Sex Chromosome Inheritance
Country: United States
Date: January 2007
Is it possible for a son to inherit an allele on
an X chromosome from his father?
The only ways a son could inherit an X chromosome from his father is if the
father's genotype was 1) XXY, or 2) he was created from a sperm that had
both an X and a Y in it.
1) There is a condition known as Klinefelter's Syndrome where a man has 3
sex chromosomes instead of 2 and he has an extra X. Most of the time these
men are sterile however and would not be able to father a child. See #2 to
learn how this condition is made.
2)Normally, males get their X from their mother and their Y from their
father. There is a situation known as nondisjunction that occurs
during anaphase I of meiosis, the time when one is making either eggs (if
you are a woman) or sperm (if you are a man). This is the time when the
parents' chromosomes are being divided in half, so that a new baby gets half
its chromosomes from each parent. Usually, an equal number of chromosomes
goes into each "half". Occasionally, one chromosome doesn't "let go" of its
partner and they to one side or the other together. This results in one
cell with too many chromosomes and another cell with too few. It is random
which of these goes to make the baby. If this happens when the X and Y are
being separated in the father and the child is made from the sperm that has
both in it, the child could get an X from the father.
The only way this could happen is if crossing over occurred between the X
and Y chromosome in the father during meiosis, but this requires homolgous
gene sequences between the X and Y chromosomes and I don't know of any genes
that are located on both the X and Y chromosomes. That being said however,
there must be some homology between the X and Y chromosomes since they do
synapse during meiosis other wise the X and Y chromosomes would assort
independently during meiosis which would result in 1/4 of the sperm carrying
an X and a Y chromosome and 1/4 carrying neither an X nor a Y chromosome.
Ron Baker, Ph.D.
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Update: June 2012