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I am learning about meiosis in biology. I've learnt that the crossing-over in metaphase I only exchanges small portions of DNA at the tips of the chromosomes. Doesn't this mean that:

  • After meiosis I, there will be two cells that will be almost opposites (for each chromosome, if one cell has the mother's chromosome, then the other will have the father's chromosome), except for the parts where crossing-over happened
  • After meiosis II, each cell produced in meiosis I will produce two similar cells (the only differences will be where crossing over happened in one but not both of the chromatids)

This means that there would be a rare chance of two of the sperm cells produced by a single meiosis fertilizing two eggs, in which case we could have:

  • Almost-identical twins
  • Very different twins, which would have less than the normal 50% of DNA in common

Am I right about this?

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Great question - I think there are two misconceptions in your question that are interfering with your understanding.

First, crossing-over occurring at just the "tips" is just for clarity in textbooks so you can easily follow the different pieces of chromosome.

Although crossover events tend to occur at particular sequences, these don't have to be at the tips. There can also be multiple crossovers on the same chromosome, or none at all. See this link for some estimates of recombination rates for different species.

Second, gamete diversity doesn't depend only on crossing-over during meiosis, it also depends on the incorporation of just a single chromosome from each pair. Humans, for example, have 23 pairs of chromosomes. There are therefore 2^23 = 8388608 different combinations of chromosomes - with NO crossing over! Therefore, even without recombination, the odds of two genetically identical sperm fertilizing two genetically identical eggs are incredibly small.

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  • $\begingroup$ excellent answer. It is surprising that these particular misconceptions are so commonly and uniformly occurring. $\endgroup$
    – user25568
    Commented Nov 4, 2016 at 16:21
  • $\begingroup$ But doesn't this random splitting of homologous chromosomes only happen in meiosis I? If there was no crossing over, then each meiosis II would produce two identical gametes. $\endgroup$
    – a8848
    Commented Nov 5, 2016 at 4:32
  • $\begingroup$ Even with no crossing over, each pair of chromosomes splits randomly, not according to parentage. Imagine you have only 4 chromosome pairs, where each pair has one from the Mother (M) and one from the Father (F) - note, these will eventually be the 'grandparents' after fertilization - they could split MMMM, MMMF, MMFM, MMFF, MFMM, MFMF, MFFM, MFFF, FMMM, FMMF, etc etc etc - a total of 2^4 = 16 possibilities. If you have 23 chromosome pairs, like, humans, it's over 8 million. And that is still with NO crossing over within the individual chromosomes. $\endgroup$
    – Bryan Krause ♦
    Commented Nov 7, 2016 at 15:32
  • $\begingroup$ Then you add in crossing over and suddenly not only is the segregation of homologous pairs random, but now the individual chromosomes are random too, and you get an incredible potential for diversity. $\endgroup$
    – Bryan Krause ♦
    Commented Nov 7, 2016 at 15:33
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Hmmm what seems to me from your statement;

"there will be two cells that will be almost opposites (for each chromosome, if one cell has the mother's chromosome, then the other will have the father's chromosome), except for the parts where crossing-over happened"

It seems to me; you're missing something.

It would happen if the haploid chromosome number n=1 and no crossingover is there.

But;

If the haploid chromosome number is more than 1; i.e 2n is more than 2 (such as in pea n=7, 2n =14 and human, n=23 and 2n=46);

then at segregation event in meiosis-1 ; any-1 of 2 homologous chromosomes will go to a doughter cell; in a probabilistic manner (and this assortment is independent from other chromosome-pairs).

Take example. If all your somatic cells have a chromosome composition Aa Bb Cc (where with each letters I meant a chromosome and not gene); then perhaps you are thinking it could be seggregated only into ABC + abc. But that is not the case.

In one cell at certain place at site of meiosis, it would be Abc + aBC; in some other cell it would be AbC + aBc; and somewhere else it would ABc + abC, and that way.

In addition; irrespective of how many pieces of chromosome you have; crossing-over will take place in probabilistic way (so; that will make different-gene-exchange at different cells at the site of meiosis.)

So ultimately you get not 2 but many more types of gametes.

Here is 2 simple drawings.

Meiosis first division

  1. Meiosis first division.

Meiosis second division

  1. Meiosis second division.

The book, Concepts of Genetics by Klug, Cummings and Spencer, 8th edition (the ebook I have), chapter 2 (mitosis and meiosis); clearly mentions the random-assortment (physical shuffle) event with examples.

here is a picture from that book.

diagram from Klug

diagram from Klug et al.

Reference: Concepts of Genetics ; Klug, Cummings and Spencer; 8th edition

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  • $\begingroup$ "In one cell at certain place at site of meiosis, it would be Abc + aBC; in some other cell it would be AbC + aBc; and somewhere else it would ABc + abC, and that way." <-- And that is exactly what I mean by "opposite cells"... If one cell inherited a particular chromosome from the mother, then we can say for sure that the other one was inherited from the father. That means that the percentage of DNA the two gametes have in common should be the same as the percentage of DNA that the parents had in common, which is less than the normal 50% between two gametes. $\endgroup$
    – a8848
    Commented Nov 5, 2016 at 4:23
  • $\begingroup$ what is that "normal 50% in common"? $\endgroup$
    – user25568
    Commented Nov 5, 2016 at 4:42
  • $\begingroup$ If you take two gametes at random (produced from different meiosises), then 50% of the DNA should be common. If you take the 4 gametes produced by meiosis, then two pairs of them will have a high percentage of common DNA and two pairs of them will be have a low percentage of common DNA. $\endgroup$
    – a8848
    Commented Nov 5, 2016 at 4:47
  • $\begingroup$ hmm but that is correct only if we focus at 1 particular pair of chromosome. When we look for more than one pairs chromosome; the probability to co-occur the parental combination in a gamete; is lesser than 50% $\endgroup$
    – user25568
    Commented Nov 5, 2016 at 4:53
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    $\begingroup$ That's not what I'm saying. I agree that it would be extremely rare to get all of the DNA from one parent into one cell and all of the DNA in common into the other cell. What I'm saying is that during meiosis I, we will always get things such as "AbCdeFG, aBcDEfg", so the two cells will have no chromosome in common (there will never be an A in both cells or an a in both cells, it will always be A a). Since they have no common chromosomes, they ressemble each other no more than the gametes of two different people. $\endgroup$
    – a8848
    Commented Nov 5, 2016 at 14:10

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