Correct.
Most likely because they are very rare compared to male hemophiliacs.
This is not only because the recessive allele is itself rare, but also because a female hemophiliac has to have a father who is himself hemophiliac. Before treatments existed, male hemophiliacs often died in childhood.
To take this further, and I’ve always been a bit hazy on the details…
Is it because the chromosomes are not simply swapping genes (or more correctly alleles) willy-nilly, but the cross over process is literally a physically crossing over of the chromosomes and whole chunks are passed between the two determined by the cross over points? So, is it a physical limitation of the chromosome itself to bend in space and cross over so that having two cross over points with only one gene inbetween would be a very rare occurance? Does the location of the centromere have any significance in this process?
Also: On a given chromosome pair do the number of cross overs follow some statistical distribution (maybe a Poisson distribution) so that we could say that X number of cross overs would occur in Y number of cases? And would that be determined by the length of the chromosome and/or some other parameters?
If that’s too big a subject to deal with in this thread, just let me know. I’m really only asking about what you know off the top of your head. Thanks in advance, as always!
If I remember right, a single sperm is unable to penetrate the ovum on its own. There needs to be hundreds or thousands of sperm pecking away at the ovum cell wall in order to “soften it up” enough that some random sperm can force its way through. I think it works because each sperm carries a tiny bit of enzyme or other “solvent” that individually is too little to do much. But when they arrive in the thousands, it adds up enough to prep the ovum for fertizilation. This is why a low sperm count can be bad for fertility. Not enough sperm reach the egg to soften its tough outer wall.
Yes, there is a physical breakage and re-linkage going on between the two chromosomes. Double crossovers occur, but the fact that they do not often just affect one gene may have more to do with the statistical probability of where breakages take place than physical limitations of the chromosome. Offhand, I don’t know what the effect of the centromere might be, if any.
Wiki article here:
There is a statistical distribution of crossovers between genes on the same chromosome, which is used to calculate recombinant frequency and was also used to map genes before DNA could be sequenced directly.
Wiki article here:
A centromere tends to repress crossing over, so recombination is more frequent at the ends of the chromosome. In addition, a crossover event will itself tend to repress other crossovers in its vicinity, so double crossover events, when they do occur, will cause entire blocks of genes to recombine rather than a single gene.
Are there identifiable traits that are known to only occur in the end sections of the chromosome? What I’m wondering is: does this result in a greater likelihood of variation in certain traits than other?
Since you have a 50/50 chance of inheriting either chromosome, what difference does it make which one an allele is located on (either before or after meiosis)?
Imagine shuffling a deck. What’s the odds of drawing the ace of spades? Now, shuffle the deck twice as long. Do the odds change?
You will find certain traits have a tendancy to be inhereted together more than others, but that’s a slightly different matter (see my note about linked genes, above). Additionally, there are regulatory genes (genes that turn other genes on or off), and where they are positions realative to othere genes on the chromosome can have some affect, but there is no requirement that the genes they regulate be located on the same chromosome in the first place.
I would say that the effect you are asking about would be tiny, if it exists at all.