Heredity: every key term you need (+ practice quiz)
56 flashcard terms for AP Biology Unit 5, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 20-question quiz — free, no account needed.
The passing of traits from parents to offspring through genes — the foundation of biological inheritance studied by genetics.
Gene
A segment of DNA that codes for a specific trait or product (usually a protein); the basic unit of heredity.
Allele
An alternate version of a gene (e.g., purple-flower allele vs. white-flower allele) that can produce different versions of a trait.
Genotype
The genetic makeup of an organism for a trait — the actual allele combination (e.g., Bb).
Phenotype
The observable physical or biochemical expression of a genotype (e.g., brown eyes) — the product of genotype and, sometimes, environment.
Homozygous
Having two identical alleles for a gene (e.g., BB or bb).
Heterozygous
Having two different alleles for a gene (e.g., Bb).
Dominant Allele
An allele whose trait is expressed in the phenotype whenever present, even in a single copy (heterozygous or homozygous).
Recessive Allele
An allele whose trait is only expressed in the phenotype when two copies are present (homozygous recessive); masked by a dominant allele.
Mendel's Pea Plant Experiments
Gregor Mendel's controlled crosses of pea plants (1860s) revealed predictable inheritance patterns, founding the science of genetics before DNA was even known.
Law of Segregation
Mendel's first law: the two alleles for a gene separate (segregate) during gamete formation, so each gamete carries only one allele per gene.
Law of Independent Assortment
Mendel's second law: alleles for different genes (on different chromosomes) segregate independently of one another during gamete formation.
Monohybrid Cross
A genetic cross tracking the inheritance of a single gene/trait, typically diagrammed with a Punnett square (e.g., Bb × Bb).
Dihybrid Cross
A genetic cross tracking the inheritance of two different genes/traits simultaneously, testing independent assortment (classic ratio 9:3:3:1).
Punnett Square
A grid tool for predicting the probability of offspring genotypes and phenotypes from a genetic cross between two known parental genotypes.
Test Cross
Crossing an individual with an unknown genotype (showing the dominant phenotype) with a homozygous recessive individual, to reveal whether the unknown is homozygous or heterozygous dominant.
P, F1, F2 Generations
P = parental generation; F1 = first filial (offspring) generation; F2 = second filial generation, produced by crossing F1 individuals.
Complete Dominance
One allele fully masks the other in heterozygotes, so the heterozygous phenotype is identical to the homozygous dominant phenotype.
Incomplete Dominance
Heterozygotes show a blended or intermediate phenotype between the two homozygous phenotypes (e.g., red × white flowers → pink).
Codominance
Both alleles are fully and separately expressed in the heterozygote, rather than blending (e.g., AB blood type showing both A and B markers).
Multiple Alleles
A gene with more than two possible alleles in the population (though each individual still has only two), such as the three alleles for human ABO blood type.
ABO Blood Type Genetics
Controlled by three alleles (IA, IB, i); IA and IB are codominant to each other, both dominant to i, producing blood types A, B, AB, and O.
Polygenic Inheritance
A trait controlled by multiple genes (each with potentially multiple alleles), producing a continuous range of phenotypes (e.g., human height, skin color).
Pleiotropy
A single gene that affects multiple, seemingly unrelated phenotypic traits (e.g., a mutation in one gene causing several symptoms of a genetic disorder).
Epistasis
When the expression of one gene is affected or masked by a different gene at another location — genes interacting rather than acting independently.
Sex-Linked Inheritance
Genes located on the sex chromosomes (typically the X in humans), producing inheritance patterns that differ between males and females.
X-Linked Recessive Traits
Because males (XY) have only one X chromosome, a single recessive allele on their X is expressed — making X-linked recessive disorders (e.g., colorblindness, hemophilia) far more common in males.
Carrier
A heterozygous individual who carries one recessive allele for a trait (often X-linked or autosomal) without expressing the disorder, but who can pass it to offspring.
Pedigree
A chart tracking the inheritance of a trait across generations of a family, used to determine inheritance patterns (dominant, recessive, sex-linked) in humans.
Reading Pedigrees — Autosomal Recessive
Trait often skips generations; two unaffected (carrier) parents can produce an affected child; affects males and females equally.
Reading Pedigrees — Autosomal Dominant
Trait typically appears in every generation; an affected individual usually has at least one affected parent; affects males and females equally.
Reading Pedigrees — X-Linked Recessive
Affects males far more often than females; an affected father cannot pass the trait to his sons (only daughters, as carriers).
Meiosis
A specialized type of cell division that produces four genetically unique haploid gametes from one diploid cell — the cellular basis of sexual reproduction and Mendel's laws.
Haploid vs. Diploid
Haploid (n) cells have one set of chromosomes (gametes); diploid (2n) cells have two sets, one from each parent (most body cells).
Homologous Chromosomes
A matching pair of chromosomes (one from each parent) carrying genes for the same traits at the same locations, though possibly different alleles.
Meiosis I
The first division: homologous chromosome pairs separate, reducing the chromosome number from diploid to haploid (the reductional division).
Meiosis II
The second division: sister chromatids separate, similar to mitosis, producing four total haploid cells from the two produced in Meiosis I.
Crossing Over
During prophase I of meiosis, homologous chromosomes exchange segments of DNA, creating new allele combinations and increasing genetic variation.
Synapsis & Tetrad
Homologous chromosomes pair tightly together (synapsis) during prophase I, forming a four-chromatid structure called a tetrad, where crossing over occurs.
Independent Assortment (Meiosis)
The random orientation of each homologous pair at the metaphase I plate means maternal and paternal chromosomes are distributed to gametes in random combinations.
Sources of Genetic Variation
Crossing over, independent assortment, and random fertilization (which of millions of possible sperm/egg combinations occurs) together generate immense genetic diversity in offspring.
Nondisjunction
An error in meiosis where homologous chromosomes (or sister chromatids) fail to separate properly, producing gametes with an abnormal chromosome number.
Aneuploidy
A condition of having an abnormal number of a particular chromosome (too many or too few), usually resulting from nondisjunction (e.g., trisomy 21 / Down syndrome).
Trisomy 21 (Down Syndrome)
A condition caused by nondisjunction producing three copies of chromosome 21 instead of the normal two — the most common human aneuploidy.
Chi-Square Test
A statistical test comparing observed genetic cross results to expected ratios (from Punnett squares), determining whether differences are due to chance or a real deviation.
Probability Rules — Multiplication
The probability of two independent genetic events BOTH occurring is the product of their individual probabilities (the 'AND' rule).
Probability Rules — Addition
The probability of EITHER of two mutually exclusive outcomes occurring is the sum of their individual probabilities (the 'OR' rule).
Environmental Effects on Phenotype
Some phenotypes are influenced by environment as well as genotype (e.g., plant height affected by sunlight/nutrients, or fur color in Himalayan rabbits by temperature).
Linked Genes
Genes located close together on the same chromosome tend to be inherited together, violating independent assortment unless separated by crossing over.
Recombination Frequency
The percentage of offspring showing new allele combinations due to crossing over; used to estimate the relative distance between linked genes on a chromosome.
Extranuclear (Cytoplasmic) Inheritance
Inheritance of genes located outside the nucleus (in mitochondria or chloroplasts), typically passed only from the mother since these organelles come from the egg.
Mitochondrial DNA Inheritance
Because sperm contribute negligible cytoplasm, mitochondrial DNA (and its traits/mutations) is inherited almost exclusively from the mother.
Genetic Disorders — Autosomal
Disorders caused by genes on non-sex chromosomes, affecting males and females equally (e.g., cystic fibrosis — recessive; Huntington's disease — dominant).
Genetic Counseling
Using pedigree analysis and probability to help individuals or couples understand their risk of passing on genetic disorders to offspring.
Karyotype
An organized image of an individual's complete set of chromosomes, used to detect chromosomal abnormalities like aneuploidy.
Sex Determination (Humans)
Human sex is determined by the sex chromosomes contributed at fertilization: XX produces female, XY produces male, with the Y chromosome from the father determining male development.