Variation - meiosis and crossing over

Can you please explain variation (specifically crossing over, independent assortment and fertilisation) in good detail that the NCEA markers will mark as excellence?

Hi 16074, welcome to studyIt.
This is such a common question, thanks for asking.
Exam questions about variation with offspring and populations usually involve explanations on how gametes are unique from the parental cells (crossing over and independent assortment occurring during meiosis) and explanations on how fertilisation leads to increased variation.

During meiosis the number of chromosomes is halved. This means during fertilisation the offspring contains half the genetic info from the mother (the egg/ova) and half from the father (sperm). This inherently increases variation when we compare sexual reproduction to asexual (cloning).

During the formation of the gametes the process of meiosis reduces the chromosome number by half. During this process there is crossing over and independent assortment. The exam marker wants you to be able to explain how crossing over and/or independent assortment make unique gametes.

For crossing over, you should explain that when the homologous chromosome line up in the middle of the cell, they can touch and swap alleles. This creates new allele combinations when end up in different gametes. Each time meiosis happens it is random which chromosomes cross over and the number of alleles shared between the homologous chromosomes is also random. This means a high rate of variability between gametes.

A previous user posted a similar question, please look at that post for diagrams to help clarify this process.

In previous years exams, to get Excellence in this type of question, students have been required to explain 2 ways sexual reproduction increases variation, therefore if you explain the halving of chromosome number, crossing over and potentially a 3rd way you should be safe. The 3rd option to explain how variation occurs could be Independent Assortment or you could explain the Random Nature of Fertilisation. I will briefly go over both ideas for you.

Independent Assortment is all about the order that the homologous chromosomes line up in (i.e. which side the maternal vs paternal chromosomes end up on). They line up independently of each other in a random nature and this leads to unique gametes.

You can see in the image above that the order of the blue and red chromosomes directly impacts the combination of chromosomes in the gametes.

Fertilisation is random. Any of the genetically unique sperm generated by a male may fertilise the genetically unique egg produced by a female. The random nature increases the likelihood of variation once again.

All of these process that occur in sexual reproduction increase variation from the parents to the offspring and make variation more common in the population/species.

Just remember to always use the context given in the question, not generic ideas. Being too general might lock you out of Merit or Excellence.

I hope this makes sense, message back if you have anymore questions.
Thanks, and good luck with your revision.

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