Re: Klinefelter syndrome and color blindness

For the purposes of this answer, XW represents a normal X chromosome with wild-type color vision, and XC represents an X chromosome that carries the red-green colorblindness mutation.

Najeeb -

Recall that Klinefelter’s Syndrome is characterized by an XXY sex chromosome karyotype. If we are talking about normal, sex-linked red-green color-blindness, then this is an X-linked mutation. So an affected male suffering from Klinefelter’s Syndrome would be “XC XC Y.” Since the father is phenotypically normal, we know that he must be “XW Y.” That means the mother MUST be “XW XC.” We know this because she is phenotypically normal, and msut therefore possess a normal X chromosome, but since her offspring has the disease, she must also possess a diseased X chromosome. Klinefelter’s disease arises during meiosis when chromosomal nondisjuction occurs. Non disjunction means that during the final split into sex cells (eggs or sperm) one cell gets more than one copy of a chromosome. During meiosis there is initially a round of DNA replication. At this point the mother’s oocyte (the cells that give rise to eggs) has two normal wild type X chromosomes (XW) and two mutated X chromosomes (XC). Meiosis should give rise to four haploid cells, but during the division one of the cells took BOTH copies of the XC chromosomes. So the egg had two copies of the XC chromosomes during fertilization, giving rise to a child with “XC XC Y” karyotype. This child would have both Klinefelter’s Syndrome and red-green colorblindness.