HUMAN MEDICAL BIOLOGY, ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedonyuk 2010

BIOLOGY

CHAPTER 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.3. MOLECULAR-GENETIC AND CELLULAR LEVELS OF LIFE ORGANIZATION

1.3.2. Structural, chemical and functional organization of eukaryotic cells

Human Karyotype

The karyotype is defined as the diploid set of cellular Chromosomes characterized by a specific set of features: chromosome number, size, shape, and structural details. This term was introduced into science in 1924 by the Ukrainian cytologist H.A. Levytsky (1878–1942). The karyotype serves as a species-specific trait and is faithfully transmitted from generation to generation through Mitosis and Meiosis.

The diploid (double) chromosome set is characteristic of somatic Cell nuclei and is designated as 2n. All chromosomes in this set exist in pairs. Chromosomes belonging to the same pair are termed homologous, whereas those from different pairs are non-homologous. Homologous chromosomes are identical in size, shape, and Structure; however, one originates from the ovum (maternal) and the other from the spermatozoon (paternal). The nuclei of Germ Cells contain half the chromosome number, known as the haploid (single) set, designated as n. This set contains only one chromosome from each diploid pair. All chromosomes in the haploid set are unique and non-homologous. Upon Fertilization, germ cells fuse, and the diploid chromosome set is restored in the zygote (n+n=2n). The chromosome sets of somatic cells in male and female individuals of the same species differ by a single pair of chromosomes. This pair determines the sex of the Organism and is referred to as sex chromosomes or heterosomes. All other chromosomes are identical in both sexes and are called autosomes. In other words, autosomes are all chromosomes except the sex chromosomes.

The karyotype is characterized by constancy and Specificity. Each species possesses a distinct karyotype established throughout evolution. The karyotypes of different species vary in chromosome number, size, shape, and structural features; nevertheless, all somatic Cells of the same species share identical chromosome sets and carry the full Complement of Genetic information characteristic of that species. For instance, the diploid set comprises 8 chromosomes in Drosophila, 48 in the cockroach, 12 in the house fly, 44 in the rabbit, and 48 in the chimpanzee. Chromosome number does not depend on the organism's level of complexity or size. The highest chromosome number (up to 1600) is found in radiolarians (Protozoans), while the lowest (2) is found in the equine roundworm (Parascaris megalocephala univalens). The species specificity of chromosomes is maintained by their capacity for self-Replication.

The normal human karyotype for both males and females consists of 46 chromosomes, comprising 22 pairs of autosomes and a single pair of sex chromosomes: XX in females and XY in males. The normal female karyotype is denoted as 46,XX, and the male karyotype as 46,XY. A normal karyotype is a prerequisite for The Development of a healthy individual. Alterations in chromosome number and structure (Mutations) lead to Chromosomal Disorders.