9700 · 16.2
The roles of genes in determining the phenotype flashcards
Revision flashcards for Cambridge 9700 The roles of genes in determining the phenotype (syllabus 16.2). Flip, recall, then mark a real past-paper question.
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What is the difference between genotype and phenotype?
**Genotype** is the genetic constitution of an organism (the set of alleles it possesses). **Phenotype** is the observable characteristics of an organism, resulting from the interaction between its genotype and the environment.
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Outline the pathway from a gene to a phenotypic trait.
A gene is transcribed into mRNA, which is then translated into a polypeptide (protein). The protein's function (e.g., as an enzyme, structural protein) determines a specific trait, thus contributing to the overall phenotype.
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Define phenotypic plasticity and provide an example.
The ability of one genotype to produce more than one phenotype when exposed to different environmental conditions. For example, *Hydrangea* flowers changing colour from pink to blue based on soil pH.
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What is the definition of epigenetics?
The study of heritable changes in gene expression that do not involve changes to the underlying DNA sequence. These changes can be influenced by the environment and can be passed on to daughter cells.
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How does DNA methylation affect gene expression?
The addition of a methyl group ($CH_3$) to cytosine bases, typically in promoter regions (CpG islands). Increased methylation (hypermethylation) usually leads to gene silencing by blocking transcription factor binding and promoting chromatin condensation.
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How does histone acetylation affect gene expression?
The addition of acetyl groups to histone tails by histone acetyltransferases (HATs). This neutralises the positive charge on histones, loosening the chromatin structure (forming euchromatin). This makes the DNA more accessible for transcription, leading to gene activation.
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What is the effect of histone deacetylation on gene expression?
The removal of acetyl groups from histone tails by histone deacetylases (HDACs). This restores the positive charge, causing chromatin to become more tightly packed (forming heterochromatin). This makes the DNA less accessible for transcription, leading to gene repression.
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Give an example of an environmental factor influencing epigenetic marks.
Diet is a key factor. In Agouti mice, a maternal diet rich in methyl donors (like folic acid) can increase methylation of the agouti gene in offspring, leading to a brown coat colour and reduced risk of obesity, compared to the yellow coat and obesity-prone phenotype of unmethylated offspring.