Genetic Variation (DP IB Biology: SL)

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)

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)

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

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.

Fusion of gametes from different parents promotes genetic variation

• Meiosis creates genetic variation between the gametes produced by an individual through crossing over and random orientation
• This means each gamete carries substantially different alleles
• During fertilisation, any male gamete can fuse with any female gamete to form a zygote
• This random fusion of gametes at fertilisation creates genetic variation between zygotes as each will have a unique combination of alleles

The different combinations of chromosomes following fertilisation

• In random fertilisation, any two gametes may fuse together
• 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 fertilization is (2²³)^2­­  = 70,368,744,177,664

Step 1: State formula for random fertilisation between any two gametes

(2ⁿ)²

Step 2: Use information from question to state haploid number

n = 8

Step 3: Substitute in figures

(2ⁿ)²

(2⁸)²

Answer  65,536