AP Biology Units 5–6 — heredity and gene expression

How traits pass between generations, and how a gene gets read once it has arrived. Five questions from each unit.

The heredity five are all worked rather than recalled. A test cross, where the point is that a heterozygote crossed with a recessive gives one to one and not three to one. A family described in words and drawn as a pedigree — an affected man whose sons are all unaffected, and whose daughters each later have an affected son — which is the pattern that identifies X-linked recessive, because a father gives his son a Y. Non-disjunction and what it produces. An incomplete-dominance cross where no red offspring are possible at all, since the white parent has no red allele to give. And two cuttings from one plant grown in different soils, which is the cleanest demonstration there is that a phenotype is not a genotype.

The expression five run from transcription and translation through a silent mutation, alternative splicing, and the lac operon, to a question about why a gene is on in one tissue and off in another.

That last question is the one the free-response section is made of. A gene is transcribed in liver cells and silent in muscle. Two explanations fit: the muscle cells lack the transcription factors the promoter needs, or the gene has been methylated. The observation predicts neither over the other. What separates them is stripping the methylation and supplying nothing else — if transcription starts, the factors were there all along.

Every pedigree, cross and organism here is invented, and every pedigree is set out in sentences, the X-linked one drawn as well. The real paper uses diagrams and gels heavily.

Zestly is an independent study tool. It is not affiliated with the College Board, which owns the AP examinations, and it is not a test center.

• Work out the offspring ratio of a described cross, including a test cross and a cross showing incomplete dominance
• Identify X-linked recessive inheritance from a family described in words, and say why autosomal dominant and Y-linked are excluded
• Explain what non-disjunction produces and why the outcome is not always trisomy
• Distinguish genotype from phenotype using a case where one genotype gives two phenotypes
• State where transcription and translation occur in a eukaryote and what each produces
• Classify a point mutation from its effect on the amino acid sequence, and say what alternative splicing makes possible
• Apply the operon model to a described situation, and design an experiment separating two explanations of a silenced gene

In an invented family, a man with a rare condition and an unaffected woman unrelated to him have two sons and two daughters, none of whom is affected. Both daughters later have an affected son of their own. No woman anywhere in the family has ever been affected. Which pattern of inheritance fits best?