chromosome

(krō′mŏ-sōm″)

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[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; SEE: centromere; SEE: chromatid; SEE: cytogenetics; SEE: dominant; SEE: gene; SEE: heredity; SEE: karyotype; SEE: mutation; SEE: recessive; SEE: telomere

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