Chromosome and genome level of hereditary material organization in porkaryotes and eukaryotes

The chromosomes play an important role in herediting. It was proved by discovering chromosome sex determination, groups of gene linking, genetic and cytological chromosome mapping. These facts were summarized in chromosome theory of inheritance.

The chromosome level of hereditary' material organization is characterized by chromosome structure. The chromosome of non cellular live forms is presented in a form of naked DNA chain (or RNA chain in some viruses). The chromosome of the prokaryotes is a naked circular DNA molecule. The chromosome of the eukaryotes is a complex of DNA with histon proteins.

The sex genetics. A large contribution to sex genetics studying was made by American scientist C. Mac-Klang in 1901-1902. He proved that the X-chromosome determines the sex of the Protentor bug. In 1959, female organisms with the chromosome set “XO” were discovered. It was concluded that the Y-chromosome determined male sex.

The organism containing same sex chromosomes is called homogametic. The organism containing different sex chromosomes is called heterogametic. The sex of future child depends on sex chromosome combination in the zygote (pic. 7.1). There are four variants of chromosome sex determination in animals by female homogamete or heterogamete. The female homogameting may have following variants: XX, XY (in mammalian and humans) and XX, XO (in bugs). The female heterogameting may have following variants: ZW, ZZ (in butterflies) and ZO, ZZ (in birds). The sex is determined by heterogametic organism.

Pic. 7.1. The sex herediting in human (by N. P. Dubinin 1963)

But sex may be determined also by a chromosome balance, so called “sex index”. Balance sex theory was suggested by K. Bridgess and R. Goldshtein in 1911. They sated that male and female sex of Drosophilla is determined by ratio of sex chromosomes to autosomes, instead of sex chromosomes combination. The genes of female organism are mostly located in X-chromosomes, whereas male organism genes are mostly located in autosomes. If ratio is X:A=1, it is female organism. If ratio is X:2A=0.5, it is male organism. If it is intermediate ratio (from 1 to 0.5), it is intersex organism. Increased ratio (3X:2A= 1.5) leads to overmatured female formation. Decreased ratio (X:3A=0.33) leads to overmatured male formation.

The balance sex theory may be used in humans. The normal female sex chromosomes to autosomes balance is XX:44A. If such balance is XO:44A, which is observed in patients with Shereshevsky-Temer syndrome, the ovarium, uterus tubes, uterus underdevelopment is founded. If patients have three X-chromosomes (XXX:44A), the secondary' sex signs expression may be broke. Normal male sex chromosomes to autosomes balance is XY:44A. The patients with Kleinfelter syndrome (XXY:44A) have unexpressed secondary' sex sings, gynecomasty, and failed spermatogenesis.

In 1949, M. Barr and G. Bertram showed that female nervous cells have a body of well stained chromatin in a nucleus, which male nervous cells haven’t. This structure was named in a name of M. Barr - Barr’s body or sex chromatin. Later it was founded that Barr’s body is inactivated X-chromosome. During first 16 days of embryo development both X-chromosomes work very actively, perhaps producing a double number of products encoded in X-chromosome. This fact is used for explanation of higher survival rate of female’s embryo.

Inactivation of one X-chromosome takes place between 10-19 days of embryonic development. Once inactivated X-chromosome preserve such structure in a line of somatic cells generations.

The traits, which are controlled by genes of sex chromosomes, are called sex linked. Sex linkage was demonstrated by T. Morgan on an example of eye color heredity in Drosophila melanogaster. So it was stated trait transmission from father to daughters and from mother to sons. More than 60 human genetic sex linked diseases have been identified. Most of them are recessive. Genes, which are on sex chromosomes, may be divided into 3 groups (pic 7.3 and 7.4).

Pic. 7.3. The genes mapping for those, which linked with X- and Y-chromosomes: black segment represent genes, which are fully linked with X-chromosome. grey segment - fully linked with Y-
chromosome, white segments - partially sex linked genes (by J. Neal, W. Shell, 1958)

Ріс. 7.4. The pedigree of family with sex linked hemophilia A herediting: I-IV - generations (by E. Harden, R. Vener, 1958)

Genes, which are in homologues regions of sex chromosomes, was named partially sex linked. There are diseases connected with partially sex linked genes. They are total colorless blindness, pigment xeroderm and others.

Genes, which are in X-chromosome region non-homologues to Y- chromosome, was named fully sex linked. There are diseases connected with fully sex linked genes. They are muscular Dushene dystrophy, hemophilia and others.

Genes, which are in Y-chromosome region non-homologues to X- chromosome, was named holandric genes (from Greek holos - whole; andros - male). There are diseases connected with holandric genes. They are hypertrichosis of ear, ichtiosis, syndactilia and others.