Genetic Variation & Meiosis
Crossing over and random orientation promote genetic variation
- Having genetically different offspring can be advantageous for natural selection and therefore increase the survival chances of a species
- Meiosis has several mechanisms that increase the genetic variation of gametes produced
- Both crossing over and random orientation result in different combinations of alleles in gametes
Crossing over
- Crossing over is the process by which non-sister chromatids exchange alleles
- Process:
- During prophase I of meiosis homologous chromosomes pair up and are in very close proximity to each other
- A pair of homologous chromosomes can be referred to as a bivalent
- At this point, there can be an exchange of genetic material (alleles) between non-sister chromatids in the bivalent
- The crossing points are called chiasmata
- This results in a new combination of alleles on the two chromosomes (these can be referred to as recombinant chromosomes)
- During prophase I of meiosis homologous chromosomes pair up and are in very close proximity to each other
- This swapping of alleles is a significant source of genetic variation because it can occur at multiple random positions along the chromosome
- Crossing over can happen anywhere along the chromosome but is more likely to occur further down the chromosome away from the centromere
Crossing over occurring between two non-sister chromatids
Random orientation
- The random orientation of homologous pairs along the equator of the cell during metaphase I result in the production of different allele combinations in daughter cells
- In prophase I, homologous chromosomes pair up and in metaphase I, they align along the equator of the cell
- Each pair can be arranged with either chromosome on top, this is completely random
- The orientation of each homologous pair is random/independent (unaffected by the orientation of any other pair)
- In anaphase I the homologous chromosomes are separated and pulled apart to different poles
- The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
- The different combinations of chromosomes in daughter cells increases genetic variation between gametes
The random orientation of homologous chromosomes leads to different genetic combinations in daughter cells
The different combinations of chromosomes following meiosis
- The number of possible chromosomal combinations resulting from random assortment is equal to 2n
- n is the number of homologous chromosome pairs or haploid number
- For humans: the number of chromosomes is 46 meaning the number of homologous chromosome pairs is 23 so the calculation would be:
- 223 = 8,388,608 possible chromosomal combinations
Worked example
Calculate how many different chromosomal combinations can result from meiosis in a plant species which has a diploid number of 16. Assume no crossing over occurs.
[1 mark]
Step 1: Use the relevant formula
2n
Step 2: Calculate the haploid number
Diploid number (2n) = 16
Haploid number (n) = 16 ÷ 2 = 8
Step 3: Substitute in figures
28 = 256
There are 256 different chromosomal combinations that can occur.