A level Biology 16.3 Gene control - Exam style question

Question

Phenotypic variation exists in natural populations. There are many causes of variation. Natural selection determines which phenotypes are advantageous.

(a) Variation in a particular characteristic can be described as either discontinuous or continuous.

Table 5.1 contains a list of statements that apply to discontinuous variation, continuous variation or both.

Complete both columns of Table 5.1. Put a tick (✓) in the box if the statement applies and leave the box empty if the statement does not apply.

Table 5.1

statement	discontinuous variation	continuous variation
often involves one gene only
environmental factors may affect gene expression
there is an additive effect of genes that contributes to the phenotype
there are distinct differences between the various forms of a characteristic

(b) There is variation in the quantity of vitamin D stored in the body.

Vitamin D has an important role in keeping bones healthy. The main storage form of vitamin D in the body is serum 25-hydroxyvitamin D (serum 25-OHD).

A study was carried out on 262 healthy women to investigate if the concentration of serum 25-OHD varied between summer and winter. The women had taken no vitamin D supplements. The age range of the women in the sample was 40 to 72 years old.

Table 5.2 shows the results of the study.

Table 5.2

group	mean concentration of serum 25-OHD /ng cm^-3	standard deviation
sampled during summer (n = 138)	32.7	7.6
sampled during winter (n = 124)	28.5	8.3
whole sample (n = 262)	30.7	8.2

Additional analysis showed that there was no significant correlation between age and serum 25-OHD concentration.

(i) Explain what is meant by standard deviation.

(ii) The t-test was used to compare the mean concentration of serum 25-OHD when sampled during the summer with the mean concentration of serum 25-OHD when sampled during the winter, as shown in Table 5.2.

Calculate the value of t using the formula provided.

\[t = \frac{| \overline{x}_1 – \overline{x}_2 |}{\sqrt{\left( \frac{s_1^2}{n_1} + \frac{s_2^2}{n_2} \right)}}\]

key to symbols:
\(\overline{x}\) = mean
\(s\) = sample standard deviation
\(n\) = sample size (number of observations)

Give your answer to four significant figures.

(iii) The critical value at the 0.0001 probability level is 3.773.

State the conclusion that can be made about the results of the study shown in Table 5.2 and explain how the result of your calculation in (b)(ii) can be used to support this conclusion.

(iv) Suggest the likely causes of variation in quantity of vitamin D stored in the body in this sample of women.

▶️ Answer/Explanation

Solution

(a)

statement	discontinuous variation	continuous variation
often involves one gene only	✓
environmental factors may affect gene expression		✓
there is an additive effect of genes that contributes to the phenotype		✓
there are distinct differences between the various forms of a characteristic	✓

Explanation: Discontinuous variation is typically controlled by a single gene (Mendelian inheritance) and shows distinct categories (like blood groups). Continuous variation is influenced by multiple genes (polygenic inheritance) and environmental factors, showing a range of phenotypes (like height). The additive effect refers to how multiple genes contribute small effects to create a continuous range.

(b)(i) Standard deviation is a measure of how spread out the data points are from the mean value.

Explanation: A small standard deviation indicates that most data points are close to the mean, while a large standard deviation shows that the data points are more spread out. It helps understand the variability in the data set.

(b)(ii) t = 4.255

Calculation:
Given:
Summer: \(\overline{x}_1 = 32.7\), \(s_1 = 7.6\), \(n_1 = 138\)
Winter: \(\overline{x}_2 = 28.5\), \(s_2 = 8.3\), \(n_2 = 124\)

Plugging into the formula:
\[t = \frac{|32.7 – 28.5|}{\sqrt{\left(\frac{7.6^2}{138} + \frac{8.3^2}{124}\right)}} = \frac{4.2}{\sqrt{(0.4186 + 0.5554)}} = \frac{4.2}{\sqrt{0.974}} = \frac{4.2}{0.9869} = 4.255\]

(b)(iii) There is a significant difference between summer and winter serum 25-OHD concentrations.

Explanation: The calculated t-value (4.255) is greater than the critical value (3.773) at the 0.0001 probability level. This means the probability that the difference occurred by chance is less than 0.01%, so we can conclude the seasonal difference is statistically significant. The higher summer values are likely due to increased sunlight exposure enabling more vitamin D synthesis.

(b)(iv) The variation is likely due to both genetic factors (like differences in vitamin D metabolism) and environmental factors (like seasonal sunlight exposure and dietary intake).

Explanation: While the study shows seasonal (environmental) effects, individual variation within each season suggests genetic differences in vitamin D absorption, metabolism, or storage. Other environmental factors could include clothing habits (skin exposure), sunscreen use, and dietary sources of vitamin D.

A level Biology 16.3 Gene control – Exam style question – Paper 4

Resources

Members

Company