Chapter 15: The Chromosomal Basis of Inheritance

Vocabulary practice flashcards covering chromosomal inheritance, sex-linked genes, genetic recombination, chromosomal alterations, epigenetics, and organelle genes.

Chromosome Theory of Inheritance

A foundational concept in genetics proposing that specific genes have specific loci on chromosomes, and that chromosomes undergo segregation and independent assortment during meiosis.

Locus (plural: Loci)

The specific physical position or location of a gene on a chromosome.

Alleles

Alternative variants or forms of a specific gene found at the same locus on homologous chromosomes.

Law of Segregation

Mendelian principle stating that the two alleles for each gene separate during gamete formation in meiosis, resulting in each gamete receiving only one allele.

Law of Independent Assortment

Mendelian principle stating that alleles of genes located on nonhomologous chromosomes assort independently of one another during gamete formation in meiosis.

Wild Type

The phenotype most commonly observed in natural populations for a specific trait, denoted with a $+$ superscript in Morgan's genetic notation (such as $w^+$).

Mutant Phenotypes

Traits that are alternative to the wild type and originate as changes or mutations in the wild-type allele, denoted with standard letter notation (such as $w$).

$SRY$ Gene

The sex-determining region on the $Y$ chromosome that directs development of the testes in an embryo.

Sex-Linked Gene

A gene located on either sex chromosome, categorized specifically as $X$-linked or $Y$-linked.

Hemizygous

The state of possessing only one copy of a gene in an otherwise diploid organism, typical of males for genes on the $X$ chromosome ($XY$).

Barr Body

A dense, condensed object inside the nucleus of female mammalian cells representing a randomly silenced and inactivated $X$ chromosome.

Linked Genes

Genes located close together on the same chromosome that tend to be inherited together as a unit in genetic crosses rather than assorting independently.

Parental Types

Offspring that exhibit phenotypes matching one of the parental phenotypes from the original $P$ generation cross.

Recombinant Types (Recombinants)

Offspring that display combinations of traits differing from both parents, produced by genetic recombination.

Crossing Over

The reciprocal exchange of genetic segments between nonsister chromatids of homologous chromosomes during prophase I of meiosis, responsible for recombining linked genes.

Linkage Map

A genetic map depicting the sequence and relative distances of genes along a chromosome based on observed recombination frequencies.

Map Unit

A unit of measurement representing a $1\%$ recombination frequency between linked genes, indicating relative distance and order rather than exact physical base pairs.

Nondisjunction

An error during meiosis or mitosis in which members of a pair of homologous chromosomes or sister chromatids fail to separate appropriately.

Aneuploidy

A chromosomal condition in which one or more chromosomes are present in an abnormal number due to the fertilization of gametes produced by nondisjunction.

Monosomic

Referring to an aneuploid zygote or organism having only one copy of a particular chromosome instead of the normal two ($2n - 1$).

Trisomic

Referring to an aneuploid zygote or organism having three copies of a particular chromosome instead of the normal two ($2n + 1$), as seen in Down syndrome.

Polyploidy

A chromosomal alteration in which an organism possesses more than two complete chromosome sets, common in plants.

Chromosomal Deletion

A structural chromosomal mutation that removes a chromosomal segment.

Chromosomal Duplication

A structural chromosomal mutation that repeats a chromosomal segment.

Chromosomal Inversion

A structural chromosomal mutation that reverses the orientation of a segment within a chromosome.

Chromosomal Translocation

A structural chromosomal mutation that moves a chromosomal segment from one chromosome to a nonhomologous chromosome.

Philadelphia Chromosome

An abnormally shortened chromosome 22 produced by reciprocal translocation with chromosome 9, responsible for chronic myelogenous leukemia (CML).

Cri du Chat

A human syndrome caused by a specific deletion in chromosome 5, resulting in severe intellectual disability and a characteristic catlike cry in infancy.

Klinefelter Syndrome

An aneuploid condition in males possessing an extra $X$ chromosome, resulting in an $XXY$ karyotype.

Turner Syndrome

A monosomic aneuploid condition in females possessing only a single $X$ chromosome ($X0$), leading to sterility.

Epigenetics

Changes in DNA chromatin structure and condensation state that regulate gene expression without altering the underlying nucleotide sequence.

Genomic Imprinting

A phenomenon in which the expression of an allele in offspring depends strictly on whether it was inherited from the mother or the father, mediated by DNA methylation.

Extranuclear (Cytoplasmic) Genes

Genes located on small circular DNA molecules in organelles like mitochondria and chloroplasts, inherited maternally because the zygote receives its cytoplasm exclusively from the egg.