Question

Bob has Klinefelter's syndrome (XXY) has normal color vision. His mother was color-blind due to a homozygous recessive mutation on the X chromosome (XbXb), but his father had normal color vision (XBY). Which parent underwent non- disjunction? Explain your answer.

Answer 1

Non dis-junction occurs when the pair of homologous chromosome fail to separate from each other.

Colour blindness is controlled by a recessive sex linked gene which is usually found on X chromosome.

Here the recessive gene responsible for color blindness has been designated as "Xb" and the gene dominant over it as "XB".

As per the above situation given in the question the Male parent or the father of Bob has underwent non-disjunction.

In normal process ( which is absent here)

The chromosome of mother XbXb (colourblind) will be divided in to two ova containing Xb in each ovum. While the chromosome of the father XBY (Normal) will be divided into XB and Y. As Bob is a male child the Y chromosome of the father will fertilize any of the ovum of the mother containing Xb and hence the new combination of the chromosome in the Bob will be XbY which will develop the colorblind disease in him.

Parent                                   Male                                    Female

XBY(Normal)                      XbXb(colorblind)

F1 generation XBXb XbY (colorblind)

(Bob)

But as per the given question it hasn't happened here.

What has happened here ?

Here only the female or mother chromosome has been divided into Xb and Xb undergoing dis-junction and contributing one Xb to each ovum. while the male chromosome hasn't undergone dis-junction and the sperm contains the parental set of chromosome XBY. So when fertilization takes place between the sperm containing XBY and ovum containing Xb, the resulting chromosome combination becomes XBXbY (depicted here as XXY) which is found in Bob. Also as the colorblind causing gene is recessive hence it is dominated by the normal gene XB and the Bob has normal vision.

Parent Male Female

XBY XbXb

resulting gamates XBY Xb & Xb

(due to non-disjunction)

After fertilization XBXbY (Bob) and Xb (remains unfertilized)

( Develops Klinefelter's syndrome)