Meiosis: Sources of Genetic Variation
- Having genetically different offspring can be advantageous for natural selection
- Meiosis has several mechanisms that increase the genetic diversity of gametes produced
- Both crossing over and independent assortment (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 meiosis I homologous chromosomes pair up and are in very close proximity to each other
- The non-sister chromatids can cross over and get entangled
- These crossing points are called chiasmata
- The entanglement places stress on the DNA molecules
- As a result of this a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
- This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes
- There is usually at least one, if not more, chiasmata present in each bivalent during meiosis
- Crossing over is more likely to occur further down the chromosome away from the centromere
Independent assortment
- Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I
- The different combinations of chromosomes in daughter cells increases genetic variation between gametes
- In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle
- Each pair can be arranged with either chromosome on top, this is completely random
- The orientation of one homologous pair is independent / unaffected by the orientation of any other pair
- The homologous chromosomes are then 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 following meiosis
- The number of possible chromosomal combinations resulting from meiosis is equal to 2n
- n is the number of homologous chromosome pairs
- For humans: the diploid number for humans is 46 then the haploid number or number of homologous chromosomes is 23 so the calculation would be:
- 223 = 8 388 608 possible chromosomal combinations
- Meiosis creates genetic variation between the gametes produced by an individual through crossing over and independent assortment
- This means each gamete carries substantially different alleles
- During fertilization any male gamete can fuse with any female gamete to form a zygote
- This random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles
- There is an almost zero chance of individual organisms resulting from successive sexual reproduction being genetically identical
The different combinations of chromosomes following fertilisation
- In random fertilisation, any two gametes may combine
- Therefore the formula to calculate the number of combinations of chromosomes after the random fertilisation of two gametes is (2n)2
- n is the haploid number and 2 is the number of gametes
- Therefore in humans, when the haploid number is 23, the number of combinations following fertilisation is (223)2 = 70368744177664
- This explains why relatives can differ so much from each other. Even with the same parents, individuals can be genetically distinct due to variation at the meiosis and fertilisation stage (as well as other possible mutations and crossing-over)
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.
Worked example
Derive a formula to calculate the number of combinations of chromosomes after the random fertilisation of an ovule and pollen nuclei from this plant species.
[2 marks]
Step 1: State formula for random fertilization between any two gametes
(2n)2
Step 2: Use information from previous question to state haploid number
n = 8
Step 3: Substitute in figures
(2n)2
(28)2
Formula is (28)2
Examiner Tip
Several sources of genetic variation have been outlined above. It is also worth remembering that genetic variation can occur on an even smaller scale than chromosomes. Mutations can occur within genes. A random mutation that takes place during DNA replication can lead to the production of new alleles and increased genetic variation.Don’t worry about the effects of crossing over when you are calculating different chromosomal combinations. This is not something you are expected to take into account when using the formulas outlined above.
