16.1 Passage of Information from Parents to Offspring

Hybridisation can be used by plant breeders to develop new varieties of crop plants.

A student investigated the ability of one species of grass, species A, to hybridise with two other species of grass, species B and species 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

Fig. 1 shows part of a grass flower stalk with several flowers.

Fig. 1

These are the main steps in the procedure used by the student:

• The anthers were removed from 40 flower stalks of species A.
• Separate plastic bags were placed around each of the flower stalks, still attached to the plant.
• Pollen from species B was transferred onto the stigmas of 20 of the flower stalks of species A.
• Pollen from species C was transferred onto the stigmas of the other 20 flower stalks of species A.
• The grains formed by fertilisation of species A, each containing a single seed, were collected.
• Hybrid embryo plants were removed from the grains and the number of chromosomes in the cells of each of the hybrid embryo plants was counted.

Identify the dependent variable in this investigation.

 [1]

(ii)

Identify the variable that was controlled by the use of plastic bags on the flower stalks of species A.

 [1]

Table 1 shows the results that the student obtained from counting the number of chromosomes in the cells of each of the hybrid embryo plants.

Table 1

hybrid embryo from cross	number of chromosomes
A × B	16
A × C	26

Explain the results of the student’s investigation, as shown in Table 1.

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

• After hybridising the grasses, 50 grains containing hybrid embryo plants were collected from each of the two crosses, A × B and A × C.
• Hybrid grass plants were grown from the grains collected.
• When the hybrid grass plants flowered, pollen containing the male gametes was collected.
• For each of the two crosses, the number of chromosomes in a random sample of 30 male gametes obtained from the pollen of the hybrid grass plants was counted.

Table 2 shows the results of this investigation.

Table 2

number of chromosomes in 30 male gametes from the hybrid grass plants produced from:
cross A × B					cross A × C
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

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

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

[1]

(ii)

The student concluded that hybrids from cross A × B could not be bred successfully.

State one piece of evidence in Table 2.2 that supports this conclusion.

[1]

The student carried out a third investigation to find out if the difference in the number of hybrid grains germinating from the two crosses was significant. The student counted the number of grains that germinated from 11 samples of 50 grains from each cross.

Table 3 shows the results of the investigation.

Table 3

sample number	number of grains that germinated
	cross A × B	cross A × C
1	30	41
2	31	43
3	32	45
4	32	45
5	34	46
6	34	46
7	34	47
8	36	48
9	36	48
10	38	49
11	37	48
mean	34
mode	34
median	34

The student calculated the mean, mode and median for the data from the cross of A × B and recorded these in Table 3.

Complete Table 3 by writing in the values of the mean, mode and median for the results from cross A × C.

[2]

(ii)

The student used the t-test to compare the means.

State one feature of the data that allows use of the t-test.

[1]

(iii)

State a null hypothesis for this t-test.

 [1]

(iv)

State why the number of degrees of freedom for this t-test is 20.

 [1]

Fig. 1 shows the life cycle of a fern plant.

Fig. 1

Explain what is indicated by the term n in Fig.1

Name the processes taking place at the stages marked A and B in Fig. 1

Describe the events at stage C in Fig. 1

Explain the importance of the process taking place at the point marked A in Fig. 1