C 14

The main steps in the procedure used by the student are described below.

The undeveloped anthers of species A were removed from 40 immature flowers.
Each of the flowers (still attached to the plant) was placed inside a separate plastic bag.
When the flowers of species A were mature, pollen from species B was transferred onto the stigmas of 20 flowers of species A.
Step 3 was repeated, transferring pollen from species C to the stigmas of the other 20 flowers of species A.
Each of the flowers of species A was then replaced inside its plastic bag.
All the fruits formed by species A were collected and the embryo plants were removed.
The number of chromosomes in the cells of the embryo plants was counted.

(a)

(i)

Identify the independent and dependent variables in this investigation.

independent variable ______
dependent variable ______

(ii)

State the variable being controlled by the use of plastic bags.

(b)

(i)

Suggest a method by which the student could have obtained and transferred the pollen from species B to species A.

(ii)

Identify one potential safety issue with the experimental method you have described.
State a suitable precaution.

(c)

The fruits of the grass species used by the student are similar in structure to maize fruits.

In steps 6 and 7 the student collected the fruits formed by species A in order to count the chromosomes in the cells of the embryos.

Suggest a procedure by which the student could:

• obtain cells of the embryo plant in which chromosomes would be present
• make the chromosomes visible so that they could be counted.

Outline the main stages of this procedure.

(d)

Table 1.1 shows the results that the student obtained from the investigation.

Table 1.1

number of chromosomes in the embryo plants
hybrid of species A and species B	hybrid of species A and species C
16	26

Explain the results of the student’s investigation as shown in Table 1.1.

(e)

In a second investigation, the student repeated the procedure to hybridise the grasses.

• The fruits were collected and 50 of each hybrid were planted.
• The number of fruits that germinated was counted.
• Pollen was collected from the flowers of the hybrid grass plants that grew from the fruits.
• The chromosomes were counted in a random sample of 30 pollen grains from each hybrid.

Table 1.2 shows the results of this investigation.

Table 1.2

number of fruits that germinated
hybrids from the crosses between A and B	hybrids from the crosses between A and C
35	46

number of chromosomes in the pollen grains
6	6	8	7	10	13	13	13	13	13
16	8	16	9	0	13	13	13	13	13
8	8	16	6	6	13	13	13	13	13
6	10	8	16	9	13	13	13	13	13
13	12	16	8	6	13	13	13	13	13
8	8	16	6	10	13	13	13	13	13

(i)

The student also counted the number of chromosomes in the female gametes of each hybrid.

Predict the number of chromosomes you would expect the student to find in the female gametes of each hybrid.

hybrid from the cross A $\times$ B ______
hybrid from the cross A $\times$ C ______

(ii)

The student concluded that hybrids from the cross of A $\times$ C could be bred successfully but hybrids from the cross of A $\times$ B could not be bred successfully.

State the evidence in Table 1.2 that supports this conclusion.

(f)

The student carried out a further investigation to find out if the difference in the germination of the two hybrid grasses was significant. The student counted the number of fruits that germinated from eleven samples of 50 fruits of each hybrid.

The student collated all the data in order to calculate the mean, mode and median of the fruit germination of both hybrids.

Table 1.3 shows the data.

Table 1.3

sample number	number of fruits germinated
	hybrid A $\times$ B	hybrid A $\times$ C
1	30	41
2	31	42
3	32	44
4	32	45
5	34	45
6	34	46
7	34	47
8	35	48
9	36	48
10	38	49
11	37	48
mean	34
mode	34
median	34

The student recorded the calculations for the hybrid from the cross of A $\times$ B in Table 1.3.

(i)

Complete Table 1.3 by writing in the values of the mean, mode and median for the hybrid from the cross of A $\times$ C.

(ii)

State a reason why the student used the $t$ -test to find out if the difference in germination of the two hybrids was significant.

(iii)

State a null hypothesis for this test.

(iv)

State why the student used 20 degrees of freedom for this test.

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Q2Planning · Analysis, Conclusions and Evaluation9M

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number of chromosomes in the pollen grains
6	6	8	7	10	13	13	13	13	13
16	8	16	9	0	13	13	13	13	13
8	8	16	6	6	13	13	13	13	13
6	10	8	16	9	13	13	13	13	13
13	12	16	8	6	13	13	13	13	13
8	8	16	6	10	13	13	13	13	13

number of chromosomes in the pollen grains
6	6	8	7	10	13	13	13	13	13
16	8	16	9	0	13	13	13	13	13
8	8	16	6	6	13	13	13	13	13
6	10	8	16	9	13	13	13	13	13
13	12	16	8	6	13	13	13	13	13
8	8	16	6	10	13	13	13	13	13

number of chromosomes in the pollen grains
6	6	8	7	10	13	13	13	13	13
16	8	16	9	0	13	13	13	13	13
8	8	16	6	6	13	13	13	13	13
6	10	8	16	9	13	13	13	13	13
13	12	16	8	6	13	13	13	13	13
8	8	16	6	10	13	13	13	13	13