The Chromosomal Theory of Inheritance and Sex Determination

Vocabulary practice flashcards covering the Chromosomal Theory of Inheritance, euchromatin vs heterochromatin, sex determination mechanisms, sex linkage, nondisjunction, and dosage compensation.

Chromosome Theory of Inheritance

The fundamental theory stating that genes are physical components of chromosomes and that chromosomes serve as the vehicles responsible for heredity.

Euchromatin

A lightly packed, gene-rich form of chromatin (composing DNA, RNA, and protein) that stains with less intensity cytologically, comprises approximately $92\%$ of the active human genome, and is frequently under active transcription.

Heterochromatin

Tightly packed, transcriptionally inactive DNA that stains intensely, characteristically found in centromeres, telomeres, and inactivated X chromosomes (Barr bodies).

Haploid ($N$)

The basic chromosome number representing one complete set of chromosomes (for example, a human sperm cell where $N = 23$).

Diploid ($2N$)

The condition of having two complete sets of chromosomes within a cell nucleus (for example, a human liver cell where $2N = 46$).

Polyploidy

A chromosomal condition in which an organism or cell possesses multiple complete sets of chromosomes, such as tetraploid ($4n$), hexaploid ($6n$), or octaploid ($8n$).

Autosomes

All chromosomes in a cell genome that are not sex chromosomes; human somatic cells contain 44 autosomes (22 pairs).

Homogametic Sex

The sex that produces gametes with identical sex chromosome complements (for example, mammalian females, which are $XX$).

Heterogametic Sex

The sex that produces gametes with two different types of sex chromosomes (for example, mammalian males, which are $XY$).

SRY Gene

The primary gene located on the Y chromosome responsible for male sex determination in mammals by directing the embryonic gonads to develop into testes.

Testis-Determining Factor (TDF)

A protein factor produced by the SRY gene that induces early embryonic gonads to differentiate into testes, which subsequently secrete testosterone.

Pseudoautosomal Region (PAR)

Homologous sequences shared at the tips of the X and Y chromosomes that undergo crossing over during meiotic division, resulting in two functional gene copies in both males and females.

MicroRNA (miRNA)

A small non-coding RNA molecule (approximately 22 nucleotides in length) that operates in RNA silencing and post-transcriptional gene regulation.

Hemizygous

The genetic state of possessing only one copy of a specific gene in an otherwise diploid organism, such as X-linked genes in male mammals ($XY$).

Duchenne Muscular Dystrophy

An X-linked recessive genetic condition causing progressive degeneration of muscle tissues, typically resulting in mortality before age 20.

Nondisjunction

The failure of homologous chromosomes or sister chromatids to separate normally during cell division, leading to offspring or gametes with abnormal chromosome numbers.

Dosage Compensation

Any genetic mechanism that equalizes the expression level of X-linked genes between sexes possessing different numbers of X chromosomes.

Barr Body

A condensed, transcriptionally inactive mass of heterochromatin inside the nucleus of female mammalian cells, representing an inactivated X chromosome.

Lyonization

The random inactivation of one X chromosome in female mammalian embryos occurring around 16 weeks post-fertilization, as formulated by Mary Lyon and Lillian Russell.

Genetic Mosaic

An individual or tissue composed of two or more genetically distinct cell types derived from a single zygote, as seen in female calico cats due to random X inactivation.