When the germ cells mature, the collective condition found in the ovary with respect to X chromosome distribution will produce the series of gametes listed on the left side of the Punnett Square. When fertilized by normal male sperm, the results are as shown. The top row consists of normal flies. Abnormal flies appear in the middle and bottom row with two of the remaining four classes lethal. This leaves two types of rather unusual fly... an XX / and an XO. Which one is the male and which one is the female? Make a guess and then click on the ANSWER button. Ok, assuming you have clicked on the ANSWER button, what does this tell you about sex determination in Drosophila? Think about this and enter your comments below. Then scroll down the window for the answer.
YOUR OPINION:
In Drosophila, sex is determined by the ratio between X chromosomes and the autosomes. The Y chromosome apparently has very little to do with sex determination in this species. Hence the XO fly has the same X to autosome ratio as a normal male. Hence the XO fly is a male with the X chromosome coming from his father. (Normally the male inherits only the Y chromosome from his father.) The XX/ fly is then a female because the X to autosome ratio is the same as in normal females. NOTE THAT AS WITH MANY GENES, THE PHENOTYPIC EFFECT PRODUCED BY A GENOTYPE IS THE RESULT OF BALANCED FORCES OPERATING BETWEEN MANY GENES. SEX DETERMINATION IS AN EXCELLENT EXAMPLE OF THIS PHENOMENON. For nondisjunctive problems in humans, flip to the next card.
