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

BIOLOGY

SECTION 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.2. Fundamentals of human genetics

Genomic Mutations

These are caused by changes in chromosome number. The types of genomic mutations include polyploidy, heteroploidy, and haploidy. Polyploidy is a genomic mutation resulting from an increase in the number of haploid chromosome sets compared to the diploid set (2n). Examples include triploidy (3n), tetraploidy (4n), pentaploidy (5n), etc. An Organism with a normal chromosome Complement is called a euploid, whereas one with a polyploid complement is a polyploid (triploid, tetraploid, pentaploid, etc.). Polyploidy arises through three main processes: 1) chromosome reduplication in non-dividing Cells; 2) fusion of somatic cells or their nuclei; 3) nondisjunction of Chromosomes during Meiosis.

Polyploid forms are frequently found among plants, which is explained by the widespread occurrence of Vegetative Reproduction in the plant kingdom. Most cultivated plants are polyploids. As a rule, polyploids feature larger Cell sizes and, accordingly, greater mass. In addition, they are more resistant to adverse conditions and capable of surviving in harsher climates. Among animals (excluding those reproducing via parthenogenesis), polyploidy is rare due to the disruption of the Chromosomal Sex-determination mechanism caused by an increased number of sex chromosomes.

In humans, polyploidy manifests as embryonic triploidy (3n): 69, XXX or 69, XXY. Triploid embryos account for 4 to 20% of all spontaneous abortions. Isolated cases of live-born triploid children have been described. Triploid newborns die within the first hours or days of life, though in rare cases they survive up to 4-7 months. They exhibit a whole range of developmental defects: hypoplasia (underdevelopment of Organs), microphthalmia (abnormally small eyes), cleft lip and palate, Syndactyly (fusion of fingers or toes), Hydrocephalus (Water on the Brain), and others.

Heteroploidy (aneuploidy) is a genomic mutation caused by A change in the number of individual chromosomes. The primary cause of heteroploidy is chromosome nondisjunction during Mitosis and Meiosis. The MAIN TYPES OF heteroploidy are trisomy, monosomy, and nullisomy. In trisomy, a chromosome pair contains three homologous chromosomes instead of two; the diploid chromosome set thus increases by one chromosome (2n+1). Such an organism is called a trisomic. The loss of a single chromosome from a pair in the diploid set is called monosomy, and the organism is a monosomic (2n-1). When two different chromosomes are absent from the diploid set, the organism is termed a double monosomic (2n-2), and when both homologous chromosomes are missing, a nullisomic. Monosomy for any of the 22 autosomal pairs in humans results in intrauterine fetal death.

Heteroploidy causes a large group of human Chromosomal Disorders, including Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13), Klinefelter syndrome (47, XXY), and Turner syndrome (45, X0). The characteristics of these syndromes are discussed below.

Haploidy is a reduction in the number of chromosome sets in somatic cells compared to the diploid set. Haploid organisms develop from a single cell with a gamete genotype, bypassing Fertilization. They are found mainly in plants, including higher plants (henbane, wheat, corn). Haploid plants differ from diploid ones by their smaller size, which is associated with a halved Gene dosage. Because haploids possess a single set of chromosomes, their phenotype expresses not only dominant genes but all recessive ones as well. Haploidy in humans is incompatible with life.