Question

Two parents can have a very large number of different potential offsprings, and it's common knowledge that the daughter chromosomes in meiosis are produced randomly, i.e. in crossover, the exact spots where the parent chromosomes are "cut" and genes are exchanged are random, with a larger probability at recombination hotspots. I can't understand how the crossover process can just be completely random, what is exactly different in each trial that leads to different outcomes? Is there something at the molecular level that actually determines the outcome (e.g. the exact positioning and the orientation of the chromosomes or other cells/molecules involved in the process)?

Answer 1

Crossover during meiosis is partly regulated by molecular mechanisms but also contains elements of randomness, contributing to genetic variation. These crossover events combined with independent assortment during metaphase I, produce a multitude of unique genetic combinations in gametes.

Step-by-step explanation:

The process of crossover during meiosis is a key source of genetic variation and is not completely random. There are molecular mechanisms that ensure the crossing-over occurs more frequently at certain regions known as recombination hotspots. During prophase I of meiosis, homologous chromosomes align and form tetrads, which consist of four chromatids. The precise locations where the chromosomes crossover is partly influenced by the intrinsic molecular features of the DNA sequences involved, but the actual sites of crossover within those regions are more random. Nevertheless, there are proteins that recognize specific DNA sequences that initiate the crossing-over process, ensuring that the exchange of DNA is tightly regulated.

During metaphase I, independent assortment further contributes to variation, with the way tetrads line up on the metaphase plate being random, leading to a variety of possible combinations. The randomness in the alignment of chromosomes, coupled with the crossovers between non-sister chromatids, gives rise to the diversity in the offspring's genetic makeup. The recombinant chromosomes are a mix of maternal and paternal genes, which did not exist prior to the crossover. Subsequently, due to these mechanisms, a vast number of genetically unique gametes can be produced. This diversity is a foundation for the evolution and adaptation of species.