Monosomy and trisomy can be represented as

Which of these is not a Mendelian disorder?

Down's syndrome in humans is due to

$A$ cross between a white-eyed female and a red-eyed male Drosophila gives red-eyed females and white-eyed males. Occasionally,the cross gives white-eyed females and red-eyed males. This is due to:

Which of the following disorders does not result in a sterile individual?

What is the cause of $Down's$ syndrome?