The advantage of meiotic division and sexual reproduction is that it promotes genetic variation in offspring. The three main sources of genetic variation arising from sexual reproduction are:
Crossing Over
Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I
As a consequence of this recombination, all four chromatids that comprise the bivalent will be genetically different
The "2^n" rule states that the number of possible chromosome combinations in a gamete due to random orientation of chromosomes during metaphase 1 of meiosis is 2^n, where n = the haploid number of chromosomes.
- Crossing over (in prophase I)
- Random assortment of chromosomes (in metaphase I)
- Random fusion of gametes from different parents
Crossing Over
Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I
- The exchange of genetic material occurs between non-sister chromatids at points called chiasmata
As a consequence of this recombination, all four chromatids that comprise the bivalent will be genetically different
- Chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinants
- Offspring with recombinant chromosomes will have unique gene combinations that are not present in either parent
The "2^n" rule states that the number of possible chromosome combinations in a gamete due to random orientation of chromosomes during metaphase 1 of meiosis is 2^n, where n = the haploid number of chromosomes.
- For humans, n = 23 so there are 2^23 possible chromosome combinations in the gamete cells. That is 8,388,608!
Random Orientation
When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is random. The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I
When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is random. The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I
- The total number of combinations that can occur in gametes is 2n – where n = haploid number of chromosomes
- Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes (223) by random orientation
- If crossing over also occurs, the number of different gamete combinations becomes immeasurable