A-Level Biology: Epistasis, Linkage and Chi-Squared

Once you can do a dihybrid cross that gives 9 : 3 : 3 : 1, A level biology starts showing you the crosses that do not. Two genes may interact so that one masks the other, or they may sit on the same chromosome and travel together. Both change the ratios, and exam questions expect you to recognise which is happening, explain it, and test the data with chi-squared. This material practises all three steps; plain monohybrid, dihybrid, codominance and Hardy–Weinberg questions are covered in the inheritance material of this category.

Epistasis is approached through biochemical pathways: if one gene codes for the enzyme that makes a yellow pigment and a second for the enzyme that turns it red, a plant that cannot make the first enzyme is colourless whatever its second gene. From that reasoning you predict and recognise the modified ratios — 9 : 3 : 4 for recessive epistasis, as in coat colour in mice, and 12 : 3 : 1 or 13 : 3 for dominant epistasis, as in fruit colour in summer squash.

Autosomal linkage is approached through test crosses. You work out what a heterozygote with completely linked genes produces, and how crossing over in prophase I creates a small number of recombinant offspring alongside the large parental classes. You identify recombinant classes from counts and distinguish autosomal linkage from sex linkage, where results differ between males and females.

The statistics tie it together. Each chi-squared item gives original counts and a critical value; you calculate expected numbers from the ratio, work out χ², use the degrees of freedom, and decide whether the data fit — for example whether a large deviation from 1 : 1 : 1 : 1 is evidence of linkage.

The material offers a multiple-choice quiz with full working in the explanations, a flashcard deck of definitions and ratios, a printable written-work sheet of eight open questions (genetic diagrams, explanations and chi-squared tests) answered by hand and checked, and an oral exam in which an examiner asks one question at a time and gives feedback at the end.

The content is based on the inheritance content of the DfE A level biology subject content, as taught in current exam-board specifications (for example AQA topic 7, inheritance). It is practice material and does not replace your own board's specification.

• Use a biochemical pathway to explain how one gene can mask the expression of another
• Predict and recognise ratios produced by recessive (9 : 3 : 4) and dominant (12 : 3 : 1, 13 : 3) epistasis
• Predict the offspring of test crosses involving autosomally linked genes
• Explain how crossing over produces recombinant offspring and identify recombinant classes
• Distinguish autosomal linkage from sex linkage
• Carry out and interpret chi-squared tests on the results of crosses

Practice material written by Zestly, based on the DfE A level biology subject content (inheritance: epistasis, autosomal linkage and the chi-squared test).