BIOL310 CH 2: Transmission Genetics & CH 3: Chromosome Heredity

Glossary-style vocabulary flashcards covering key terms in transmission genetics and chromosome heredity from the notes.

Allele

Variant form of a gene; a gene typically has two alleles in a diploid individual.

Homozygous

Having identical alleles for a given gene (e.g., AA or aa).

Heterozygous

Having two different alleles for a given gene (e.g., Aa).

Dominant allele

An allele that masks the effect of the other allele in the phenotype when present.

Recessive allele

An allele whose phenotype is masked by a dominant allele in a heterozygote.

Genotype

The genetic makeup of an individual; the combination of alleles (e.g., AA, Aa, aa).

Phenotype

The observable trait or trait signature of an organism.

Pure-breeding (true-breeding)

Plants that, when selfed or crossed, produce offspring identical to the parent.

Monohybrid cross

Cross involving only one gene; often yields a 3:1 phenotypic ratio in F2.

Dihybrid cross

Cross involving two genes; typically yields a 9:3:3:1 phenotypic ratio in F2.

Trihybrid cross

Cross involving three genes; yields eight possible gametes (2^3) for the F1.

Reciprocal cross

Crosses where the sexes of the parents are reversed to test maternal or paternal effects.

Test cross

Cross between an unknown genotype and a homozygous recessive to reveal its genotype.

Replicate cross

Repetition of a cross to obtain many offspring for reliable data.

Phenotypic ratio 3:1

Common ratio observed in F2 for a monohybrid cross; dominant to recessive.

Genotypic ratio 1:2:1

Ratio of genotypes (e.g., AA:Aa:aa) in F2 of a monohybrid cross.

Punnett square

A grid used to predict the genotype and phenotype combinations in a cross.

Gamete

A haploid reproductive cell (sperm or egg) carrying one allele from each gene.

Zygote

Diploid cell formed by the fusion of gametes during fertilization.

Meiosis

Cell division producing four genetically diverse haploid gametes; reduces chromosome number.

Mitosis

Cell division that yields two genetically identical diploid daughter cells.

M-phase

The mitotic phase where cell division occurs (mitosis and cytokinesis).

Interphase

Part of the cell cycle when the cell grows and DNA is replicated; not part of M phase.

G1 phase

First gap phase; cell growth and initial gene expression.

S phase

DNA synthesis phase; replication of chromosomes.

G2 phase

Second gap phase; preparation for cell division.

G0 phase

Quiescent state; non-dividing cells may enter G0 and not progress.

Mitosis stages

Prophase, prometaphase, metaphase, anaphase, telophase; followed by cytokinesis.

Prophase I

Meiosis I stage where chromosomes condense, and homologs pair (synapsis) begins.

Leptotene

Meiosis I substage when chromosomal condensation starts.

Zygotene

Meiosis I substage with initiation of synapsis between homologs.

Pachytene

Meiosis I substage where crossing over occurs between nonsister chromatids.

Diplotene

Meiosis I substage where homologs separate slightly and chiasmata are visible.

Diakinesis

Meiosis I substage with maximally condensed chromosomes and spindle formation.

Metaphase I

Homologous chromosome pairs align at the metaphase plate; crossing over completed.

Anaphase I

Homologous chromosomes separate to opposite poles; sister chromatids stay together.

Telophase I

Nuclei reform; cytokinesis may divide cytoplasm; results in haploid cells.

Meiosis II

Second division: separates sister chromatids to form four haploid gametes.

Sister chromatids

Identical copies of a replicated chromosome held together at the centromere.

Kinetochore

Protein structure at centromere where microtubules attach to pull chromosomes.

Centrosome

Organizes the spindle poles; origin of spindle fibers.

Spindle apparatus

Microtubule-based structure that separates chromosomes during cell division.

Cohesin

Protein complex that holds sister chromatids together until anaphase.

Chiasmata

Visible site of crossing over between nonsister chromatids in meiosis.

Crossing over

Exchange of genetic material between homologous chromosomes during meiosis.

Synaptonemal complex

Protein bridge that holds homologous chromosomes together during prophase I.

Tetrad

Pair of homologous chromosomes each composed of two sister chromatids.

Nondisjunction

Failure of chromosomes to separate properly during cell division.

Autosome

Non-sex chromosome; humans have 22 pairs of autosomes.

Sex chromosome

Chromosomes that determine sex (X and Y in humans).

X-linked inheritance

Traits associated with genes on the X chromosome.

X-linked recessive

Recessive traits expressed mainly in males; often skipped generations.

X-linked dominant

Dominant traits expressed in both sexes; no skipping typically.

Y-linked inheritance

Traits transmitted from father to son via Y chromosome.

Hemizygous

Males are hemizygous for X-linked genes (only one X).

SRY

Sex-determining region on the Y chromosome; initiates testes development.

PARS

Pseudoautosomal regions on X and Y that pair and recombine during meiosis.

XIST

Long noncoding RNA gene that coats and inactivates the X chromosome (X-inactivation).

Barr body

Condensed inactive X chromosome visible in female cell nuclei.

Lyon hypothesis

Random X-chromosome inactivation as a mechanism of dosage compensation.

Dosage compensation

Mechanisms that balance expression of sex-linked genes between sexes.

X-autosome ratio

In Drosophila, the ratio of X chromosomes to autosomes determines sex.

X/A ratio

Ratio used in Drosophila to determine sex (1.0 female, 0.5 male).

ZW system

Sex-determination system in which females are heterogametic (ZW) and males are homogametic (ZZ).

SRY function

Gene on the Y chromosome that initiates male development in mammals.

X-linked inheritance in humans

Patterns of inheritance for genes on the X chromosome; different in males and females.

Mosaicism (female mammals)

Females are cellularly mosaic due to random X-inactivation, expressing maternal or paternal X in different cells.

X-inactivation (XCI)

One X chromosome in each female cell becomes inactivated to balance dosage.

Barr body visualization

Inactivated X chromosome appears as a dense, Barr body in the nucleus.

Mutation type: wild type vs mutant

Wild type is the common/reference phenotype; mutant differs due to a mutation.

Sbe1

Gene for starch-branching enzyme; affects amylose/amylopectin; wrinkled seeds when mutant.

Le

Gene for gibberellin biosynthesis; affects plant height (tall vs short).

Sgr

Gene involved in chlorophyll breakdown; affects seed color (green vs yellow).

bHLH

Transcription factor gene; controls pigment pathways (flower color).

Seed shape

Round vs wrinkled; often linked to starch composition via Sbe1.

Seed color

Green vs yellow; influenced by Sgr in peas.

Flower color

Purple vs white; influenced by bHLH in peas.

Mendel’s first law

Law of segregation: alleles separate into gametes and join randomly.

Mendel’s second law

Law of independent assortment: alleles at different loci assort independently.

Chi-square test

Statistical method to compare observed and expected frequencies.

P-value

Probability that observed deviation could occur by chance; used to test hypotheses.

Degrees of freedom

n - 1, where n is the number of outcome classes in a chi-square test.

Binomial probability

Probability of a given number of successes in a fixed number of independent trials.

Pascal’s triangle

A shortcut tool for binomial coefficients in probability.

Product rule

Probability of independent events occurring together equals the product of their probabilities.

Sum rule

Probability of mutually exclusive outcomes sums up their probabilities.

Conditional probability

Probability of an outcome given that another outcome has occurred.

Autosome vs sex chromosome distinction

Autosomes are non-sex chromosomes; sex chromosomes determine sex.

Chromosome theory of heredity

Genes are carried on chromosomes, and chromosome behavior reflects heredity.

Morgan

Thomas Hunt Morgan; demonstrated sex-linked inheritance using Drosophila.

Nondisjunction in humans

Error in meiosis leading to aneuploidies (e.g., Turner, Klinefelter).