Unit 5 · Heredity

Meiosis ● (core concept) — The process that ensures the formation of haploid gamete cells (daughter cells) in sexually reproducing diploid organisms.

Haploid ● (core concept) — Having one set of chromosomes (1n); gametes are haploid.

Diploid ● (core concept) — Having two sets of chromosomes; somatic cells of sexually reproducing organisms are diploid.

Gamete ● (core concept) — A haploid reproductive cell formed by meiosis.

Homologous chromosome ● (core concept) — A chromosome pair member — one maternal, one paternal — that pair up and align together during meiosis I.

Sister chromatid ● (core concept) — One of the two identical copies of a duplicated chromosome, connected to its sister at the centromere. In anaphase I sister chromatids remain attached while homologous chromosomes separate; in anaphase II the sister chromatids are pulled apart.

Centromere ● (core concept) — The region where two sister chromatids are connected to each other. In anaphase II, proteins at the centromeres break down so the sister chromatids can be pulled apart toward opposite poles.

Meiotic spindle ● (core concept) — The spindle apparatus of microtubule fibers that forms during meiosis to move chromosomes: spindle fibers pull homologous chromosomes toward opposite poles in anaphase I, and pull apart sister chromatids attached at the centromere in anaphase II.

Meiosis I ● (core concept) — The first meiotic division: in prophase I homologous chromosomes pair, synapsis occurs and chiasmata may form; in metaphase I homologous pairs align at the metaphase plate; in anaphase I homologs separate while sister chromatids stay attached; telophase I yields two haploid daughter cells.

Meiosis II ● (core concept) — The second meiotic division: sister chromatids connected at the centromere attach to the spindle, align at the metaphase plate, and are pulled apart in anaphase II, producing four haploid daughter cells, each with an unduplicated chromatid.

Synapsis ● (core concept) — The pairing of homologous chromosomes during prophase I of meiosis.

Chiasma ● (core concept) — Structures that may form between paired homologous chromosomes during synapsis, where crossing over occurs.

Metaphase plate ● (core concept) — The equatorial plane of the cell along which homologous pairs (meiosis I) or chromosomes (meiosis II) align.

Kinetochore ● (core concept) — The structure on each chromatid that attaches to a microtubule extending from the poles during metaphase II.

Cytokinesis ● (core concept) — Division of the cytoplasm — via a cleavage furrow in animal cells or a cell plate in plant cells — that completes each meiotic division, yielding two haploid daughter cells at the end of meiosis I and four haploid daughter cells at the end of meiosis II.

Mitosis and meiosis ● (core concept) — Mitosis and meiosis are similar in their use of a spindle apparatus to move chromosomes, but differ in the number of cells produced and the genetic content of the daughter cells.

Crossing over ● (core concept) — During prophase I of meiosis, non-sister chromatids exchange genetic material (recombination), increasing genetic diversity among the resultant gametes.

Random assortment ● (core concept) — The independent alignment of homologous chromosome pairs during meiosis I, so each gamete receives an assortment of maternal and paternal chromosomes.

Genetic diversity ● (core concept) — Variation in genetic makeup generated by sexual reproduction in eukaryotes: crossing over, random assortment of chromosomes during meiosis, and subsequent fertilization of gametes. Genetic diversity is crucial to the survival of a species.

Nondisjunction ● (core concept) — Incorrect separation of homologous chromosomes in meiosis I or sister chromatids in meiosis II, producing gametes that are no longer haploid.

Fertilization ● (core concept) — The fusion of two haploid gametes, restoring the diploid number of chromosomes and increasing genetic variation by creating new combinations of alleles in the zygote.

Allele ● (core concept) — A variant form of a gene.

Law of segregation ● (core concept) — Mendel's law stating that the two alleles of a gene separate during gamete formation.

Law of independent assortment ● (core concept) — Mendel's law stating that genes on different chromosomes assort independently during gamete formation.

Monohybrid cross ● (core concept) — A cross analyzing a single-gene trait, used to determine whether alleles are dominant or recessive.

Dihybrid cross ● (core concept) — A cross analyzing two gene traits at once.

Test cross ● (core concept) — A cross of an individual of unknown genotype with a homozygous recessive individual, used to reveal the unknown genotype.

Genotype ● (core concept) — The set of alleles inherited for one or more genes by an individual organism; can be homozygous or heterozygous for each gene.

Phenotype ● (core concept) — The observable expression of inherited traits.

Homozygous ● (core concept) — Having two identical alleles for a gene.

Heterozygous ● (core concept) — Having two different alleles for a gene.

Dominant allele ● (core concept) — An allele whose phenotype is expressed even when paired with a different allele.

Recessive allele ● (core concept) — An allele whose phenotype is masked when paired with a dominant allele.

Punnett square ● (core concept) — A diagram used to predict the genotypes and phenotypes of parents and offspring in a cross.

Pedigree ● (core concept) — A family-tree chart used to predict patterns of inheritance (autosomal, genetically linked, sex-linked) and whether an allele is dominant or recessive.

Zygote ● (core concept) — The diploid cell formed when two haploid gametes fuse during fertilization; it carries new combinations of alleles from both parents, increasing genetic variation in the population.

Rules of probability ● (core concept) — CED equations used to analyze the passing of single-gene traits from parent to offspring: if A and B are mutually exclusive, P(A or B) = P(A) + P(B); if A and B are independent, P(A and B) = P(A) × P(B).

Chi-square test ○ — A statistical test used to determine whether observed phenotypic ratios statistically differ from the ratios predicted by Mendel's laws; it requires stating a null hypothesis and then deciding whether to reject or fail to reject it.

Null hypothesis ○ — In hypothesis testing (e.g., chi-square), the statement related to the experimental variables in question — for example, that observed offspring ratios match the predicted ratios; the test concludes by rejecting or failing to reject it.

Genetic linkage ● (core concept) — Genes located on the same chromosome, which tend to segregate together during meiosis.

Genetic mapping ● (core concept) — Using the probability that linked genes segregate together during meiosis to calculate the map distance (map units) between them on a chromosome.

Map unit ● (core concept) — The unit of genetic map distance between two linked genes on a chromosome, calculated from the probability that the genes segregate together during meiosis.

Codominance ● (core concept) — An inheritance pattern in which the phenotype of both alleles is expressed, so the heterozygote has a different phenotype than either homozygote.

Incomplete dominance ● (core concept) — An inheritance pattern in which neither allele can mask the other, so the heterozygote phenotype is a blended version of the dominant and recessive phenotypes.

Sex-linked trait ● (core concept) — Traits determined by genes on the sex chromosomes (X- or Y-linked). Their inheritance patterns can often be predicted from pedigrees.

Pleiotropy ● (core concept) — A phenomenon in which the expression of a single gene results in multiple traits or effects; these traits therefore do not segregate independently.

Non-nuclear inheritance ● (core concept) — Inheritance of traits determined by chloroplast or mitochondrial DNA, which do not follow simple Mendelian rules. In animals mitochondria are usually transmitted by the egg, and in plants mitochondria and chloroplasts are transmitted in the ovule — so such traits are typically maternally inherited.

Phenotypic plasticity ● (core concept) — The ability of an individual genotype to produce different phenotypes under different environmental conditions; environmental conditions influence gene expression.

Environmental phenotype examples ○ — Illustrative (suggested, not required) CED examples of environmental effects on phenotype: height and weight in humans, flower color based on soil pH, seasonal fur color in arctic animals, sex determination in reptiles, increased UV on melanin production, and mating-type effects on pheromone production in yeast and other fungi.