homologous sex chromosomes

[Aggie2]

I know in humans that the size of the regions of homology on the X and Y chromosomes are very small. In other organisms (e.g., guppies) the X and Y chromosomes are comparable in size. Does that mean that, other than the SRY region, these chromosomes act like autosomes? Would they possess comparable genes and two alleles for each of those genes (at specific loci)?

Thank you,
Aggie2

[david23]

No actually the Y chromosome aside from the SRY region has very little genes. Some useful, some not. The difference between x and y are therefore huge. As you probably noticed that most x-linked diseases are genes that are defective on the X. Those genes are essential and not available on Y

[Aggie2]

david23,

Thank you for your reply.

The paucity of genes on Y is certainly true for humans and species with truncated Y chromosomes. I'm wondering about certain fish (e.g., poecilia reticulata), where the Y definitely contains genes/alleles which control color patterns and other male ornamental characteristics. In this case the sex chromosomes are of equal size and crossing over does appear to take place. Some have suggested that the sex chromosomes may function as pseudo-autosomes in these animals.

Any thoughts?

Aggie2

[MrMistery]

Well, first of all i have a question: Are you sure that animal has a SRY gene? Because not all species have the same way of determining sex.

Second: size has nothing to do with it. Saying that they have big homologous regions just because they are similar in size makes as much sense as saying that a my house and the tree next to it have many things in common because they have the same height. However, if crossing over occurs(which can be tested, by construction of a linkage map) then it is obvious that they do have homologous regions.
I am afraid that if you do not have results of crossings(or better yet a physical map/DNA sequence) there isn't much you can say about the situation.

[Aggie2]

Mr. Cheese,

Are you sure that animal has a SRY gene?

Yes

Saying that they have big homologous regions just because they are similar in size makes as much sense as saying that a my house and the tree next to it have many things in common because they have the same height.

I didn't say that they have big homologous regions. I'm not sure how pertinent your analogy is, but if you are referring to differences in euchromatin and heterochromatin, I don't believe that is known. BTW, if that's what you wanted to know, you might have just asked.

However, if crossing over occurs(which can be tested, by construction of a linkage map) then it is obvious that they do have homologous regions.

Funny, I thought I made myself quite clear. Crossing over does occur. In fact, this organism is one of the first where this was demonstrated. Perhaps your comments would be more helpful if you actually knew something about this organism.

Thanks so much for your help.

Aggie2

[david23]

I think we established that SRY is needed to turn on other genes, especially genes used to produce testosterone which then turns on other genes that control male characteristics. So these other genes, where are they located was what I never really learned too much in depth. So are you are saying they are located on Y as well?

I mean what I can say about it is that because of the difference in chromosome formation of X and Y, there should be significant difference in the contents between the two as well.