Question

What type of abnormality in the sperm that fertilizes a normal egg will produce an XXY zygote? What process can lead to the formation of this abnormal gamete?

Answer 1

Sex chromosome abnormalities may occur due to chromosomal mutations or problems that occur during meiosis. In normal zygotes, haploid male (Y) and haploid female (X) chromosomes or two haploid female chromosomes pair to form a diploid zygote (XY or XX, respectively).

XXY zygote formation is an abnormality known as Klinefelter syndrome (KS). This occurs due to nondisjunction of the sister chromatids during meiosis. In this disorder, male babies have an extra X chromosome. The extra X chromosome comes from the father or mother nearly equally.

The abnormality in the sperm is nondisjunction of paternal meiosis 1. The relevant nondisjunction in meiosis 1 occurs when homologous chromosomes, in this case, X and Y fail to separate producing a sperm with an X and Y chromosome. Fertilizing a normal (X) egg with this sperm produces an XXY offspring. However, KS can also occur by nondisjunction of maternal meiosis 1. Thus when the XX egg fertilizes a normal (Y) sperm, it produces an XXY zygote.

KS is diagnosed by the genetic test known as Karyotype as the zygote has 47 chromosomes, instead of 46.