Meiosis
A specialized cell division that produces four haploid gametes from one diploid parent cell. Requires two rounds of division — meiosis I and meiosis II.
MeiosisGamete
A reproductive cell (sperm or egg) with half the chromosome number of body cells. Created by meiosis. Fuses with another gamete during fertilization.
MeiosisDiploid (2n)
A cell with two complete sets of chromosomes — one from each parent. Human body cells are diploid (2n = 46).
MeiosisHaploid (n)
A cell with one complete set of chromosomes. Human gametes are haploid (n = 23).
MeiosisHomologous chromosomes
A pair of chromosomes — one from each parent — that have the same genes in the same order but may carry different alleles.
MeiosisSister chromatids
Two identical copies of one chromosome, joined at the centromere after DNA replication. Separated during meiosis II (and mitosis).
MeiosisSynapsis
The pairing of homologous chromosomes during prophase I of meiosis. The result is called a tetrad or bivalent (four chromatids close together).
MeiosisTetrad
A structure of two paired homologous chromosomes (four chromatids total) formed during prophase I. The site where crossing over happens.
MeiosisMeiosis I
The first meiotic division. Homologous chromosomes pair up, cross over, and separate to opposite poles. Reduces chromosome number from 2n to n.
MeiosisMeiosis II
The second meiotic division. Sister chromatids separate (like mitosis). Produces four haploid cells from the two haploid cells of meiosis I.
MeiosisCrossing over
The exchange of DNA segments between homologous chromosomes during prophase I. Occurs at chiasmata. Major source of genetic variation.
VariationChiasma (plural: chiasmata)
A point where two homologous chromatids physically connect during crossing over. Visible as an X shape under the microscope.
VariationIndependent assortment
Random orientation of each homologous pair at metaphase I, so each gamete gets a random mix of maternal and paternal chromosomes. Mendel's second law.
VariationRandom fertilization
Any sperm can fertilize any egg. Combined with crossing over and independent assortment, this produces enormous genetic diversity in offspring.
VariationGenetic recombination
The production of offspring with combinations of alleles different from either parent. Results from crossing over and independent assortment.
VariationGene
A specific segment of DNA that codes for a particular protein or trait. Located at a specific position (locus) on a chromosome.
MendelianAllele
A different version of a gene. For each gene, an individual carries two alleles — one from each parent.
MendelianGenotype
The combination of alleles an individual has for a gene (e.g., AA, Aa, or aa).
MendelianPhenotype
The observable trait that results from a genotype (e.g., brown eyes). Different genotypes can produce the same phenotype.
MendelianDominant allele
An allele whose phenotype is expressed in heterozygotes. Conventionally shown with a capital letter (A).
MendelianRecessive allele
An allele whose phenotype only appears in homozygotes (two copies). Shown with a lowercase letter (a).
MendelianHomozygous
Having two identical alleles for a gene (AA or aa).
MendelianHeterozygous
Having two different alleles for a gene (Aa).
MendelianLaw of segregation
Mendel's first law: the two alleles for a gene separate during gamete formation, so each gamete carries only one allele.
MendelianLaw of independent assortment
Mendel's second law: alleles of different genes assort into gametes independently of one another (for genes on different chromosomes).
MendelianPunnett square
A grid used to predict the genotype and phenotype ratios of offspring from a genetic cross.
MendelianMonohybrid cross
A cross involving one gene. A standard heterozygous × heterozygous monohybrid cross produces a 3:1 phenotype ratio.
MendelianDihybrid cross
A cross involving two genes. A standard cross between two double heterozygotes (AaBb × AaBb) produces a 9:3:3:1 ratio.
MendelianTest cross
A cross between an individual showing the dominant phenotype (unknown genotype) and a homozygous recessive. Used to determine whether the unknown is homozygous or heterozygous.
MendelianIncomplete dominance
Heterozygotes show an intermediate phenotype between the two homozygotes (e.g., red × white snapdragons → pink).
Non-MendelianCodominance
Both alleles are fully expressed in the heterozygote (e.g., ABO blood type IA IB = AB).
Non-MendelianMultiple alleles
More than two possible alleles for a gene in a population, even though each individual still has only two. ABO blood type has three alleles: IA, IB, and i.
Non-MendelianPolygenic inheritance
A trait controlled by multiple genes. Produces continuous variation (e.g., human height, skin color).
Non-MendelianPleiotropy
When a single gene affects multiple unrelated traits. Example: sickle cell allele affects red blood cells, but also impacts spleen, heart, and resistance to malaria.
Non-MendelianLinked genes
Genes located on the same chromosome. They tend to be inherited together — violating Mendel's law of independent assortment unless crossing over separates them.
Non-MendelianSex chromosomes
Chromosomes that determine sex. In humans, females are XX and males are XY.
Sex LinkageAutosome
Any chromosome that is NOT a sex chromosome. Humans have 22 pairs of autosomes plus one pair of sex chromosomes.
Sex LinkageX-linked inheritance
Inheritance pattern of genes on the X chromosome. X-linked recessive traits (e.g., hemophilia, colorblindness) appear far more often in males.
Sex LinkageCarrier
A heterozygous individual who carries a recessive allele but doesn't show the trait. Important in X-linked inheritance — females can be carriers; XY males generally can't.
Sex LinkagePedigree
A family tree showing the inheritance of a trait across generations. Used to determine inheritance patterns and predict offspring.
Sex LinkageNondisjunction
A failure of chromosomes (or chromatids) to separate properly during meiosis. Produces gametes with extra or missing chromosomes.
ChromosomalTrisomy
Having three copies of a chromosome instead of two. Trisomy 21 causes Down syndrome.
ChromosomalEnvironmental effects on phenotype
The principle that genotype alone doesn't determine phenotype — temperature, diet, light, and other factors also play a role. Examples: Himalayan rabbit fur, hydrangea flower color.
Chromosomal