PCB 3063C Genetics test 1

Gene

A DNA sequence that contributes to a functional product and can influence an inherited characteristic.

Gene expression

The use of genetic information to produce a functional RNA or protein; protein-coding genes use transcription then translation.

Chromosome

A structure made of DNA and proteins that carries genes.

Genome

All of the chromosomes and DNA sequences an organism/species possesses.

Karyotype

An organized photographic/digital display of a cell's chromosomes, usually arranged in homologous pairs.

Metaphase

Mitotic stage where duplicated chromosomes align at the metaphase plate.

Mitosis order

Prophase → prometaphase → metaphase → anaphase → telophase → cytokinesis.

Diploid (2n)

Cell with two sets of chromosomes; most animal somatic cells.

Haploid (n)

Cell with one set of chromosomes; gametes are haploid.

Meiosis

One round of DNA replication followed by two divisions, typically producing four genetically nonidentical haploid cells.

Locus

The specific position of a gene on a chromosome.

Why meiosis is required

Meiosis reduces 2n to n before fertilization so chromosome number is restored instead of doubled.

Dominant phenotype

In a true-breeding contrasting cross, the phenotype shown by all F1 offspring under complete dominance.

Recessive phenotype

The hidden phenotype in a true-breeding contrasting cross; can reappear in later generations.

Dihybrid phenotype ratio

AaBb × AaBb with complete dominance and independent assortment → 9:3:3:1.

Forked-line / multiplication method

Break a multigene cross into separate one-gene probabilities, then multiply independent probabilities.

Repeated offspring probability

Independent births: multiply the single-offspring probability for each specified outcome (example: 3/4 × 3/4 = 9/16).

Pedigree

A family chart tracking a trait across generations; use affected status, sex distribution, skipped generations, and parent-offspring patterns.

Monohybrid phenotype ratio

Aa × Aa → 3 dominant : 1 recessive.

Monohybrid genotype ratio

Aa × Aa → 1 AA : 2 Aa : 1 aa.

Genotype

An individual's allele combination.

Phenotype

An observable or measurable characteristic resulting from genotype and environment.

X-linked inheritance

A gene located on the X chromosome but absent from the Y chromosome.

Hemizygous

Having only one copy of an X-linked gene, as males do for most X-linked genes.

X-linked carrier cross

Carrier female XᴬXᵃ × normal male XᴬY → 1/2 of sons affected.

X-linked dominant

Heterozygous affected female × unaffected male → 1/2 of daughters affected and 1/2 of sons affected.

Affected X-linked recessive female

Because she is XᵃXᵃ, she must receive a mutant X from both parents; if mother is unaffected, mother is a carrier.

X-inactivation

Heterozygous females can be cellular mosaics because different cells may keep different X chromosomes active; homozygous disease females still lack a normal allele.

Pleiotropy

One gene influences multiple phenotypic traits.

Epistasis

The genotype at one gene masks or modifies expression of another gene.

Penetrance

The percentage of individuals with a particular genotype who express the associated phenotype.

Genomic imprinting

Expression depends on whether an allele was inherited from the mother or father.

DNA methylation

A parent-specific epigenetic mark that can alter gene expression without changing the DNA sequence.

Maternal effect

The mother's genotype determines the offspring's early phenotype because she deposits RNAs/proteins into the egg.

Extranuclear DNA in mammals

Located in mitochondria; mitochondria have a small genome separate from nuclear chromosomes.

Cytoplasmic/extranuclear inheritance

Inheritance of mitochondrial or chloroplast genes; mitochondrial inheritance in mammals is usually maternal.

Gene linkage

Genes physically located on the same chromosome tend to be inherited together.

Map distance

Approximately additive when a gene lies between two loci: outside distance = left distance + right distance.

Maximum observable recombination

A two-point cross usually cannot show more than 50% recombinant offspring, even when loci are >50 map units apart.

Linkage group

All genes on one chromosome; humans have 22 autosomal linkage groups plus X and Y groups.

Linkage vs recombination

Linkage = genes tend to transmit together; crossing over can break linkage and produce recombination.

Useful mapping organism

Short generation time, many offspring, controlled crosses; visible markers also help.

Distance and recombination

Farther genes have more crossover opportunity, so recombination frequency increases with distance until ~50%.

Recombination frequency

RF = recombinant offspring ÷ total offspring × 100; map distance in cM/map units ≈ RF.

Deletion

A portion of a chromosome is missing.

Duplication

A portion of a chromosome is repeated.

Inversion

A chromosome segment is reversed; total DNA amount may remain unchanged.

Translocation

A segment of one chromosome becomes attached to a nonhomologous chromosome.

Nondisjunction

Failure of homologous chromosomes or sister chromatids to separate properly during anaphase.

Genetic variation

Differences in DNA, alleles, or chromosomes among members of the same species.

Paralog

Homologous genes within the same species, commonly from gene duplication.

Ortholog

Homologous genes in different species that diverged after speciation.

Telomere

Repetitive DNA-protein structure at chromosome ends that protects chromosome integrity.

Centromere

Internal chromosome region where kinetochores form and spindle microtubules attach.

Endopolyploidy

Some cells/tissues contain extra complete chromosome sets while the organism is otherwise diploid.

Polytene chromosome

Giant bundle formed by repeated DNA replication without cell division, leaving many chromosome copies aligned together.

Trisomy

One extra copy of a particular chromosome; 2n + 1.

Monosomy

One chromosome missing; 2n − 1.