Do a brother and a sister share less genes then two brothers or two
sisters because they have different sex chromosomes.

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Clarification of Question by arkin-ga on 09 Dec 2003 21:26 PST

Thanks for responding.I have read that during the process of
separation,the male y chromosome cannot easily swab information with
the x chromosome,so i was wondering if that would affect how much
different-sex siblings are related.Thanks again for responding.

No and yes.  Is the measurement of more or less shared genes based on
phenotypic expression, genotypic expression or just chromosomal
homology?  Phenotypically you would think that less genes are shared
by a brother and sister because the two sexes are so different. 
Genotypically, sex does not determine the amount of homologous gene
expression between siblings.  In the human, you could say that less
genes are shared between two brothers because the Y chromosome encodes
less genes than the X chromosome

Yup, masterryan has it right. 

I just wanted to add that it is chance that determines the degree that
two siblings will share genetic similarites (alleles). Every person
has two copies of every gene (excluding male sex chromosomes).  We get
these copies from our parents. When eggs and sperm are formed in the
during a process called meiosis there is a step where each chromosome
will curl up with its matching chromosome. While they are curled up
together, pieces of the chromosomes will swap from one to the other.
This happens very randomly, and is controlled only by chance. This
swapping step is called "crossing over".

Crossing over is very important in determining which alleles of your
parents' genes you will recieve.  You only get one copy of every gene
from your mom and one copy of every gene from your dad. Your parents
actually have two copies of each gene so it's up to chance which one
of theirs you will get. This is how two full blood siblings could
share more genetic information than other full blood siblings. If, by
chance, you and a sibling get most of the same genes from each parent,
then you wil be more geneticly similar that another sibling that got a
different set of genes.

Because the sex chromosomes are a reletively small subset of the whole
complement of genetic information, the difference between two siblings
of opposite gender may not necessarily be greater than two siblings of
the same gender who, by chance, got very different gene sets.

hopper