I’ve been reading a lot on the human genome lately, and it got me thinking. Say if I took a man, and were to replicate all 23 of his chromosome pairs to produce a monozygotic twin - a clone - but I replace the Y chromosome with a second copy of his X (or with the X from his father, or the other X from the mother), what would happen?
Obviously, this is not longer a perfect clone, but I’m curious just how genetically similar the not-quite-a-clone (sibling?) would turn out. Eye color, skin color would likely be the same, but how correlated would the personality be? The IQ? Would this even produce a valid (healthy) fetus which could be carried to birth with no defects or diseases?
Tangentially related, if the two were to conceive a child (grossness not withstanding), would crossover always 100% produce the same result, as the two input chromosomes are identical?
Different sex monozygotic twins are extremely rare but they have happened naturally in a few cases. The explanation for the phenomenon is too long to quote but it is presented fairly well here:
No. The chromosomes of eggs and sperm undergo a kind of semi-random exchange between chromosome pairs when they are created; that’s why a given pair of parents don’t just produce an endless string of identical children.
Ignoring the sex chromsomes for a moment, the clones each have two different copies of each autosome, call them A and B. There are usually 1 or 2 crossovers per chromosome, and the crossovers in a sperm and egg will almost always be in different places (the fact that the parents are clones does not significantly influence this).
The expected genotype of the offspring of the clones will be:
for 50% of the genome, heterozygous AB, identical to the parents;
for 25% of the genome, homozygous AA;
for 25% of the genome, homozygous BB.
The genotype will switch between these states several times along the length of each chromosome, corresponding to the crossover points in sperm or egg. Children will not be identical to one another, the particular regions with each genotype will vary with random independent assortment of chromosomes and different crossover points in each sperm/egg, but the overall expected proportion of the genome with each genotype is the same for all children.
The children are obviously heavily inbred, with deleterious recessive alleles in half the genome exposed in the homozygous state. They are twice as inbred as the offspring of a non-identical brother and sister.
Even without crossovers, the offspring of such a pair still wouldn’t be identical (to each other or to the parent). You have two copies of your Chromosome 1, one each from your mother and from your father, and the two are different. Your clone would also have those two. Now, your children could get your mother’s copy from one parent and your father’s copy from the other, but it’s just as likely that they’d get matching chromosomes from you and your clone. Put it all together, and there’s only a 1 in 4 million chance that any given child would be identical to one of you.
No, this mixes up mitosis and meiosis. If eggs and sperm were made by mitosis, this would be it. But they are made by meiosis. The important differences with meisosis, of course along with the reduction to having only one copy of each chromosome instead of two, is that the chromosomes undergo recombination (before the split to single chromosome gamete ) Recombination means semirandom swapping of parts of the chromosomes between them.
No, it doesn’t. This conversation is solely about meiosis. Chronos was talking about the random independent assortment of chromosomes during meotic reduction. He was pointing out that this alone (even without the additional variation from crossover recombination) gives 2^N possible haploid genotypes for the gametes when there are N chromosomes.