Genetics with the Fundamentals of Breeding - M.P. Myhun - 2008

CHAPTER V. Genetic Foundations of Ontogeny

5.2. General Patterns in Individual Development

The ontogeny of any multicellular Organism begins with Fertilization. The penetration of the egg by a spermatozoon triggers the Cleavage of the zygote and The Development of the new organism it originates. In accordance with the laws of genetics, it inherits specific species and individual physiological traits, as well as psychological characteristics in the case of humans.

Zygote cleavage initiates a vast number of mitoses, and consequently, Cells that enter the stage of differentiation and tissue formation in the new embryo. As a result, cells lose their functional uniformity. Within each tissue formed by respective Cell groups, a unique internal environment emerges where only those GENES OF THE genotype function that enable the developing tissue to perform its specific role in the developing organism.

To elucidate the Genetic foundations of individual development, several general patterns confirmed by research across various branches of biology are taken into account, namely:

- the ontogeny of Multicellular Organisms represents a complex sequence of Gene-controlled changes (the regulated realization of a genetic program under specific conditions);

- during Embryogenesis, every organism passes through developmental phases characteristic of its ancestors to varying degrees, which indicates that the general developmental plan of organisms is genetically predetermined;

- the individual development of multicellular organisms is based on the genetic determination and differentiation of distinct cell clones arising from zygote cleavage. Differentiation is the acquisition of The ability to perform specific Functions associated with cell specialization. Cell determination is a developmental stage after which a cell loses the capacity to differentiate in a direction not provided for by genetic determination;

- the differentiation of somatic Cells and Tissues can be reversible or irreversible depending on the degree of genetic specialization of the nuclei and Cytoplasm within the cells. Occasionally, an individual cell or a group of cells from differentiated tissue is capable of regenerating an entire organism. This cellular capability is termed totipotency;

- the processes of growth, differentiation, and morphogenesis characteristic of development are uneven (some genes function actively at first, followed by the activation of others);

- so-called critical periods exist in the ontogeny of both animals and plants;

- cell determination and differentiation are not associated with quantitative or qualitative alterations of the genomes.

The development of any multicellular organism belonging to highly organized eukaryotes can be divided into several stages or periods.

1. Embryonic Development of the organism. During this time, a multicellular organism is formed from a single cell—the zygote.

2. Postembryonic period of GROWTH AND DEVELOPMENT from birth to sexual maturity. This period becomes subject to natural Selection and determines the organism's ability to perform its primary function—reproduction.

3. Period of sexual reproduction—characterized by peak physiological activity of the organisms and the generation of viable offspring.

4. Period of senescence—the loss of reproductive capacity leading ultimately to the death of the organism.

The last two periods are often difficult to separate. In many animal species (such as sturgeon and salmon Fishes, certain insects, etc.), parental organisms die after spawning or laying zygotes, meaning the senescence period is bypassed; the same applies to annual and biennial plants.