# Are chromosomes from each parent split between gametes with equal probability (esp. in humans)?

I've recently read a little on Wikipedia about genetics, but I can't find a direct answer to this question.

My rough understanding is this:

• Both males and females have pairs of chromosomes, one copy from each parent
• Both produce gametes (eggs in females, sperm in males)
• The gamete is produced during something called 'meiosis' to take only one set of genes

What I can't find an answer to though is whether that selection is completely random, such that every chromosome in a gamete has an equally likely chance of being taken from the male parent set and the female parent set.

Another way of putting it, perhaps: Would the range of 100% male parent to 100% female parent chromosome selection in the gamete be a binomial distribution?

My background is computing, so please assume very little knowledge of biology.

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To clarify on your question: Imagine a species with two pairs of chromosomes, with one member of each pair coming from each parent. So there is one copy of chromosome 1 from the father (1F) and one from the mother (1M) and the same for chromosome 2 (2M and 2F) with one member of each pair going in to a gamete. Do you want to know whether the 1M+2M or 1F+2F combinations would be more common in the gametes than the 1M+2F or 1F+2M combinations? – rg255 Jul 5 '13 at 10:40
Not quite - what I want to know is whether nM is just as likely to be in the gamete as nF. Another way of putting it (maybe): Is it correct to say (in a human for instance) that a gamete has a 1 in 2^23 (~1 in 8 million) chance of having entirely the nM or nF set of chromosomes? Also, in that extreme case, would such a gamete be indistinguishable from a gamete from the parent? Say, a male producing a sperm with 23 chromosomes all from the male parent. – Marcos Scriven Jul 5 '13 at 10:48
I have heard of some research being done in to whether the chromosomes associate in such a way but can't remember who was doing it, also you might want to look at meiotic drive elements. However recombination occurs between chromosomes so the chromosome in a gamete is also likely to be a cocktail of the parental ones - here is a wiki section that should help, I might write an answer this weekend for you en.wikipedia.org/wiki/Intragenomic_conflict#Nuclear_genes – rg255 Jul 5 '13 at 11:06

Other answers are correct in their own terms, but because of homologous recombination during meiosis, the idea of maternal and paternal chromosomes becomes meaningless. Any chromosome in a gamete will be a mosaic of the maternal and paternal versions. In this sense the chromosome is not the unit of inheritance.

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Ah.... that's really helpful, I didn't realise the chromosomes themselves were mixed up like that. – Marcos Scriven Jul 5 '13 at 11:05
Excellent point. I can't believe I forgot to mention that. – user3934 Jul 5 '13 at 11:09
Have marked as the answer - but one other question in relation to this, if I may. This 'mosaic', what would be the 'unit' of this? Is it at the gene level? So a resulting chromosome would be a mixture of genes from each parent chromosome pair? – Marcos Scriven Jul 5 '13 at 11:21
@MarcosScriven no it is not at the gene level, it is usually in a much larger scale than that, resulting in the mix of groups of genes. This is a process that has nothing to do with genes really, if genes are the paragraphs in a book, meiotic recombination consists of mixing up the pages. This means that genes can be split as well. – terdon Jul 5 '13 at 12:27

Yes, for all intents and purposes, the assorting of chromosomes during meiosis is random. In fact, it's difficult to imagine a mechanism by which the chromosomes would assort in any non-random manner (i.e., how would the maternal chromosomes "identify" each other so as to assort together during meiosis?)

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That was indeed my intuition, but I wanted to comfirm it. My only thought was perhaps there could be a case where a chromosome from one parent may have some characteristic (however minor) that made it more likely to 'follow' another chromosome from the same parent. Another way to put it - I recall reading about how during development symmetry is even possible (it was above me, but something to do with ion flow). I wondered if there was some similarly subtle mechanism possible in this case. – Marcos Scriven Jul 5 '13 at 11:03

Yes, every chromosome in gamete has an equal probability of being taken from the male parent set and the female set.

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Thanks for answer. While I'm here is there a technical term for 'male parent chromosome set'? – Marcos Scriven Jul 5 '13 at 10:54

As far as we know, chromosomes tend to be equally distributed across normal gameteogenesis, following Mendel's second law of independent assortment. However some versions of DNA repair and chromosomal condensation can malfunction and result in more versions of one parents genes in a cell than another. Two phenomena which are exceptions to the equal parental ratio are Gene Conversion and Chromosomal Fusions.

Although a much smaller scale than chromosome segregation, Gene Conversion is a case of unequal allele transmission. While repairing double strand breaks during meiosis, one parent's DNA may be used as the repair template instead of the original version. This phenomena distorts the 1:1 allele ratio on the scale of hundreds of base pairs.

Robertson translocations are when two nonhomologous chromosomes fuse into one. Chromosomes which were once separate, segregate together into the same gamete. These cases can serve as exceptions to Mendel's second law of independent assortment.

There are also several cases of transmission distortion, while these cases may not affect the segregation of chromosomes during meiosis, they do cause unequal fertilization. The t-haplotype in male house mice, is passed on to up to 95% of offspring! The mechanism is still under investigation, but it appears that this gene when expressed in sperm, kills all other sperm without the t gene. The Sd gene is a similar element in Drosophila. A great book on selfish genetic elements and intragenomic conflict is Genes in Conflict by Burt and Trivers.

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