This Chromosomal theory of Inheritance was proposed by Walter Sutton and Theodor Boveri (1902). Following are the main point of Chromosomal theory of Inheritance –
- Gametes serve as the bridge between two successive generations.
- Male and female gametes play an equal role in contributing hereditary components of future Generation.
- Only the nucleus of sperm combines with ovum Thus the hereditary information is contained in the nucleus.
- Chromatin in the nucleus is associated with the cell division in the form of chromosomes.
- Any type of deletion or addition in the chromosome can cause structural and functional changes in living beings.
- A sort of Parallelism is observed between Mendelian factors and chromosomes.
- A number of genes or Mendelian factors are found in each chromosome.
- Determination of sex in most of the animals and plant is affected by specific chromosomes these chromosomes are called sex chromosomes.
Parallelism between Gene and chromosome
- Chromosomes are also transferred from one generation to the next as in the case of genes (Mendelian factor)
- The number of chromosomes is fixed in each living species. These are found as homologous pairs in diploid cells, one chromosome from father and the other contributed by the mother constitute a homologous pair.
- Before cell division each chromosome as a whole and the alleles of genes get replicated and are separated during mitotic division.
- Meiosis takes place during gamete formation .Homologous chromosomes form synapses during prophase-I stage which in later course get separated and transferred to daughter cell. Each gamete or a haploid cell has only one allele of each gene present in the chromosome.
- A characteristic diploid number is again established by the union of the two haploid gametes.
Cytoplasmic Inheritance (Correns)
Inheritance of characters which are controlled by cytogene or cytoplasm is called cytoplasmic inheritance. Genes which are present in cytoplasm called cytogene or plasmagene or extra nuclear gene.
Total cytogene present in cytoplasm is called plasmon. A gene which is located in the nucleus is called karyogene.
–Inheritance of cytogene in higher plants only through the female because female gamete has more cytoplasm..
-The male gamete of higher plant is called male nucleus it has very minute (equivalent to nil) cytoplasm so male gamete only inherited karyogene.
– Thus inheritance of cytogene only through female (also called maternal inheritance)
-If there is reciprocal cross in this condition then results may be effected.
Types of Cytoplasmic Inheritance
Maternal effect depending indirectly on nuclear genes and involving no known cytoplasmic hereditary unit called as predetermination in this maternal effect is determined before fertilization.
Example of Predetermination
Shell coiling in snail (limnea peregra). In shell coiling of snail can be dextral (coiling to the right) or sinistral (coiling to the left) this direction of coiling is genetically controlled.The dextral coiling depending upon dominant allele D and sinistral coiling depending upon recessive allele so the dextral is DD, Dd and sinistral is dd.
Cytoplasmic inheritance involving dispensable and infective hereditary particle in cytoplasm which may or may not depend on nuclear genes called as Deuteromodification.
Example:- Sigma particle in drosophila theses particles are virus like particles which are present in drosophila and related to co2 sensitivity inheritance of sigma particle takes place through the egg cytoplasm.
Cytoplasmic inheritance involving essential organelles like chloroplast and mitochondria called as organellar Genetics.
Example :- Plastid inheritance in mirabilis jalapa-cytoplasmic inheritance first discovered by Correns in Mirabilis jalapa. In mirabilis jalapa plastid inheritance ie. Branch colour is example of cytoplasmic inheritance.