Question

The following pedigree shows a person affected with the sex chromosome aneuploidy called Klinefelter syndrome (XXY). Recall that human aneuploidy is caused by nondisjunction during meiosis. A and B are two codominant alleles of an x linked gene; the phenotypes of individuals for this x linked gene are shown. In which parent of the affected person did nondisjuction occur, and was the nondisjuction event in meiosis I or II? Check all that are possible.

- Father; Meiosis I

- Father; Meiosis II

- Mother; Meiosis I

- Mother; Meiosis II

Answer 1

FAILURE OF SEGREGATION OF CHROMATIDS DURING CELL DIVISION CYCLE RESULTS IN THE GAIN OR LOSS OF CHROMOSOSMES. THIS IS CALLED ANEUPLOIDY.

KLINFELTERS SYNDROME IS A GENETIC DISORDER CAUSED DUE TO THE PRESENCE OF AN ADDITIONAL COPY OF X CHROMOSOME RESULTING IN TO A KARYOTYPE OF 47 XXY INSTEAD OF 46 XY OR 46 XX.SUCH INDIVIDUALS ARE STERILE.

THE EXTRA CHROMSOME IS DUE TO THE NON DISJUNCTION DURING MEOSIS 1 .MEOSIS 1 IS AGAIN DIVIDED IN TO PROPHASE 1 , METAPHASE 1 , ANAPHASE 1 AND TELOPHASE 1.

DURING PROPHASE 1 CROSSING OVER OCCURS BETWEEN NON SISTER CHROMATIDS ONE PATERNAL CHROMOSOSME AND ONE MATERNAL CHROMOSOME.

DURING METAPHASE 1 CHROMOSOEMS ALIGN AT THE EQUITORILA PLATE.

DURING ANAPHASE 1 EACH DIAD   IS  PULLED TOWARDS THE OPPOSITE POLES  OF THE DIVIDING CELL. THIS SEPERATION PROCESS IS KNOWN AS DISJUNCTION . ANY FAILURE IN THIS SEPERATION PROCESS RESULTS IN NONDISJUNCTION .

DURING TELOPHASE 1 CELL DIVIDES IN TO TWO.

NONDISJUNCTION RESULTS IN MOVING OF TWO CHROMTIDS TO A SINGLE CELL INSTEAD OF MOVING IN TO SEPERATE CELLS. THIS NONDISJUNCTION MAY OCCUR IN SPERM OR EGG . THAT IS BOTH MOTHER AND FATHER HAVE EQUAL CHANCES OF NONDISJUNCTION DURING MEOSIS 1.

EVENTS IN MEOSIS 2 IS JUST LIKE MITOSIS . MEOSIS 2 IS KNOWN AS EQUATIONAL DIVISION. NONDISJUNCTION DOESNOT OCCUR DURING MEOSIS 2

SO THE NONDISJUNCTION MAY OCCURED IN FATHER DURING MEOSIS 1 OR IN MOTHER DURING MEOSIS 1.