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Origins of genetic variation

Let’s look at 3 broad categories of source of genetic variation: mutation, meiosis and fertilisation.

DNA mutation occurs spontaneously and creates a direct change in the genetic material. Single bases may be changed, added or removed, or even large chunks of chromosomes can be swapped between them or duplicated. Whole chromosomes, too, can shift around.

Mutations are a random occurrence during DNA replication and the rate of mutation is influenced by external factors such as UV radiation. There are different types of mutation:

1. Deletion where a nucleotide base is deleted. AGTCA becomes AGCA.
2. Substitution where a nucleotide base is replaced by another. AGTCA becomes AGTCG.
3. Insertion where a nucleotide base is added as extra. AGTCA becomes ATGTCA.

Meiosis adds to genetic diversity via independent assortment of homologous chromosomes and crossing over. This creates gamete cells that are each different from the next, with millions of possible combinations of chromosomes. A brief review of these two events:

How does meiosis achieve genetic diversity without which you would actually look *just* like your siblings?

10 words: Independent Assortment of Homologous Chromosomes, &
Genetic Recombination by Crossing Over

What an unnecessary mouthful. You still have to learn them though.

Independent assortment of homologous chromosomes means that in meiosis I, when the original diploid line-up a.k.a. XX xx XX XX xx XX xx xx becomes X x X X x X x x in 2 resulting cells, which big X’s and which small x’s end up with each other in each cell is random. Pretty simple concept.

XX xx

If you split the homologous chromosomes, you get Xx in 2 cells. The idea is that there is no rule saying that black must go with black, and red must go with red. You can end up with Xx and Xx, or Xx and Xx with an equal probability. What can I say, genetics likes being a bit random.

Not assigning the expected chromosome or chromatid during meiosis is called chromosome non-disjunction and results in a cell with a different number of chromosomes.

Genetic recombination by crossing over is a lot more interesting. It’s like a bowl of spaghetti. Homologous chromosomes snuggle each other and exchange parts in the process:

Did I mention how important it is to use accurate scientific terminology in the exams? The process is called synapsis, during which mutual exchange of genetic information occurs.

Independent fertilisation means that of these millions of different possible cells, only 2 actually take part in fertilisation. This event creates an additional level of genetic diversity.

Ok byeeeeeeeeee





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