AP Biology Unit 5 Heredity: Beyond Mendel (Patterns, Environment, and Chromosomes)

Non-Mendelian genetics

Umbrella term for inheritance patterns that do not follow Mendel’s simplest rules (one gene/two alleles, complete dominance, independent assortment); Mendel’s rules are special cases within a broader framework.

Incomplete dominance

Inheritance pattern in which neither allele fully masks the other; the heterozygote shows an intermediate phenotype between the two homozygotes (often yielding a 1:2:1 phenotypic ratio in heterozygote × heterozygote crosses).

Codominance

Inheritance pattern in which both alleles are fully and distinctly expressed in the heterozygote, so both phenotypes appear at the same time (not intermediate).

Multiple alleles

Situation where a single gene has more than two allele forms in a population (though each individual still carries only two alleles—one on each homologous chromosome).

ABO blood group system

Human blood type system with three alleles (A, B, O) where A and B are codominant with each other and O is typically recessive; AB individuals express both A and B antigens on red blood cells.

Lethal allele

Allele that causes death in a particular genotype (often the homozygous state), removing that genotype from the observed living offspring and distorting expected Mendelian ratios (e.g., 2:1 among survivors).

Pleiotropy

When one gene influences multiple traits because its product participates in multiple tissues or pathways; a single mutation can cause a syndrome with seemingly unrelated effects.

Epistasis

Gene interaction where one gene masks, modifies, or enables the expression of another gene (often via pathways), causing dihybrid phenotypic ratios to deviate from 9:3:3:1.

Polygenic inheritance

Trait determination by many genes, each contributing a small (often additive) effect; commonly produces a wide range of phenotypes rather than discrete categories.

Continuous variation

Phenotypic pattern showing a spectrum (range) of values, often associated with polygenic traits and environmental influence; frequently forms a bell-shaped distribution in populations.

Penetrance

The proportion of individuals with a particular genotype who actually display the associated phenotype (helps explain “messy” pedigrees).

Expressivity

The degree or intensity with which a genotype is expressed in individuals who show the phenotype; varies among affected individuals.

Phenotype

Observable expression of traits (what you can measure or categorize), influenced by genotype and often by environmental conditions.

Genotype

An individual’s allele combination (genetic makeup) underlying a trait.

Environmental effects on phenotype

External conditions (e.g., temperature, diet, pH, stress) that influence gene expression, protein function, or development, changing the observed trait without necessarily changing the DNA sequence.

Epigenetic regulation

Heritable changes in gene expression that do not involve changes to the DNA nucleotide sequence (e.g., DNA methylation patterns); environmental factors can influence these marks.

Linked genes

Genes located close together on the same chromosome that tend to be inherited together, so they do not assort independently unless separated by crossing over.

Crossing over (recombination)

Exchange of chromosome segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis, creating new allele combinations and breaking up linkage.

Recombination frequency

Percent of offspring that are recombinant types: (number of recombinant offspring ÷ total offspring) × 100%; higher values suggest genes are farther apart, lower values suggest closer linkage.

Testcross

Cross of an individual (often heterozygous for one or more genes) with an individual homozygous recessive; used to infer genotype and detect linkage by analyzing offspring ratios.

X-linked inheritance

Inheritance of a gene located on the X chromosome; recessive X-linked traits appear more often in males (XY) because they have only one X, and affected males pass their X to all daughters but to none of their sons.

Hemizygous

Having only one copy of a gene in a diploid organism (e.g., males are hemizygous for X-linked genes), so the single allele present is expressed.

X-inactivation (Barr body)

Dosage compensation mechanism in mammals where one X chromosome in female cells is largely inactivated, forming a Barr body; can cause mosaic expression in heterozygous females for X-linked traits.

Nondisjunction

Meiotic error where homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate properly, producing gametes with extra or missing chromosomes and potentially leading to abnormal chromosome numbers after fertilization.

Mitochondrial (cytoplasmic) inheritance

Inheritance of traits encoded by mitochondrial DNA; typically maternal in animals because the egg contributes most of the cytoplasm (and mitochondria), so affected mothers can pass the trait to all offspring while affected fathers usually do not.