Rucete ✏ AP Biology In a Nutshell
12. Non-Mendelian Genetics
This chapter explores genetic inheritance patterns that do not follow classic Mendelian laws, including linked genes, polygenic traits, nonnuclear inheritance, and environmental effects on phenotype.
Linked Genes
• Genes located close together on the same chromosome are called linked genes.
• They tend to be inherited together more often than genes on separate chromosomes.
• Crossing over during prophase I can separate linked genes, especially if they are far apart.
• The farther apart two genes are, the higher the recombination frequency.
• Recombination frequency (%) ≈ distance between genes in map units.
Sex-Linked Genes
• Genes located on sex chromosomes (especially the X chromosome) follow unique inheritance patterns.
• Males (XY) are more likely to express X-linked recessive traits because they have only one X chromosome.
• Females (XX) need two copies of the recessive allele to express the trait.
• Pedigrees showing more affected males than females often indicate sex-linked recessive traits (e.g., hemophilia, color blindness).
• X-linked dominant traits are rare but affect all daughters of affected males.
Multiple Gene Inheritance (Polygenic Inheritance)
• Some traits (e.g., height, eye color) are controlled by multiple genes acting together.
• Each dominant allele may have an additive effect on the phenotype.
• Many genotypes can lead to the same intermediate phenotype.
• These traits exhibit continuous variation in a population.
Nonnuclear Inheritance
• Mitochondria and chloroplasts contain their own DNA (mtDNA, cpDNA).
• Inheritance of these genes is maternal because the egg contributes most of the cytoplasm.
• Traits controlled by nonnuclear genes are passed from mother to offspring of both sexes.
• Males with the trait do not pass it on to their children.
• This differs from sex-linked traits, which are nuclear and can be inherited from either parent.
Phenotype = Genotype + Environment
• Environmental conditions can influence how genes are expressed.
• Example: hydrangea flower color changes with soil pH; UV light increases melanin production in humans.
• This ability of genotypes to produce different phenotypes in response to environmental conditions is called phenotypic plasticity.
In a Nutshell
Not all traits follow simple Mendelian inheritance. Linked genes, sex-linked traits, polygenic inheritance, nonnuclear DNA, and environmental factors all affect how traits are passed on and expressed. Understanding these exceptions helps explain the full complexity of heredity.