Dihybrid crosses, linkage and chi-square

Genetics questions on the AP Biology exam are rarely about a single gene with a dominant and a recessive allele. They ask for the probability of a combination of traits from a two- or three-gene cross, the distance between two linked genes from a table of offspring, a chi-square test that decides whether two genes assort independently, or the pattern behind an unusual family history. This material trains those quantitative and pattern-reading skills from topics 5.3 and 5.4 of the course.

The probability questions use the product rule: treat each gene as its own cross, then multiply. You find the chance of a double-recessive offspring from two dihybrid parents, the fraction of tall plants with white flowers from a cross in which only one parent is heterozygous for flower color, the chance of a triple-recessive offspring in a three-gene test cross, and the expected count in each class of a dihybrid test cross when the genes are unlinked.

The linkage questions start from real-looking data. From a test cross of 1,000 fruit flies with two large parental classes and two small recombinant classes, you calculate the recombination frequency and convert it to map units. From three pairwise distances you place three genes in order. From a dihybrid test cross whose counts are far from 1:1:1:1, you run the full chi-square test with the correct degrees of freedom and conclude that independent assortment is rejected — a pattern consistent with linkage, never a proof.

The remaining questions cover inheritance that departs from Mendel's model: the probability that a son of a carrier mother has an X-linked recessive trait; the reverse situation in birds, where females are ZW and so it is daughters who show a Z-linked recessive trait; codominance in the ABO blood group system, where an AB parent and an O parent can have only type A or type B children; a family history in which a disorder passes from mothers to all their children and never from fathers, the signature of mitochondrial inheritance; and pleiotropy.

The material offers a quiz and a flashcard deck. Each explanation shows the working, so you can see where a tempting wrong answer comes from, such as using 1 degree of freedom for four classes or counting only one recombinant class. The flashcards review the product rule, map units, recombination frequency, degrees of freedom, test crosses, independent assortment, codominance, pleiotropy, X-linked and ZW inheritance and mitochondrial inheritance.

• Apply the product rule to dihybrid and trihybrid crosses
• Calculate recombination frequency and map units from test-cross data and order linked genes
• Perform a chi-square test on a dihybrid test cross and interpret it in terms of linkage
• Predict offspring for X-linked and Z-linked recessive traits
• Recognize codominance, mitochondrial inheritance and pleiotropy from described data

Practice material written by Zestly, based on the College Board AP Biology course framework (topics 5.3 Mendelian genetics and 5.4 non-Mendelian genetics) and the chi-square table of the AP Biology equations and formulas sheet.