A Quick Look at Genes in Cytoplasm
Figure 4. An example of maternal-only cytoplasmic inheritance, contrasting nuclear inheritance in Figure 2 where genes were inherited equally from each parent. Something has blocked the paternal DNA and the genes in this case only come from the mother. Base image from passel2.unl.edu.
Still wanting to know more about their white-leaved lilac, our plant breeders peruse past research and find that the white leaves in some plants are caused by mutant chloroplasts. A change in the organelle DNA prevents those cells from producing the green pigment.
They also find research, performed with 4-o’clock flowers, which reported that white leaves, caused by mutant chloroplasts, could only be passed on by the mother parent (Miko 2008). One of the breeders is confused by this because it doesn’t follow the genetic law that half of the DNA should come from each parent. Finding some free time, he digs deeper into the subject.
As we have seen, the plant breeder knew about nuclear DNA as well as cell organelles, but he had not yet heard about cytoplasmic inheritance.
Cytoplasmic inheritance is defined as the inheritance of organelle DNA from the parents.
Cytoplasmic inheritance differs from “plain old” nuclear genetics because it does not follow the laws of gene inheritance which state that half of the genes will come from each parent. Instead, depending on the species, DNA found in the organelles can come exclusively from the mother, father, or an unequal mixture of both (Figure 4).
The breeder finds several research experiments reporting that almost all of the organelle DNA in certain groups of plants is inherited from the mother.
If fact, in one study involving 295 species found that 238 exhibited maternal inheritance (Zhang and Liu 2003). The confusing part for the plant breeder is, if all plant cells, including pollen, contain organelles, what blocked the father’s organelle DNA from entering the egg?