(krō'mŏ-sōm‵ )

[ chrom- + -some ]

A linear strand made of DNA (and associated proteins in eukaryotic cells) that carries genetic information. Chromosomes stain deeply with basic dyes and are esp. conspicuous during mitosis. The normal diploid number of chromosomes is constant for each species. For humans, the diploid number is 46 (23 pairs in all somatic cells). In the formation of gametes (ovum and spermatozoon), the number is reduced to one half (haploid number); i.e., the ovum and sperm each contain 23 chromosomes. Of these, 22 are autosomes and one is the sex chromosome (X or Y). At fertilization, the chromosomes from the sperm unite with the chromosomes from the ovum. The sex of the embryo is determined by the sperm. The ovum always contributes an X chromosome. The sperm may contribute an X or a Y chromosome. An embryo with XX chromosomes will be female; an embryo with XY chromosomes will be male.

SEE: Barr body; centromere; chromatid; cytogenetics; dominant; gene; heredity; karyotype; mutation; recessive; telomere.

accessory c.An unpaired sex chromosome.

SEE: sex chromosome.

band ed c.A chromosome specially stained to delineate band s of various widths on its regions or loci. This facilitates analysis and investigation of genes and gene-related illnesses.

bivalent c.A double chromosome resulting from the conjugation of two homologous chromosomes in synapsis, which occurs during the first meiotic division.

homologous c.One of a pair of chromosomes that contain genes for the same traits; one is maternal in origin, the other paternal.

Philadelphia c.An abnormal chromosome 22 in which there is translocation of the distal portion of its long arm to chromosome 9. It is found in leukocyte cultures of many patients with chronic myelocytic leukemia. The Philadelphia chromosome was the first chromosomal change found to be characteristic of a human disease.

sex c.One of two chromosomes, the X and Y chromosomes, that determine sex in humans and that carry the genes for sex-linked characteristics.

somatic c.Autosome.

X c.One of the sex chromosomes; women have two (XX) present in all somatic cells, and men have one (XY). Characteristics transmitted on the X chromosome are said to be X-linked or sex-linked. The human X chromosome, sequenced in 2005, has approximately 1100 genes.

Y c.The male-determining member of a pair of human chromosomes (XY). The human Y chromosome, sequenced in 2023, has fewer than 200 genes.