Genetic Variation

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•  Crossing over and random orientation promotes genetic variation

•  Fusion of gametes from different parents promotes genetic variation

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 (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 


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

  • 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

assortment and variation

Random Fertilisation

The fusion of two haploid gametes results in the formation of a diploid zygote

  • This zygote can then divide by mitosis and differentiate to form a developing embryo

As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes

  • Identical twins are formed after fertilisation, by the complete fission of the zygote into two separate cell masses

random fertilisation