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Meiosis help alevel

During crossing over, in meiosis it says that alleles from the maternal and paternal chromosomes are switched.

Would this mean, for example if I produce gametes, the crossing over would occur between MY mom and dads alleles in my chromosomes? And this allows genetic variation? Honestly i can’t get my head around this because why else would there be paternal chromosome in the process of producing (for example) egg cells?because this was all taken out of my GCSE content, I never really learned this. So yes, I did pull out a GCSE book to understand thisimage.jpgin the 3rd blue box
(edited 8 months ago)
Original post by harlz_chalamet
During crossing over, in meiosis it says that alleles from the maternal and paternal chromosomes are switched.

Would this mean, for example if I produce gametes, the crossing over would occur between MY mom and dads alleles in my chromosomes? And this allows genetic variation? Honestly i can’t get my head around this because why else would there be paternal chromosome in the process of producing (for example) egg cells?because this was all taken out of my GCSE content, I never really learned this. So yes, I did pull out a GCSE book to understand thisimage.jpgin the 3rd blue box

Okay so think about it this way, everyone has 23 chromosomes from their mother and 23 from their father, right? This doesn’t apply to egg cells because they only have 23 chromosomes in total and you can’t split 23 in half and get an equal number from 2 parents. It’s not from the father of the child but the father of the person with the egg cell, does that make sense?

So this is a person with an egg cell. When meiosis happens, the cell duplicates, and then goes through2 divisions to form a haploid cell with only 23 chromosomes. The maternal and paternal chromosomes can switch and are present in the egg cell because it’s not the paternal DNA of the sperm, but the paternal DNA of the father of the person with the gamete. Since gamete cells only have 23 chromosomes, the maternal and paternal chromosomes can arrange in any way - some gametes will, have 16 maternal and 7 paternal chromosomes, some will have 13 maternal and 10 paternal etc etc it’s not equal because it’s a random selection of the person’s DNA.

I know it’s super confusing but I hope this helps!
(edited 8 months ago)
Reply 2
Original post by harlz_chalamet
During crossing over, in meiosis it says that alleles from the maternal and paternal chromosomes are switched.

Would this mean, for example if I produce gametes, the crossing over would occur between MY mom and dads alleles in my chromosomes? And this allows genetic variation? Honestly i can’t get my head around this because why else would there be paternal chromosome in the process of producing (for example) egg cells?because this was all taken out of my GCSE content, I never really learned this. So yes, I did pull out a GCSE book to understand thisimage.jpgin the 3rd blue box


It might be easier to visualize this as separate alleles rather than the normal 'thread' diagrams you see.

Imagine an animal that reproduces sexually, but only has two pairs of chromosomes, the first with four genes, the second with just three. Imagine also that the maternal chromosome only has dominant forms of the gene (alleles in other words), and the paternal chromosome only recessive alleles. This will make it a lot easier to see how the switching works. So here is the adult cell:


Maternal 1 Paternal 1
A a
B b
C c
D d

Maternal 1 Paternal 2
E e
F f
G g

Before meiosis occurs, the chromosomes replicate, so there are two identical copies of each chromosome. They are So now the adult cell looks like this, with each strand of the chromosome joined in the middle in the characteristic 'X' shape:

Maternal 1 Paternal 1
A A a a
B B b b
C C c c
D D d d

Maternal 1 Paternal 2
E E e e
F F f f
G G g g

If we just pulled apart these chromosomes we could make an egg cell with half the number of original chromosomes (so two rather than two pairs), but they would all be identical, not ideal for variation. So during prophase 1 the two strands of the chromosome (known as chromatids) crossing-over. One chromatid from the maternal and one paternal chromatid swap sections of DNA, so you could end up with this if there were three changes.
Maternal 1 Paternal 1
A A a a
B B b b
C c C c
D D d d

Maternal 1 Paternal 2
E e E e
F F f f
G g G g

Now each strand is unique, so variation has increased. The use of 'paternal' here is possibly confusing, since it refers to the original source of the pairs of chromosome you received.

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