The roles of genes in determining the phenotype — practice questions

1. Epigenetics involves heritable changes in gene expression that occur without altering the underlying DNA sequence. This means individuals with identical genotypes (e.g., identical twins or clones) can still show phenotypic variations. [1]
2. One key mechanism is DNA methylation, where methyl groups are added to cytosine bases, particularly in promoter regions. Increased methylation generally silences genes by preventing transcription factors from binding, thus reducing or stopping gene expression. [1]
3. Another mechanism is histone modification, such as acetylation or methylation of histone tails. For example, histone acetylation loosens the chromatin structure, making DNA more accessible for transcription and 'activating' gene expression. Conversely, deacetylation condenses chromatin, repressing gene expression. [1]
4. These epigenetic marks are influenced by environmental factors (e.g., diet, stress, toxins) throughout an individual's life. Different environmental exposures lead to varying patterns of gene silencing or activation, resulting in distinct protein profiles and thus observable phenotypic differences despite identical genetic codes. [1]

(a) Calculation of percentage decrease:

1. Find the absolute decrease in expression: $\text{Decrease} = \text{Standard Expression} - \text{High-Fat Expression}$ $\text{Decrease} = 120 - 30 = 90 \text{ arbitrary units}$
2. Calculate the percentage decrease relative to the standard diet group: $\text{Percentage Decrease} = \frac{\text{Decrease}}{\text{Original Expression}} \times 100\%$ $\text{Percentage Decrease} = \frac{90}{120} \times 100\%$ $\text{Percentage Decrease} = 0.75 \times 100\% = 75\%$

Final Answer: The percentage decrease in gene expression is 75%. [2]

(b) Explanation of the relationship:

1. The data shows a correlation: as average promoter methylation increased from 15% to 65%, the relative mRNA expression decreased from 120 to 30 units. [1]
2. This occurs because increased methylation (hypermethylation) of the gene's promoter region leads to gene silencing. The methyl groups physically block transcription factors from binding to the DNA and/or recruit proteins that cause the chromatin to condense. This makes the gene inaccessible to RNA polymerase, preventing transcription and thus reducing the production of mRNA. [1]