AP Syllabus focus:
‘Autosomal, genetically linked, and sex-linked inheritance patterns can often be predicted from data, including family pedigrees.’
Inheritance patterns describe how traits pass through families. AP Biology emphasises recognising autosomal versus sex-linked patterns and using pedigree evidence to infer likely genotypes and predict risk in offspring.
Core idea: tracking alleles through pedigrees
A pedigree is a family diagram showing phenotypes across generations; it helps infer whether a trait is autosomal or sex-linked, and whether it is likely dominant or recessive.
Practice Questions
FAQ
If some individuals with the dominant genotype do not show the phenotype, the trait may appear to “skip” generations.
This can lead to misclassification unless multiple relatives and confirmed genotypes are considered.
Females can be affected if they inherit two recessive alleles, for example:
an affected father and a carrier mother
an affected mother and an affected father
Skewed X-inactivation can also influence severity.
Clear male-to-male transmission (affected father and affected son) strongly indicates the gene is not on the X chromosome.
It suggests autosomal inheritance or Y-linkage (if only males are affected).
Random X-inactivation can create mosaic expression in heterozygous females.
If inactivation is skewed, a “carrier” female may show mild or even substantial symptoms, complicating phenotype-based genotype calls.
Y-linked traits show strict father-to-son transmission with no affected females.
Male-limited autosomal traits can appear only in males but do not require father-to-son transmission; unaffected fathers can have affected sons if the allele is inherited autosomally.
