Heredity: every key term you need (+ practice quiz)
88 flashcard terms for AP Biology Unit 5, written to match the course framework. Read them here, drill them as flashcards, or take the 47-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.
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).
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.
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.
Product Rule for Dihybrids
For AaBb × AaBb, P(aabb) = 1/4 × 1/4 = 1/16; treat each gene as an independent monohybrid cross and multiply.
Sum Rule Example
P(homozygous at gene A) = P(AA) + P(aa) = 1/4 + 1/4 = 1/2, because the outcomes are mutually exclusive.
Chi-Square Formula
χ² = Σ (observed − expected)² / expected. Degrees of freedom = number of phenotype classes − 1; compare to critical value at p = 0.05.
Interpreting Chi-Square
If χ² is less than the critical value (3.84 for 1 df, 7.82 for 3 df), fail to reject the null — the data fit the expected ratio.
9:3:3:1 Modifications by Epistasis
Recessive epistasis gives 9:3:4 (Labrador coat), dominant epistasis gives 12:3:1; both indicate one gene masks another.
Map Units (Centimorgans)
1 map unit = 1% recombination frequency; genes 50+ map units apart assort as if unlinked because crossovers between them are near-certain.
Three-Point Cross Logic
The rarest recombinant class results from double crossovers, revealing which gene is in the middle.
Complementation Test
Crossing two recessive mutants: wild-type offspring means the mutations are in different genes; mutant offspring means the same gene.
X Inactivation (Barr Body)
In female mammals one X is randomly silenced in each cell early in development, producing mosaics such as calico cats.
Hemizygous
Males have only one allele for X-linked genes, so any recessive X-linked allele is expressed — colorblindness and hemophilia are far more common in males.
Nondisjunction Timing
Failure in meiosis I yields all four gametes abnormal (two n+1, two n−1); failure in meiosis II yields two normal, one n+1, one n−1.
Chromosomal Structural Mutations
Deletion, duplication, inversion, and translocation alter chromosome structure; a Philadelphia chromosome (9;22 translocation) causes chronic myelogenous leukemia.
Norm of Reaction
The range of phenotypes a single genotype can produce across environments — e.g., hydrangea flower color varies with soil pH.
Maternal Inheritance Pattern
Mitochondrial disorders pass from an affected mother to all her children; affected fathers pass them to none.
Genomic Imprinting
Some genes are expressed only from the maternal or paternal copy because of methylation; Prader-Willi and Angelman result from the same deletion on different parental chromosomes.
Meiosis vs. Mitosis Outcome
Mitosis: 2 identical diploid cells; meiosis: 4 genetically distinct haploid cells via crossing over and independent assortment (2ⁿ combinations, 2²³ ≈ 8.4 million in humans).
With n homologous pairs, meiosis can produce 2^n chromosomally different gametes from orientation alone — 2^23 ≈ 8.4 million in humans, before crossing over is counted.
Chromosomal Basis of Segregation
Mendel's law of segregation is the physical result of homologs separating at anaphase I; the law of independent assortment reflects their random metaphase I orientation.
Synapsis and the Chiasma
During prophase I homologs pair along their lengths, and crossing over at chiasmata physically exchanges segments, creating recombinant chromatids not seen in either parent.
Random Fertilization
Any of ~8 million egg types can be fertilized by any of ~8 million sperm types, producing over 70 trillion zygote combinations even before crossing over is included.
Incomplete Dominance
Heterozygotes show an intermediate phenotype, as in pink snapdragons from red and white parents. The F2 ratio is 1:2:1 phenotypically as well as genotypically.
Codominance
Both alleles are fully and separately expressed in the heterozygote — MN blood type and roan coat color show both products, not a blend.
Multiple Alleles: ABO
Three alleles circulate in the population (I^A, I^B, i). I^A and I^B are codominant with each other and both dominant to i, giving four blood-type phenotypes.
Pleiotropy
One gene affects several apparently unrelated traits — sickle-cell allele influences blood cell shape, pain crises, spleen function, and malaria resistance.
Polygenic Inheritance
Several genes contribute additively to one trait, producing a continuous bell-shaped distribution such as human height or skin pigmentation.
Penetrance and Expressivity
Penetrance is the fraction of genotype carriers showing any phenotype; expressivity is how strongly it appears. Both explain pedigrees that seem to skip carriers.
Test Cross Logic
Crossing an unknown dominant-phenotype individual with a homozygous recessive reveals the unknown genotype: any recessive offspring proves the parent was heterozygous.
Reading Pedigrees for Mode
Two unaffected parents with an affected child means recessive. If affected individuals are mostly male and inheritance passes through carrier mothers, suspect X-linked recessive.
Lethal Alleles
Some alleles kill homozygotes, so a heterozygote x heterozygote cross gives a 2:1 ratio instead of 3:1 — as with the yellow coat allele in mice.
Trisomy 21 and Maternal Age
Nondisjunction of chromosome 21 produces three copies. Its frequency rises with maternal age because oocytes remain arrested in prophase I for decades.
Karyotype Analysis
Stained, ordered metaphase chromosomes reveal number and gross structure, detecting aneuploidy and large deletions, translocations, or inversions — but not single-base mutations.
ZW Sex Determination
In birds and some other animals the female is ZW and the male ZZ, so sex-linked inheritance patterns are the mirror image of the mammalian XY system.