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A measured characteristic can reflect both genes and environment. Choose representative samples and compare molecular evidence without claiming that one short sequence describes an entire genome.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Design random sampling to reduce bias
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Random sampling is intended chiefly to reduce:
Selection bias. Representative selection is different from instrument precision.
After watching 2: Standard deviation describes:
Spread of observations around a mean. It is a measure of dispersion.
What you'll be able to do
Choose individuals through a procedure that gives eligible members an appropriate chance of selection, such as random coordinates or random identifiers. Define the population and sampling frame before collecting data. Selecting only the largest leaves or easiest-to-reach organisms introduces observer bias.
A larger sample can improve estimation but does not automatically remove systematic bias. Replication across genuinely independent organisms or sites matters; repeatedly measuring one leaf is not the same as sampling many leaves. Keep relevant environmental and developmental factors comparable where the question concerns genetic differences.
A mean summarises central tendency, while standard deviation describes the spread of observations around that mean. Two samples can have the same mean and different variability, or different means with considerable overlap. The range uses only the extremes and can be strongly affected by outliers.
Interpret means and standard deviations and follow this board’s mathematical requirements. Error bars must be identified as SD, standard error or confidence intervals before interpretation. Overlap of SD bars alone is not a definitive significance test of a difference between means.
DNA or mRNA base sequences and amino-acid sequences of homologous proteins can help investigate diversity. Use aligned, comparable regions and count differences consistently. Greater similarity can support closer relationships, but conclusions depend on the region, sampling and the underlying evolutionary model.
Phenotypes can differ because of environment as well as genotype, while the degenerate code means DNA differences need not always change an amino-acid sequence. Direct DNA comparison therefore reveals information that a visible trait or one protein sequence can miss. Conversely, a matching short DNA fragment does not establish identity of the entire genome.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.