Non-Mendelian genetics: linkage, sex linkage and other patterns

What the exam asks

In Section I (Multiple Choice), expect stimulus sets featuring a dihybrid testcross table or a multi-generation pedigree. You will calculate recombination frequencies, order genes on a chromosome map, or rule out modes of inheritance using specific parent-offspring pairs.

In Section II (Free Response), especially Q1 (Interpreting and Evaluating Experimental Results), Q5 (Analyze Model), and Q6 (Analyze Data), you may be asked to:

• Calculate a recombination frequency and construct a linear linkage map.
• Justify why a cross shows genetic linkage rather than independent assortment (often alongside a chi-square test).
• Identify and explain X-linked or mitochondrial inheritance from a pedigree.
• Predict phenotypic ratios for crosses involving incomplete dominance, codominance, or sex linkage.

Core ideas

Genetic linkage and mapping

Genes located close together on the same chromosome are linked and tend to be inherited together during meiosis, violating Mendel’s law of independent assortment. Linked alleles can only be separated by crossing over between homologous chromosomes during prophase I.

In a dihybrid testcross ($AaBb \times aabb$), independent assortment produces a $1:1:1:1$ ratio ($50\%$ recombinant offspring). If two genes are linked, parental-type offspring appear in high numbers and recombinant-type offspring appear in low numbers ($< 50\%$).

Because the probability of a crossover between two loci is proportional to the physical distance between them, $1\%$ recombination frequency equals $1\text{ map unit (mu)}$ (or $1\text{ centimorgan, cM}$).

Sex-linked inheritance

In organisms with $XX/XY$ sex determination, genes on the sex chromosomes show distinct inheritance patterns:

• X-linked traits: Males ($XY$) are hemizygous—they have only one copy of X-linked genes. Any recessive allele on a male’s X chromosome ($X^aY$) is expressed in his phenotype. Fathers pass their X chromosome to all daughters ($XX$) and their Y chromosome to all sons ($XY$), so a father can pass an X-linked allele to his son.