Learn about the difference between Nuclear Traits and Extra-Nuclear Traits.
1. Reciprocal Differences:
Differences in the results of reciprocal crosses would suggest a deviation from the pattern of Mendelian autosomal gene transmission. According to Mendelian inheritance, the chromosome complement in male and female gametes obtained from the same species would be similar; reciprocal crosses should give same results (♀A x♂ B =♀B x ♀’A).
The only exception to this expectation is sex- linked inheritance which can be explained on the basis of transmission of sex chromosomes.
If sex linkage is ruled out, differences in the result of reciprocal crosses would indicate that one parent (maternal) is exerting a greater influence than the other on a particular trait. This is because cytoplasm does not divide in a precise manner like the chromosomes during the process of cell division during gametogenesis. Female gametes usually contribute more cytoplasm to the zygote.
Consequently for characters having cytoplasmic control, differences in reciprocal crosses are observed.
As shown in the Fig. 10.1, if two strains A and B respectively having genotypes AA and BB and cytoplasm’s a and b are crossed reciprocally, we will get two hybrids AB (a) and AB (b) [cytoplasm is indicated in parentheses].
In case of maternal inheritance, AB (a) and AB (b), despite having same nuclear genotype, will differ. AB (a) will resemble strain A or AA (a) and AB (b) will resemble strain B or BB (b). Since such effects are solely produced by cytoplasm of the egg, they are described as maternal inheritance (uniparental inheritance).
2. Lack of Segregation:
Irregular Segregation; Somatic Segregation; Lack of Mendelian segregation and characteristic Mendelian ratios that depend on chromosomal transmission in meiosis would suggest extra-chromosomal transmission. Cytoplasmic genes inherited from both the parents sometimes give rise to irregular segregation ratios. They generally show somatic segregation during mitosis, a feature uncommon to nuclear genes.
3. Lack of Chromosomal Location:
The chromosomal genes occupy particular loci and specifically linked to other genes. The failure to find linkage to known nuclear genes may rule out chromosomal inheritance and suggests extra-nuclear inheritance.
4. Association with Organellar DNA:
The cytoplasmic inheritance or extra-chromosomal inheritance is defined as non-Mendelian inheritance, usually involving DNA in replicating cytoplasmic organelles, such as mitochondria and chloroplastids. The presence of DNA in cell- organelles, found outside the nucleus, is a strong evidence to suggest that genetic information does exist in cytoplasm also.
5. Transfer of Nuclear Genome through Backcrosses:
The transfer of nucleus of a variety to the cytoplasm’ of another variety through repeated backcrossing, results in lines having nucleus and cytoplasm from two different varieties. A comparison of these lines with original lines having nucleus and cytoplasm of same variety demonstrates cytoplasmic effects on these traits.